6.3: Geology

Standardize the order of geological processes and respond to them for each planet

What is Geology?

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.

Figure \(\PageIndex{1}\) A schematic view of the rock cycle. The rock cycle takes place both above and below the Earth’s surface. The rock deepest beneath the earth’s surface, and under extreme heat and pressure, is metamorphic rock. This metamorphic rock can melt and become magma. When magma cools below the earth’s surface, it becomes “intrusive igneous rock.” If magma cools above the earth’s surface, it is “extrusive igneous rock” and becomes part of the outcrop. The outcrop is subject to weathering and erosion, and can be moved and redeposited around the earth by forces such as water and wind. As the outcrop is eroded, it becomes sediment which can be buried, compacted, and cemented beneath the Earth’s surface to become sedimentary rock. As sedimentary rock gets buried deeper and comes under increased heat and pressure, it returns to its original state as metamorphic rock. Rocks in the rock cycle do not always make a complete loop. It is possible for sedimentary rock to be uplifted back above the Earth’s surface and for intrusive and extrusive igneous rock to be reburied and become metamorphic rock.

In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma. As we’ll see in more detail below, magma is rock that is hot to the point of being entirely molten, with a temperature of between about 800° and 1300°C, depending on the composition and the pressure.

Figure \(\PageIndex{2}\) Magma forming pahoehoe basalt at Kilauea Volcano, Hawaii.

Earth Science is the study of all aspects of our planet Earth. Earth Science is not just about the molten lava, icy mountain peaks, steep canyons and towering waterfalls of the continents. Earth Science includes the atmosphere and oceans. The field also looks out into the solar system, galaxy, and universe. Earth scientists seek to understand the beautiful planet on which we depend (figure 1).

Figure 1

Different branches of Earth Science study one particular part of Earth. Since all of the branches are connected, specialists work together to answer complicated questions. Let’s look at some important branches of Earth Science.

Geology is the study of the solid Earth. Geologists study how rocks and minerals form. The way mountains rise up is part of geology. The way mountains erode away is another part. Geologists also study fossils and Earth’s history. There are many other branches of geology. There is so much to know about our home planet that most geologists become specialists in one area. For example, a mineralogist studies minerals, as seen in (figure 2).

Figure 2. (A) Mineralogists focus on all kinds of minerals. (B) Seismographs are used to measure earthquakes and pinpoint their origins.

Some volcanologists brave molten lava to study volcanoes. Seismologists monitor earthquakes worldwide to help protect people and property from harm (figure 2). Paleontologists are interested in fossils and how ancient organisms lived. Scientists who compare the geology of other planets to Earth are planetary geologists. Some geologists study the Moon. Others look for petroleum. Still others specialize in studying soil. Some geologists can tell how old rocks are and determine how different rock layers formed. There is probably an expert in almost anything you can think of related to Earth!

Geologists might study rivers and lakes, the underground water found between soil and rock particles, or even water that is frozen in glaciers. Earth scientists also need geographers who explore the features of Earth’s surface and work with cartographers, who make maps. Studying the layers of rock beneath the surface helps us to understand the history of planet Earth (figure 3).

Figure 3. These folded rock layers have bent over time. Studying rock layers helps scientists to explain these layers and the geologic history of the area.

In its broadest sense, geology is the study of Earth—its interior and its exterior surface, the minerals, rocks and other materials that are around us, the processes that have resulted in the formation of those materials, the water that flows over the surface and through the ground, the changes that have taken place over the vastness of geological time, and the changes that we can anticipate will take place in the near future. Geology is a science, meaning that we use deductive reasoning and scientific methods to understand geological problems. It is, arguably, the most integrated of all of the sciences because it involves the understanding and application of all of the other sciences: physics, chemistry, biology, mathematics, astronomy, and others. But unlike most of the other sciences, geology has an extra dimension, that of time—deep time—billions of years of it. Geologists study the evidence that they see around them, but in most cases, they are observing the results of processes that happened thousands, millions, and even billions of years in the past. Those were processes that took place at incredibly slow rates—millimeters per year to centimeters per year—but because of the amount of time available, they produced massive results.

Geology is displayed on a grand scale in mountainous regions, perhaps nowhere better than the Rocky Mountains in Canada (Figure \(\PageIndex{1}\)). The peak on the right is Rearguard Mountain, which is a few kilometers northeast of Mount Robson, the tallest peak in the Canadian Rockies (3,954 meters). The large glacier in the middle of the photo is the Robson Glacier. The river flowing from Robson Glacier drains into Berg Lake in the bottom right. There are many geological features portrayed here. The sedimentary rock that these mountains are made of formed in ocean water over 500 million years ago. A few hundred million years later, these beds were pushed east for tens to hundreds of kilometers by tectonic plate convergence and also pushed up to thousands of meters above sea level. Over the past two million years this area—like most of the rest of Canada—has been repeatedly glaciated, and the erosional effects of those glaciations are obvious.

The Robson Glacier is now only a small remnant of its size during the Little Ice Age of the 15th to 18th centuries, and even a lot smaller that it was just over a century ago in 1908. The distinctive line on the slope on the left side of both photos shows the elevation of the edge of the glacier a few hundred years ago. Like almost all other glaciers in the world, it receded after the 18th century because of natural climate change, is now receding even more rapidly because of human-caused climate change.

Figure \(\PageIndex{1}\) Rearguard Mountain and Robson Glacier in Mount Robson Provincial Park, BC. Left: Robson Glacier in 2012. Right: Robson Glacier circa 1908.  (left): © Steven Earle. CC BY. (right): A.P. Coleman. Public domain. Source: Arthur P. Coleman Collection at Victoria University Library.

Geology is also about understanding the evolution of life on Earth; about discovering resources such as water, metals and energy; about recognizing and minimizing the environmental implications of our use of those resources; and about learning how to mitigate the hazards related to earthquakes, volcanic eruptions, and slope failures. All of these aspects of geology, and many more, are covered in this textbook.

What are scientific methods?

There is no single method of inquiry that is specifically the “scientific method”; furthermore, scientific inquiry is not necessarily different from serious research in other disciplines. The most important thing that those involved in any type of inquiry must do is to be skeptical. As the physicist Richard Feynman once said: the first principle of science is that “you must not fool yourself—and you are the easiest person to fool.” A key feature of serious inquiry is the creation of a hypothesis (a tentative explanation) that could explain the observations that have been made, and then the formulation and testing (by experimentation) of one or more predictions that follow from that hypothesis.

For example, we might observe that most of the cobbles in a stream bed are well rounded (see photo above), and then derive the hypothesis that the rocks are rounded by transportation along the stream bed. A prediction that follows from this hypothesis is that cobbles present in a stream will become increasingly rounded as they are transported downstream. An experiment to test this prediction would be to place some angular cobbles in a stream, label them so that we can be sure to find them again later, and then return at various time intervals (over a period of years) to carefully measure their locations and roundness.

A critical feature of a good hypothesis and any resulting predictions is that they must be testable. For example, an alternative hypothesis to the one above is that an extraterrestrial organization creates rounded cobbles and places them in streams when nobody is looking. This may indeed be the case, but there is no practical way to test this hypothesis. Most importantly, there is no way to prove that it is false, because if we aren’t able to catch the aliens at work, we still won’t know if they did it!

What Do Geologists Do?

Geologists are involved in a range of widely varying occupations with one thing in common: the privilege and responsibility of studying this fascinating planet. In Canada, many geologists work in the resource industries, including mineral exploration and mining and energy exploration and extraction. Other major areas where geologists work include hazard assessment and mitigation (e.g., assessment of risks from slope failures, earthquakes, and volcanic eruptions); water supply planning, development, and management; waste management; and assessment of geological issues in the forest industry, and on construction projects such as highways, tunnels, and bridges. Most geologists are employed in the private sector, but many work for government-funded geological organizations, such as the Geological Survey of Canada or one of the provincial geological surveys. And of course, many geologists are involved in education at the secondary and the post-secondary levels.

Some people are attracted to geology because they like to be outdoors, and it is true that many geological opportunities involve fieldwork in places that are as amazing to see as they are interesting to study. But a lot of geological work is also done in offices or laboratories. Geological work tends to be varied and challenging, and for these reasons and many others, geologists are among those who are the most satisfied with their employment.

Figure \(\PageIndex{1}\) Geologists examining ash-layer deposits at Kilauea Volcano, Hawaii.  © Steven Earle. CC BY.

In Canada, most working geologists are required to be registered with an association of professional geoscientists. This typically involves meeting specific post-secondary educational standards and gaining several years of relevant professional experience under the supervision of a registered geoscientist. More information can be found at Engineers and Geoscientists British Columbia.

Rocks and Minerals

The rest of this chapter is devoted to a brief overview of a few of the important aspects of physical geology, starting with minerals and rocks. This is followed by a review of Earth’s internal structure and the processes of plate tectonics, and an explanation of geological time.

The Earth is made up of varying proportions of the 90 naturally occurring elements—hydrogen, carbon, oxygen, magnesium, silicon, iron, and so on. In most geological materials, these combine in various ways to make minerals. Minerals will be covered in some detail in Chapter 2, but here we will briefly touch on what minerals are, and how they are related to rocks.

A mineral is a naturally occurring combination of specific elements that are arranged in a particular repeating three-dimensional structure or lattice.^[1] The mineral halite is shown as an example in Figure \(\PageIndex{1}\).

Figure \(\PageIndex{1}\) The lattice structure and composition of the mineral halite (common table salt).  © Steven Earle. CC BY.

In this case, atoms of sodium (Na: purple) alternate with atoms of chlorine (Cl: green) in all three dimensions, and the angles between the bonds are all 90°. Even in a tiny crystal, like the ones in your salt shaker, the lattices extend in all three directions for thousands of repetitions. Halite always has this composition and this structure. Note: Element symbols (e.g., Na and Cl) are used extensively in this book. , you will find a list of the symbols and names of the elements common in minerals and a copy of the periodic table. Please use those resources if you are not familiar with the element symbols.

There are thousands of minerals, although only a few dozen are mentioned in this book. In nature, minerals are found in rocks, and the vast majority of rocks are composed of at least a few different minerals. A close-up view of granite, a common rock, is shown in Figure \(\PageIndex{2}\). Although a hand-sized piece of granite may have thousands of individual mineral crystals in it, there are typically only a few different minerals, as shown here.

Figure \(\PageIndex{2}\) A close-up view of the rock granite and some of the minerals that it typically contains (H = hornblende (amphibole), Q = quartz and F = feldspar). The crystals range from about 0.1 to 3 millimeters (mm) in diameter. Most are irregular in outline, but some are rectangular.  © Steven Earle. CC BY.

Rocks can form in a variety of ways. Igneous rocks form from magma (molten rock) that has either cooled slowly underground (e.g., to produce granite) or cooled quickly at the surface after a volcanic eruption (e.g., basalt). Sedimentary rocks, such as sandstone, form when the weathered products of other rocks accumulate at the surface and are then buried by other sediments. Metamorphic rocks form when either igneous or sedimentary rocks are heated and squeezed to the point where some of their minerals are unstable and new minerals form to create a different type of rock. An example is schist.

A critical point to remember is the difference between a mineral and a rock. A mineral is a pure substance with a specific composition and structure, while a rock is typically a mixture of several different minerals (although a few types of rock may include only one type of mineral). Examples of minerals are feldspar, quartz, mica, halite, calcite, and amphibole. Examples of rocks are granite, basalt, sandstone, limestone, and schist.

Key Takeaway: Know the difference between minerals and rocks!If you are currently taking a geology course, you’ll likely be asked more than once to name a mineral or a rock that has specific characteristics or composition, or was formed in a specific environment. Please make sure that if you’re asked for a rock name that you don’t respond with a mineral name, and vice versa. Confusing minerals and rocks is one of the most common mistakes that geology students make.

There are three types of rocks: igneous, sedimentary and metamorphic. Each of these types is part of the rock cycle. Through changes in conditions one rock type can become another rock type. Or it can become a different rock of the same type.

Figure 1. The different colors and textures seen in this rock are caused by the presence of different minerals.

What Are Rocks?

A rock is a naturally formed, non-living earth material. Rocks are made of collections of mineral grains that are held together in a firm, solid mass (figure 1).

How is a rock different from a mineral? Rocks are made of minerals. The mineral grains in a rock may be so tiny that you can only see them with a microscope, or they may be as big as your fingernail or even your finger (figure 1).

Figure 2. A pegmatite from South Dakota with crystals of lepidolite, tourmaline, and quartz (1 cm scale on the upper left).

Rocks are identified primarily by the minerals they contain and by their texture. Each type of rock has a distinctive set of minerals. A rock may be made of grains of all one mineral type, such as quartzite. Much more commonly, rocks are made of a mixture of different minerals. Texture is a description of the size, shape, and arrangement of mineral grains. Are the two samples in figure 2 the same rock type? Do they have the same minerals? The same texture?

Figure 3. Rock samples.

Sample	Minerals	Texture	Formation	Rock Type
Sample 1	plagioclase, quartz, hornblende, pyroxene	Crystals, visible to the naked eye	Magma cooled slowly	Diorite
Sample 2	plagioclase, hornblende, pyroxene	Crystals are tiny or microscopic	Magma erupted and cooled quickly	Andesite

Table 1

As seen in table 1, these two rocks have the same chemical composition and contain mostly the same minerals, but they do not have the same texture. Sample 1 has visible mineral grains, but Sample 2 has very tiny or invisible grains. The two different textures indicate different histories. Sample 1 is a diorite, a rock that cooled slowly from magma (molten rock) underground. Sample 2 is an andesite, a rock that cooled rapidly from a very similar magma that erupted onto Earth’s surface.

Rocks are classified into three major groups according to how they form. Rocks can be studied in hand samples that can be moved from their original location. Rocks can also be studied in outcrop, exposed rock formations that are attached to the ground, at the location where they are found.

Types of Rocks

Igneous Rocks

Magma is molten rock inside the earth. It is the source of all igneous rock. Because the earth was largely molten at its origin, magma may be considered the beginning of the rock cycle. Igneous rocks contain information about how they originate. By carefully analyzing igneous rocks and interpreting the information they contain, we can deduce processes that take place within the earth and we can understand volcanic processes that take place on the earth’s surface.

The study of igneous rocks enables us to understand the igneous part of geologic history. For example, at the end of the Triassic period, 245 million years ago, the greatest mass extinction ever known took place, wiping out more life forms on earth than the mass extinction that led to the demise of dinosaurs 65 million years ago at the end of the Cretaceous. At the end of the Triassic, a huge amount of basalt erupted onto the earth. Many geologists think that the gases and particles released into the atmosphere by those eruptions may have been a major factor in the end of Triassic mass extinction. Those scientists are studying the information contained in the basalts of that age to further test their hypotheses.

Igneous rocks contain three essential sources of information: their minerals, their overall chemical composition, and their igneous texture. Igneous rock names are based on specific combinations of these features. Igneous rocks also contain isotopic information that is used in determining absoloute ages and in further characterizing the origin of the magma. Special equipment and expertise is required to conduct isotopic and precise chemical analyses. Fortunately, with some basic training and practice anyone can learn to identify the minerals, composition and texture of an igneous rock; name the rock; and interpret key information about its origins.

All igneous rocks, other than pure volcanic glass, contain minerals. The minerals provide details on the chemical composition of the rock, and on the conditions in which the magma originated, cooled, and solidified. Geologists conduct chemical analyses of minerals to determine the temperatures and pressures at which they formed and to identify the dissolved gases and chemical elements that were present in the magma.

Most magmas are predominantly silicate liquids, composed largely of silica tetrahedra that have not yet bonded together to become silicate minerals. The chemical composition of an igneous rock tells us about the origin of the magma, beginning with which type of rock melted within the earth to form the magma in the first place, and how deep in the earth the melting occurred. Once magma has formed inside the earth, its composition may be modified. Minerals can grow from the magma and separate from it, changing the chemistry of the remaining liquid. Or, one body of magma can mix with another that has a different composition.

Magmas come in a range of compositions, from rich in silica and poor and iron and magnesium (felsic) to moderate in silica and high in iron and magnesium (mafic). Felsic igneous rocks, as a whole rock, tend to have light colors or shades: white, pink, light brown, light gray. Mafic igneous rocks, on the whole, tend to be dark colored, commonly black or dark gray. Most mafic magma originates by melting of rocks in the mantle that are extremely rich in iron and magnesium. Felsic magma usually originates in the crust or by the shedding of mafic minerals as magma rises through the crust.

The igneous texture tells us how the magma cooled and solidified. Magma can solidify into igneous rock in several different ways, each way resulting in a different igneous texture. Magma may stay within the earth, far below ground level, and crystallize into plutonic igneous rock (also known as intrusive igneous rock). Or, magma may flow out onto surface of the earth as a lava flow. Another way that igneous rock forms is by magma erupting explosively into the air and falling to earth in pieces known as pyroclastic material, also called tephra. Lava flows and pyroclastic material are volcanic igneous rock (also known as extrusive igneous rock).

The igneous texture of a rock is not how it feels in your hand, not whether it is rough or smooth. The igneous texture describes whether the rock has mineral crystals or is glassy, the size of the mineral grains, and the rock’s porosity (empty spaces).

This basics page focuses on igneous rocks and gives you the background needed to understand the terms used in the igneous rock classification table.

There are two main types of igneous rocks: (1) plutonic (intrusive) rocks, which form by solidification of molten rock deep within the earth, and (2) volcanic (extrusive) rocks, which solidify from molten rock erupted to the surface. Volcanic rocks break down into two more categories: (a) lava flows and (b) tephra (pyroclastic material).

Igneous rocks are classified on the basis of their composition and their texture. Magma, and the igneous rock it becomes, has a range of chemical compositions.For example, basalt is a mafic lava flow rock which originates from melting of the upper mantle. The way that magma turns into a solid rock gives it a distinctive igneous texture. For example, magma that becomes a pluton by slowly crystallizing (growing minerals) within the crust will develop a very different texture from magma that becomes an ash flow tuff as a result of semi-molten volcanic ash spewing across a landscape and then settling down and welding itself together into solid rock.

Igneous Rock Textures

The texture of an igneous rock results from the cooling, crystallization, and solidification history of the magma that formed it. Once you know the texture of an igneous rock, you can usually deduce from the texture whether it was intrusive or extrusive, lava flow or pyroclastic.

Texture in this context is not whether the rock feels rough or smooth to the touch. Igneous texture terms have objective definitions that refer only to igneous rocks.

Volcanic Rocks

Let us start with textures associated with rocks formed by lava flows. Magmas that erupt as lava onto the earth’s surface cool and solidify rapidly. Rapid cooling results in an aphanitic igneous texture, in which few or none of the individual minerals are big enough to see with the naked eye. This is sometimes referred to as a fine-grained igneous texture.

Some lava flows, however, are not purely fine-grained. If some mineral crystals start growing while the magma is still underground and cooling slowly, those crystals grow to a large enough size to be easily seen, and the magma then erupts as a lava flow, the resulting texture will consist of coarse-grained crystals embedded in a fine-grained matrix. This texture is called porphyritic.

If lava has bubbles of gas escaping from it as it solidifies, it will end up with “frozen bubble holes” in it. These “frozen bubble holes” are called vesicles, and the texture of a rock containing them is said to be vesicular.

If so many bubbles are escaping from lava that it ends up containing more bubble holes than solid rock, the resulting texture is said to be frothy. Pumice is the name of a type of volcanic rock with a frothy texture.

If lava cools extremely quickly, and has very little water dissolved in it, it may freeze into glass, with no minerals (glass by definition is not a mineral, because it does not have a crystal lattice). Such a rock is said to have a glassy texture. Obsidian is the common rock that has a glassy texture, and is essentially volcanic glass. Obsidian is usually black.

Now let us briefly consider textures of tephra or pyroclastic rocks. Like lava flow rocks, these are also extrusive igneous rocks. However, instead of originating from lava that flowed on the earth’s surface, tephra is volcanic material that was hurled through the air during a volcanic eruption.

A pyroclastic rock made of fine-grained volcanic ash may be said to have a fine-grained, fragmental texture. Volcanic ash consists mainly of fine shards of volcanic glass. It may be white, gray, pink, brown, beige, or black in color, and it may have some other fine crystals and rock debris mixed in. The term “fine-grained, fragmental” is easy to confuse with the term fine-grained (aphanitic). An equivalent term that is less ambiguous is tuffaceous. Rocks made of volcanic ash are called tuff.

A pyroclastic rock with many big chunks of material in it that were caught up in the explosive eruption is said to have a coarse-grained, fragmental texture. However, a better word that will avoid confusion is to say it has a brecciated texture, and the rock is usually called a volcanic breccia. The bigger chunks of material in a volcanic breccia are more than 1 cm (5/8 inch) across, and sometimes are much bigger.

Plutonic Rocks

When magma cools slowly underground and solidifies there, it usually grows crystals big enough to be seen easily with the naked eye. These visible crystals comprise the whole rock, not just part of it as in a porphyritic, fine-grained igneous rock. The texture of an igneous rock made up entirely of crystals big enough to be easily seen with the naked eye is phaneritic. Phaneritic texture is sometimes referred to as coarse-grained igneous texture. Granite, the most well known example of an intrusive igneous rock, has a phaneritic texture.

Sometimes an intrusion of magma that is crystallizing slowly underground releases large amounts of hot water. The water is released from the magma as extremely hot fluid with lots of chemical elements dissolved in it. This hydrothermal fluid gets into cracks and voids in the earth’s crust, and as it cools it may grow very large minerals from the dissolved chemical elements. A rock consisting of such large minerals is said to have a pegmatitic texture, which means the average mineral size is greater than 1 cm in diameter (and sometimes is much larger). The name of an igneous rock with a pegmatitic texture is pegmatite. Pegmatites are commonly found in or near the margins of bodies of granite.

Igneous Rock Compositions

The most common igneous compositions can be summarized in three words: mafic (basaltic), intermediate (andesitic), and felsic (granitic).

Felsic composition is higher in silica (SiO₂) and low in iron (Fe) and magnesium (Mg). Mafic composition is higher in iron and magnesium and lower in silica. Intermediate compositions contain silica, iron, and magnesium in amounts that are intermediate to felsic and mafic compositions.

Composition and Color

Composition influences the color of igneous rocks. Felsic rocks tend to be light in color (white, pink, tan, light brown, light gray). Mafic rocks tend to be dark in color (black, very dark brown, very dark gray, dark green mixed with black). The color distinction comes from the differences in iron and magnesium content. Iron and, to a lessor extent, magnesium give minerals a darker color. Intermediate igneous rocks tend to have intermediate shades or colors (green, gray, brown).

The association between color and composition is useful because before you can name and interpret an igneous rock you need to determine both its texture AND its composition. If you have an aphanitic igneous rock, which has no crystals big enough to see without a microscope, you can estimate its composition based on its color: pink or nearly white, felsic; medium gray, intermediate; very dark or black, mafic.

This color rule works most of the time but there are two problems that you need to keep in mind. First, the rule does not work for glassy igneous rocks. Obsidian, which is volcanic glass, is usually black, even though it has a felsic composition. That is because a tiny amount of iron, too little to color minerals very darkly, can color glass darkly.

The second problem is that when igneous rocks have been exposed to air and water for a long time, they start to weather, which changes their color. Geologists working in the field carry a rock hammer, so they can break off the weathered, outer parts of rocks to see the “fresh,” unweathered rock inside.

If you can see and identify the minerals in an igneous rock, you can gain further information about the igneous composition. Igneous rocks with quartz in them are usually felsic. Igneous rocks with olivine in them are usually mafic. Igneous rocks with neither quartz nor olivine in them are most commonly intermediate.

Origins of Igneous Rocks

Once you have determined the texture and composition of an igneous rock, you can name it and you can also say something important about how it formed. For example, a coarse-grained, felsic igneous rock is not only a granite, it is an intrusive igneous rock that formed from slow cooling and crystallization of a body of magma within the earth’s crust. The intrusion of large bodies of granite – batholiths – is usually part of the origin of a mountain range. Similarly, a fine-grained, mafic igneous rock is not only a basalt, it is an extrusive igneous rock that formed from rapid cooling and crystallization of a lava flow at earth’s surface.

How to Identify Igneous Rocks

Igneous rocks can be distinguished from sedimentary rocks by the lack of beds, lack of fossils, and lack of rounded grains in igneous rocks, and the presence of igneous textures. A granite, for example, can be distinguished from a sandstone because rather than being a mixture of weathered, rounded grains compressed and cemented together, granite consists of a small number of minerals in shiny black, white, or pink colors, with excellent crystal forms, grown together into a completely interlocking pattern. Sandstones, by contrast, have sedimentary bedding (layers) and consist of rounded grains with some spaces between the grains, which you can see with a hand lens or magnifying glass.

Igneous rocks can be distinguished from most regional metamorphic rocks by the lack of foliation (layering) in igneous rocks. Unfoliated metamorphic rocks lack igneous textures and usually contain minerals not found in igneous rocks.

Granite may look like gneiss at first glance, but granite has no layering, no preferred orientation of the minerals. The minerals in a granite grow randomly in all directions, rather than tending to grow parallel to each other.

Igneous rocks are classified on the basis of their texture and their composition. See the previous sections for descriptions of the different igneous textures and compositions.

The igneous rock classification tables that accompany this section are arranged on the basis of igneous textures first, and further broken down on the basis of igneous composition. Remember that igneous composition is estimated on the basis of color: light = felsic composition, medium = intermediate composition, and dark = mafic composition.

Igneous rocks form from cooling magma. Magma that erupts onto Earth’s surface is lava, as seen in figure 4. The chemical composition of the magma and the rate at which it cools determine what rock forms as the minerals cool and crystallize.

Figure 4. This flowing lava is molten rock that will harden into an igneous rock.

Igneous rocks form from the cooling and hardening of molten magma in many different environments. These rocks are identified by their composition and texture. More than 700 different types of igneous rocks are known.

Magma Composition

The rock beneath the Earth’s surface is sometimes heated to high enough temperatures that it melts to create magma. Different magmas have different composition and contain whatever elements were in the rock that melted. Magmas also contain gases. The main elements are the same as the elements found in the crust. Table 1 lists the abundance of elements found in the Earth’s crust and in magma. The remaining 1.5% is made up of many other elements that are present in tiny quantities.

Table 1. Elements in Earth’s Crust and Magma
Element	Symbol	Percent
Oxygen	O	46.6%
Silicon	Si	27.7%
Aluminum	Al	8.1%
Iron	Fe	5.0%
Calcium	Ca	3.6%
Sodium	Na	2.8%
Potassium	K	2.6%
Magnesium	Mg	2.1%
Total		98.5%

Whether rock melts to create magma depends on several factors:

• Temperature: Temperature increases with depth, so melting is more likely to occur at greater depths.
• Pressure: Pressure increases with depth, but increased pressure raises the melting temperature, so melting is less likely to occur at higher pressures.
• Water: The addition of water changes the melting point of rock. As the amount of water increases, the melting point decreases.
• Rock composition: Minerals melt at different temperatures, so the temperature must be high enough to melt at least some minerals in the rock. The first mineral to melt from a rock will be quartz (if present) and the last will be olivine (if present).

The different geologic settings that produce varying conditions under which rocks melt will be discussed in the “Plate Tectonics” chapter.

As a rock heats up, the minerals that melt at the lowest temperatures will melt first.Partial melting occurs when the temperature on a rock is high enough to melt only some of the minerals in the rock. The minerals that will melt will be those that melt at lower temperatures. Fractional crystallization is the opposite of partial melting. This process describes the crystallization of different minerals as magma cools.

Bowen’s Reaction Series indicates the temperatures at which minerals melt or crystallize (figure 1). An understanding of the way atoms join together to form minerals leads to an understanding of how different igneous rocks form. Bowen’s Reaction Series also explains why some minerals are always found together and some are never found together.

Figure 1. Bowen's Reaction Series

Follow this link to see a diagram illustrating Bowen’s Reaction Series.

This excellent video that explains Bowen’s Reaction Series in detail.

If the liquid separates from the solids at any time in partial melting or fractional crystallization, the chemical composition of the liquid and solid will be different. When that liquid crystallizes, the resulting igneous rock will have a different composition from the parent rock.

Intrusive and Extrusive Igneous Rocks

Igneous rocks are called intrusive when they cool and solidify beneath the surface. Intrusive rocks form plutons and so are also called plutonic. A pluton is an igneous intrusive rock body that has cooled in the crust. When magma cools within the Earth, the cooling proceeds slowly. Slow cooling allows time for large crystals to form, so intrusive igneous rocks have visible crystals. Granite is the most common intrusive igneous rock (see figure 2 for an example).

Figure 2. Granite is made of four minerals, all visible to the naked eye: feldspar (white), quartz (translucent), hornblende (black), and biotite (black, platy).

Igneous rocks make up most of the rocks on Earth. Most igneous rocks are buried below the surface and covered with sedimentary rock, or are buried beneath the ocean water. In some places, geological processes have brought igneous rocks to the surface. Figure 3 below shows a landscape in California’s Sierra Nevada made of granite that has been raised to create mountains.

Figure 3. California's Sierra Nevada is intrusive igneous rock exposed at Earth's surface.

Igneous rocks are called extrusive when they cool and solidify above the surface. These rocks usually form from a volcano, so they are also called volcanic rocks (figure 4).

Figure 4. Extrusive igneous rocks form after lava cools above the surface.

Extrusive igneous rocks cool much more rapidly than intrusive rocks. There is little time for crystals to form, so extrusive igneous rocks have tiny crystals (figure 5).

Figure 5. Cooled lava forms basalt with no visible crystals. Why are there no visible crystals?

Some volcanic rocks have a mixed texture. A rock such as an andesite may have large crystals set within a matrix of tiny crystals. In this case, the magma cooled enough to form some crystals before erupting. Once erupted, the rest of the lava cooled rapidly. This is called porphyritic texture.

Cooling rate and gas content create other textures (see figure 6 for examples of different textures). Lavas that cool extremely rapidly may have a glassy texture. Those with many holes from gas bubbles have a vesicular texture.

Figure 6. Different cooling rate and gas content resulted in these different textures.

Igneous Rock Classification

Igneous rocks are classified by their composition, from felsic to ultramafic. The characteristics and example minerals in each type are included in table 2.

Table 2. Properties of Igneous Rock Compositions
Composition	Color	Density	Minerals
Felsic	Light	Low	Quartz, orthoclase feldspar
Intermediate	Intermediate	Intermediate	Plagioclase feldspar, biotite, amphibole
Mafic	Dark	High	Olivine, pyroxene
Ultramafic	Very dark	Very high	Olivine

Table 3. Silica Composition and Texture of Major Igneous Rocks
Type	Amount of Silica	Extrusive	Intrusive
Ultramafic	<45%	Komatiite	Peridotite
Mafic	45-52%	Basalt	Gabbro
Intermediate	52-63%	Andesite	Diorite
Intermediate-Felsic	63-69%	Dacite	Granodiorite
Felsic	>69% SiO2	Rhyolite	Granite

Some of the rocks in the table 3 were pictured earlier in this chapter. Look back at them and, using what you know about the size of crystals in extrusive and intrusive rocks and the composition of felsic and mafic rocks, identify the rocks in the following photos in figure 7:

Figure 7. These are photos of A) rhyolite, B) gabbro, C) peridotite, and D) komatiite.

 

Sedimentary Rocks

This basics page focuses on sedimentary rocks, which are sediments that were turned into solid rock by geologic processes. Sedimentary rocks contain information about what was occurring on earth’s surface at the place and time the sediments were deposited. In some cases sedimentary rocks contain fossils, which provide information about what was living at a certain place and time on earth.

There are two main groups of sedimentary rocks: chemical and clastic. Clastic is sometimes called detrital. Each type of sedimentary rock is formed when sediments lithify (turn into rock). Chemical sediments are sediments that precipitate from solution, for example salt crystals that grow at the bottom of an evaporating body of water. Clastic sediments are solid pieces of weathered and eroded rocks or minerals, for example sand on a beach.

This page provides the background needed to understand the terms used in the sedimentary rock classification table at the end of this page.

Lithification - Sediments to Sedimentary Rocks

Sedimentary rocks are rocks made of lithified sediment. Sediments are grains of rocks, minerals, or mineraloids deposited on the surface of the earth. Reflect on the rock cycle for an indication of the relationships between the rocks that erode to become sediments and sedimentary rocks. For sediment to become sedimentary rock, it usually undergoes burial, compaction, and cementation.

Clastic sedimentary rocks are the result of weathering and erosion of source rocks, which turns them into pieces—clasts—of rocks and minerals. Once they become pieces, these clasts are free to move away from their source rock and they usually do. They are most often transported by water and deposited as layers of sediment.

The burial stage of lithification involves the deposition of more sediment layers top of those that had been deposited earlier. In a sedimentary basin where sediment is being deposited, it is common for subsidence (lowering) of the basin to be taking place, either because the crust and lithosphere beneath it are subsiding into the mantle to some extent, or because the surrounding uplands are undergoing uplift relative to the basin, or both. This allows thousands of feet of burial, in some cases tens of thousands of feet of burial, to occur.

As sediments are buried, the weight of overlying material exerts pressure, causing compaction of the sediments. The pressure, known as lithostatic pressure, “squeezes” the sediments from all sides into a smaller volume. Lithostatic pressure packs the sediment grains closer together and reduces the porosity – space between the sediment grains.

Some chemical sedimentary rocks are rock as soon as the sediments have been deposited by crystallization of minerals from substances dissolved in water, at the earth’s surface. Examples include rock salt and other evaporite deposits. These sediments of salt crystals and other minerals form sedimentary rock without having to undergo burial and compaction.

During burial and compaction, sediments will undergo some amount of cementation. Cementation refers to the growth of new minerals between the sediment grains. These new minerals bind the sediment grains together. One form of cementation is growth of quartz rims on the surfaces of pre-existing quartz grains in the sediment. This new mineral growth is a result of water in the pore spaces that dissolves and precipiates quartz. A second common cementing mineral is hematite, a red or rust-colored iron oxide mineral, which precipitates onto the sediment grains from a combination of dissolved iron and oxygen from water in the pore spaces. A third common cementing mineral is calcite, which also precipitates from ions dissolved in the water in the pore spaces during lithification. Although there are other cementing minerals, quartz, hematite, and calcite are common cementing minerals that grow between or on the surfaces of the original sedimentary grains.

Generalized steps from source to sedimentary rock:
weathering >> erosion >> transport >> deposition >> burial >> compaction >> cementation >> sedimentary rock

Minerals and Sedimentary Rocks

Any type of rock containing any type of mineral will undergo weathering and erosion at the earth’s surface. However, some minerals are more stable than others in earth’s surface environments and are more likely to be found in sedimentary rocks.

Grains of clastic sediment, which are called clasts are winnowed and modified during the weathering-to-deposition process. Weathering of minerals will gradually eliminate the physically weaker and chemically more reactive minerals, increasing the relative abundance of more resistant minerals. Quartz tends to become increasingly abundant during the process, due to its common occurrence in the source rocks combined with its hardness and lack of cleavage, which makes it resistant to breaking down physically. Quartz is not easily dissolved or chemically altered, so it is resistant to breakdown by chemical reactions as well. That is why beach sand is often more rich in quartz than any other mineral. Sediments rich in quartz are considered mineralogically “mature” because they have been subjected to a longer interval of physical and chemical modification during the erosion-to-deposition process. Mature sediments are deposited farther from their source rocks in space and/or time than immature sediments.

Feldspars are the most common type of mineral in earth’s crust and are also abundant in many clastic sediments and sedimentary rocks. Although feldspar is a fairly hard mineral, it does cleave (split apart) and is chemically reactive, especially in the presence of water. The most abundant product of chemical reaction of feldspar and water is clay minerals. During the erosion-to-deposition process, clastic sediments lose feldspar and gain a larger proportion of clay. Other minerals such as amphiboles, micas, and carbonates are relatively soft and chemically reactive and tend to be scarce or absent as sediment grains in mature clastic sediments, although calcite may be present in clastic sedimentary rocks as a secondary, cementing mineral that grew during lithification.

Minerals in chemical sedimentary rocks precipitate from water and usually remain in place or are not transported far before lithification. Such mineral sediments are subjected to little, if any, erosion and transportation. Therefore, the minerals in chemical sedimentary rocks are not winnowed during the weathering-to-deposition process as are the minerals in clastic sedimentary rocks. In some cases, during the formation of chemical sediments, the minerals may change as a result of chemical reactions. For example, dolostone is a chemical sedimentary rock that forms in certain coastal environments by alteration of precipitated calcite to dolomite.

Besides minerals, mineraloid solids occur in some chemical sedimentary rocks. For example, the carbonaceous material in coal is an organic mineraloid rather than a mineral. Another example, opal, is a chemical sedimentary rock that does not have a fully developed crystal lattice and therefore is a mineraloid.

Sedimentary Rock Textures

In clastic sediments the sedimentary texture includes the grain size, rounding, and sorting of the grains, all of which are related to what happened to the sediment during the weathering-to-deposition process.Because the processes that lead to the formation of chemical sedimentary rocks do not involve the weather-to-deposition process, there is no widely agreed-upon texture scheme that applies to chemical sedimentary rocks.

Clastic Textures

Clastic sedimentary textures are described in terms of the size of the sediment grains, how round they are, and how well they are sorted.

Grain Size

The diameter or width of a clastic sediment grain determines its grain size. Specific ranges of grain size have specific names.

• Gravel is an overall name for large sediment grain size, which includes boulder, cobble, and pebble.
• Sand includes sediment grains ranging in size from 2mm to 0.625 mm.
• Silt is the name of a sediment grain that range in size from 0.625 mm to 0.0039 mm.
• Mud is the smallest grain size and is also known as clay. It is important to distinguish the grain size called clay from the mineral called clay. Clay sized grains are too small to see individually without the aid of a microscope.

Rounding

Clastic sediment grains can be round, angular, or in-between (subangular or subrounded). Breccia is a clastic sedimentary rock that by definition consists largely of angular grains of pebble size or larger. Conglomerate, another sedimentary rock, consists largely of rounded grains of pebble size or larger. The schematic diagram below shows classes of rounding, from the left: angular, subangular, subrounded, rounded. Not shown are very angular and well-rounded, which are less common.

Sorting

The extent to which all the grains are the same size is known as sorting. If all the grains are the same size, they are well sorted. Some sandstones are well sorted, and some are not. Most conglomerates are poorly sorted, and consist of a mixture of grain sizes ranging from sand to pebble. The schematic diagrams below represent poorly-sorted, moderately-sorted, and well-sorted sediments.

Other Aspects of Texture

Other aspects of clastic sedimentary texture include the packing of the grains, the porosity of the rock, and the hardness of the rock as a whole.

The packing of the grains applies only to poorly sorted sediments in which the finer clasts form a matrix around the coarser clasts. If the large grains are touching each other, the packing is said to be clast-supported. If the coarse grains are separate and not touching each other, with the finer-grained sediment between them, the packing is said to be matrix-supported.

The porosity of a rock or sediment is the amount of empty space between the grains of sediment.

The hardness of the rock refers to how easily it breaks apart. Shale is harder than claystone, for example.

How to Identify Sedimentary Rocks

Clastic

The common names of clastic sedimentary rocks—conglomerate, sandstone, siltstone, and shale—are based solely on grain size. However, there are more precise names within these broader categories that are based on clastic sediment features other than grain size. Breccia is a clastic sedimentary rock distinguished from conglomerate by the angularity of its clasts.

A clastic sedimentary rock simply made of sand-sized grains qualifies as sandstone, regardless of what minerals the grains are made of. More complete names for various sandstones are based on their mineral composition. Some sandstones are made almost entirely of quartz. Sandstone made virtually entirely of quartz is called arenite. Other sandstones have lots of feldspar mixed with quartz. Such sandstones are called arkose. Other sandstones are a mixture of feldspar, quartz, clay, and small fragments of “lithics,” dark-appearing minerals and rock fragments, which represent mineralogically immature sediments. Such sandstones are technically known as lithic wackes, although geologists often call them by their old name, graywackes, and the rocks are sometimes informally described as “dirty sandstones.”

In using the sedimentary rock classification table that accompanies this section, you will see that the clastic sedimentary rocks are classified on the basis of grain size. Sandstones are rocks made of sand-size grains. Shale is layered sedimentary rock made of fine mud-size grains too small to see with the naked eye.

Chemical

The key to naming chemical sedimentary rocks is the minerals from which they are made. For example, all limestones consist mostly of the mineral calcite. Coal is made of carbon. Rock salt is made of salt minerals such as halite. Gypsum rock is made of the mineral gypsum. Chalcedony is made of microcrystalline quartz, quartz grains so tiny that they cannot be distinguished even with a standard optical microscope.

Settings for the Origin of Sedimentary Rocks

The minerals, textures, structures, and fossils in sedimentary rocks are used to infer what was happening on earth biologically, chemically, physically—in sum geologically—at the time and place the sediments were originally deposited.

Figure 10. The White House of the USA is made of a sedimentary rock called sandstone.

The White House (shown in the figure 10) is the official home and workplace of the President of the United States of America. Why do you think the White House is white? If you answered, “Because it is made of white rock,” you would be only partially correct. Construction for the White House began in 1792. Its outside walls are made of the sedimentary rock sandstone. This sandstone is very porous and is easily penetrated by rainwater. Water damage was common in the early days of construction for the building. To stop the water damage, workers covered the sandstone in a mixture of salt, rice, and glue, which help to give the White House its distinctive white color.

Sediments

Sandstone is one of the common types of sedimentary rocks that form from sediments. There are many other types. Sediments may include:

• fragments of other rocks that often have been worn down into small pieces, such as sand, silt, or clay.
• organic materials, or the remains of once-living organisms.
• chemical precipitates, which are materials that get left behind after the water evaporates from a solution.

Rocks at the surface undergo mechanical and chemical weathering. These physical and chemical processes break rock into smaller pieces. Physical weathering simply breaks the rocks apart. Chemical weathering dissolves the less stable minerals. These original elements of the minerals end up in solution and new minerals may form. Sediments are removed and transported by water, wind, ice, or gravity in a process called erosion (figure 11). Much more information about weathering can be found in the “Weathering and Formation of Soil” chapter. Erosion is described in detail in the “Erosion and Deposition” chapter.

Figure 11. Water erodes the land surface in Alaska’s Valley of Ten Thousand Smokes.

Streams carry huge amounts of sediment (figure 12). The more energy the water has, the larger the particle it can carry. A rushing river on a steep slope might be able to carry boulders. As this stream slows down, it no longer has the energy to carry large sediments and will drop them. A slower moving stream will only carry smaller particles.

Figure 12. A river dumps sediments along its bed and on its banks.

Sediments are deposited on beaches and deserts, at the bottom of oceans, and in lakes, ponds, rivers, marshes, and swamps. Avalanches drop large piles of sediment. Glaciers leave large piles of sediments, too. Wind can only transport sand and smaller particles. The type of sediment that is deposited will determine the type of sedimentary rock that can form. Different colors of sedimentary rock are determined by the environment where they are deposited. Red rocks form where oxygen is present. Darker sediments form when the environment is oxygen poor.

Sedimentary Rock Formation

Accumulated sediments harden into rock by lithification, as illustrated in figure 13. Two important steps are needed for sediments to lithify.

1. Sediments are squeezed together by the weight of overlying sediments on top of them. This is called compaction. Cemented, non-organic sediments become clastic rocks. If organic material is included, they are bioclastic rocks.
2. Fluids fill in the spaces between the loose particles of sediment and crystallize to create a rock by cementation.

Figure 13. This cliff is made of sandstone. Sands were deposited and then lithified.

The sediment size in clastic sedimentary rocks varies greatly (see table 4).

Table 4. Sedimentary rock sizes and features
Rock	Sediment Size	Other Features
Conglomerate	Large	Rounded
Breccia	Large	Angular
Sandstone	Sand-sized
Slitstone	Silt-sized, smaller than sand
Shale	Clay-sized, smallest

When sediments settle out of calmer water, they form horizontal layers. One layer is deposited first, and another layer is deposited on top of it. So each layer is younger than the layer beneath it. When the sediments harden, the layers are preserved. Sedimentary rocks formed by the crystallization of chemical precipitates are called chemical sedimentary rocks. As discussed in the “Earth’s Minerals” chapter, dissolved ions in fluids precipitate out of the fluid and settle out, just like the halite in figure 14.

Figure 14. The evaporite, halite, on a cobble from the Dead Sea, Israel.

Biochemical sedimentary rocks form in the ocean or a salt lake. Living creatures remove ions, such as calcium, magnesium, and potassium, from the water to make shells or soft tissue. When the organism dies, it sinks to the ocean floor to become a biochemical sediment, which may then become compacted and cemented into solid rock (figure 15).

Figure 15. Fossils in a biochemical rock, limestone, in the Carmel Formation in Utah.

Table 5 shows some common types of sedimentary rocks.

Sedimentary rocks form by the compaction and cementing together of sediments, broken pieces of rock-like gravel, sand, silt, or clay (figure 5). Those sediments can be formed from the weathering and erosion of preexisting rocks. Sedimentary rocks also include chemical precipitates, the solid materials left behind after a liquid evaporates.

Figure 5. This sedimentary rock is made of sand that is cemented together to form a sandstone.

Metamorphic Rocks

A metamorphic rock used to be some other type of rock, but it was changed inside the Earth to become a new type of rock. The word metamorphism comes from ancient Greek words for “change” (meta) and “form” (morph). The type of rock that a metamorphic rock used to be, prior to metamorphism, is called the protolith. During metamorphism the mineral content and texture of the protolith are changed due to changes in the physical and chemical environment of the rock. Metamorphism can be caused by burial, tectonic stress, heating by magma, or alteration by fluids. At advanced stages of metamorphism, it is common for a metamorphic rock to develop such a different set of minerals and such a thoroughly changed texture that it is difficult to recognize what the protolith was.

A rock undergoing metamorphism remains a solid rock during the process. Rocks do not melt during most conditions of metamorphism. At the highest grade of metamorphism, rocks begin to partially melt, at which point the boundary of metamorphic conditions is surpassed and the igneous part of the rock cycle is entered.

Even though rocks remain solid during metamorphism, fluid is generally present in the microscopic spaces between the minerals. This fluid phase may play a major role in the chemical reactions that are an important part of how metamorphism occurs. The fluid usually consists largely of water.

Metamorphic rocks provide a record of the processes that occurred inside Earth as the rock was subjected to changing physical and chemical conditions. This gives the geologist literally “inside information” on what occurs within the Earth during such processes as the formation of new mountain ranges, the collision of continents, the subduction of oceanic plates, and the circulation of sea water into hot oceanic crust. Metamorphic rocks are like probes that have gone down into the Earth and come back, bringing an record of the conditions they encountered on their journey in the depths of the Earth.

Factors that Control Metamorphism

The reason rocks undergo metamorphism is that the minerals in a rock are only stable under a limited range of pressure, temperature, and chemical conditions. When rocks are subjected to large enough changes in these factors, the minerals will undergo chemical reactions that result in their replacement by new minerals, minerals that are stable in the new conditions.

Chemical Composition of the Protolith

The type of rock undergoes metamorphism is a major factor in determing what type of metamorphic rock it becomes. In short the identify of the protolith plays a big role the identity of the metamorphic rock. A fluid phase may introduce or remove chemical substances into or out of the rock during metamorphism, but in most metamorphic rock, most of the atoms in the protolith are be present in the metamorphic rock after metamorphism; the atoms will likely be rearranged into new mineral forms within the rock. Therefore, not only does the protolith determine the initial chemistry of the metamorphic rock, most metamorphic rocks do not change their bulk (overall) chemical compositions very much during metamorphism. The fact that most metamorphic rocks retain most of their original atoms means that even if the rock was so thoroughly metamorphosed that it no longer looks at all like the protolith, the rock can be analyzed in terms of its bulk chemical composition to determine what type of rock the protolith was.

If the protolith is an arenite, made mostly of the mineral quartz (SiO₂), metamorphism cannot turn the rock into a marble, which is made of the mineral calcite (CaCO₃). In fact, as a result of metamorphism, a pure quartz arenite will become quartzite. It is still made of quartz, but the quartz has recrystallized during metamorphism, filling in most of the pore space of the arenite with new quartz growth and becoming a denser, harder rock. The reason pure arenite becomes quartzite is that the mineral quartz is stable over a wide range of pressures and temperatures. Under most metamorphic conditions quartz will simply recrystallize, overgrow the existing quartz grains with more quartz, and reorient its quartz crystals to become a new rock type made of quartz.

Many protoliths have chemical compositions consisting of more than three chemical elements, and most protoliths are made of minerals that do not remain stable in the conditions encountered during metamorphism. Such protoliths undergo chemical reactions during metamorphism that replace the protolith minerals with new metamorphic minerals, made of the atoms from the protolith minerals rearranged into new mineral structures.

Temperature

Temperature is another major factor of metamorphism. There are two ways to think about how the temperature of a rock can be increased as a result of geologic processes.

If rocks are buried within the Earth, the deeper they go, the higher the temperatures they experience. This is because temperature inside the Earth increases along what is called the geothermal gradient, or geotherm for short. Therefore, if rocks are simply buried deep enough enough sediment, they will experience temperatures high enough to cause metamorphism. This temperature is about 200ºC (approximately 400ºF).

Tectonic processes are another way rocks can be moved deeper along the geotherm. Faulting and folding the rocks of the crust, can move rocks to much greater depth than simple burial can.

Yet another way a rock in the Earth’s crust can have its temperature greatly increased is by the intrusion of magma nearby. Magma intrusion subjects nearby rock to higher temperature with no increase in depth or pressure.

The upper limit of metamorphism, beyond which igneous conditions occur, is the temperature and pressure at which partial melting of the rocks begins. This limit varies greatly, depending on the pressure, the chemical composition of the rocks, and the presence of a fluid phase. With water present in the fluid, some types of rock begin melting, if the pressure is high enough, at temperatures of about 600 ºC (approximately 1100 ºF) at the low end. Other types of rock, if there is no fluid in the rock to lower the melting temperature, will remain solid and continue undergoing metamorphism to over 1000 ºC (approximately 1800 ºF).

Pressure

Pressure is a measure of the stress, the physical force, being applied to the surface of a material. It is defined as the force per unit area acting on the surface, in a direction perpendicular to the surface.

Lithostatic pressure is the pressure exerted on a rock by all the surrounding rock. The source of the pressure is the weight of all the rocks above. Lithostatic pressure increases as depth within the Earth increases and is a uniform stress—the pressure applies equally in all directions on the rock.

If pressure does not apply equally in all directions, differential stress occurs. There are two types of differential stress.

Normal stress compresses (pushes together) rock in one direction, the direction of maximum stress. At the same time, in a perpendicular direction, the rock undergoes tension (stretching), in the direction of minimum stress.

Shear stress pushes one side of the rock in a direction parallel to the side, while at the same time, the other side of the rock is being pushed in the opposite direction.

Differential stress has a major influence on the the appearance of a metamorphic rock. Differential stress can flatten pre-existing grains in the rock, as shown in the diagram below.

Metamorphic minerals that grow under differential stress will have a preferred orientation if the minerals have atomic structures that tend to make them form either flat or elongate crystals. This will be especially apparent for micas or other sheet silicates that grow during metamorphism, such as biotite, muscovite, chlorite, talc, or serpentine. If any of these flat minerals are growing under normal stress, they will grow with their sheets oriented perpendicular to the direction of maximum compression. This results in a rock that can be easily broken along the parallel mineral sheets. Such a rock is said to be foliated, or to have foliation.

Fluids

Any open space between the mineral grains in a rock, however microscopic, may contain a fluid phase. Most commonly, if there is a fluid phase in a rock during metamorphism, it will be a hydrous fluid, consisting of water and things dissolved in the water. Less commonly, it may be a carbon dioxide fluid or some other fluid. The presence of a fluid phase is a major factor during metamorphism because it helps determine which metamorphic reactions will occur and how fast they will occur. The fluid phase can also influence the rate at which mineral crystals deform or change shape. Most of this influence is due to the dissolved ions that pass in and out of the fluid phase. If during metamorphism enough ions are introduced to or removed from the rock via the fluid to change the bulk chemical composition of the rock, the rock is said to have undergone metasomatism. However, most metamorphic rocks do not undergo sufficient change in their bulk chemistry to be considered metasomatic rocks.

Time

Most metamorphism of rocks takes place slowly inside the Earth. Regional metamorphism takes place on a timescale of millions of years. Metamorphism usually involves slow changes to rocks in the solid state, as atoms or ions diffuse out of unstable minerals that are breaking down in the given pressure and temperature conditions and migrate into new minerals that are stable in those conditions. This type of chemical reaction takes a long time.

Grades of Metamorphism

Metamorphic grade refers to the general temperature and pressure conditions that prevailed during metamorphism. As the pressure and temperature increase, rocks undergo metamorphism at higher metamorphic grade. Rocks changing from one type of metamorphic rock to another as they encounter higher grades of metamorphism are said to be undergoing prograde metamorphism.

Low-grade metamorphism takes place at approximately 200–320 ºC and relatively low pressure. This is not far beyond the conditions in which sediments get lithified into sedimentary rocks, and it is common for a low-grade metamorphic rock to look somewhat like its protolith. Low grade metamorphic rocks tend to characterized by an abundance of hydrous minerals, minerals that contain water within their crystal structure. Examples of low grade hydrous minerals include clay, serpentine, and chlorite. Under low grade metamorphism many of the metamorphic minerals will not grow large enough to be seen without a microscope.

Medium-grade metamorphism takes place at approximately at 320–450 ºC and at moderate pressures. Low grade hydrous minerals are replaced by micas such as biotite and muscovite, and non-hydrous minerals such as garnet may grow. Garnet is an example of a mineral which may form porphyroblasts, metamorphic mineral grains that are larger in size and more equant in shape (about the same diameter in all directions), thus standing out among the smaller, flatter, or more elongate minerals.

High-grade metamorphism takes place at temperatures above about 450 ºC. Micas tend to break down. New minerals such as hornblende will form, which is stable at higher temperatures. However, as metamorphic grade increases to even higher grade, all hydrous minerals, which includes hornblende, may break down and be replaced by other, higher-temperature, non-hydrous minerals such as pyroxene.

During high-grade metamorphism the minerals tend to grow larger. Some varieties of valuable gemstones, such as rubies, emeralds, and jade, come from high grade metamorphic rocks. At the highest metamorphic grade, if the temperature gets high enough, the rock will start to melt, entering the next stage of the rock cycle, the igneous stage. The temperature at which melting begins ranges from about 600 ºC to over 1000 ºC depending on the rock and fluids in the rock.

Index Minerals

Index minerals, which are indicators of metamorphic grade. In a given rock type, which starts with a particular chemical composition, lower-grade index minerals are replaced by higher-grade index minerals in a sequence of chemical reactions that proceeds as the rock undergoes prograde metamorphism. For example, in rocks made of metamorphosed shale, metamorphism may prograde through the following index minerals:

• chlorite characterizes the lowest regional metamorphic grade
• biotite replaces chlorite at the next metamorphic grade, which could be considered medium-low grade
• garnet appears at the next metamorphic grade, medium grade
• staurolite marks the next metamorphic grade, which is medium-high grade
• sillimanite is a characteristic mineral of high grade metamorphic rocks

Index minerals are used by geologists to map metamorphic grade in regions of metamorphic rock. A geologist maps and collects rock samples across the region and marks the geologic map with the location of each rock sample and the type of index mineral it contains. By drawing lines around the areas where each type of index mineral occurs, the geologist delineates the zones of different metamorphic grades in the region. The lines are known as isograds.

Types of Metamorphism

Regional Metamorphism

Regional metamorphism occurs where large areas of rock are subjected to large amounts of differential stress for long intervals of time, conditions typically associated with mountain building. Mountain building occurs at subduction zones and at continental collision zones where two plates each bearing continental crust, converge upon each other.

Most foliated metamorphic rocks—slate, phyllite, schist, and gneiss—are formed during regional metamorphism. As the rocks become heated at depth in the Earth during regional metamorphism they become ductile, which means they are relatively soft even though they are still solid. The folding and deformation of the rock while it is ductile may greatly distort the original shapes and orientations of the rock, producing folded layers and mineral veins that have highly deformed or even convoluted shapes. The diagram below shows folds forming during an early stage of regional metamorphism, along with development of foliation, in response to normal stress.

The photograph below shows high-grade metamorphic rock that has undergone several stages of foliation development and folding during regional metamorphism, and may even have reached such a high temperature that it began to melt.

Contact Metamorphism

Contact metamorphism occurs to solid rock next to an igneous intrusion and is caused by the heat from the nearby body of magma. Because contact metamorphism is not caused by changes in pressure or by differential stress, contact metamorphic rocks do not become foliated. Where intrusions of magma occur at shallow levels of the crust, the zone of contact metamorphism around the intrusion is relatively narrow, sometimes only a few m (a few feet) thick, ranging up to contact metamorphic zones over 1000 m (over 3000 feet) across around larger intrusions that released more heat into the adjacent crust. The zone of contact metamorphism surrounding an igneous intrusion is called the metamorphic aureole. The rocks closest to the contact with the intrusion are heated to the highest temperatures, so the metamorphic grade is highest there and diminishes with increasing distance away from the contact. Because contact metamorphism occurs at shallow to moderate depths in the crust and subjects the rocks to temperatures up to the verge of igneous conditions, it is sometimes referred to as high-temperature, low-pressure metamorphism. Hornfels, which is a hard metamorphic rock formed from fine-grained clastic sedimentary rocks, is a common product of contact metamorphism.

Hydrothermal Metamorphism

Hydrothermal metamorphism is the result of extensive interaction of rock with high-temperature fluids. The difference in composition between the existing rock and the invading fluid drives the chemical reactions. The hydrothermal fluid may originate from a magma that intruded nearby and caused fluid to circulate in the nearby crust, from circulating hot groundwater, or from ocean water. If the fluid introduces substantal amounts of ions into the rock and removes substantial amounts of ions from it, the fluid has metasomatized the rock—changed its chemical composition.

Ocean water that penetrates hot, cracked oceanic crust and circulates as hydrothermal fluid in ocean floor basalts produces extensive hydrothermal metamorphism adjacent to mid-ocean spreading ridges and other ocean-floor volcanic zones. Much of the basalt subjected to this type of metamorphism turns into a type of metamorphic rock known as greenschist. Greenschist contains a set of minerals, some of them green, which may include chlorite, epidote, talc, Na-plagioclase, or actinolite. The fluids eventually escape through vents in the ocean floor known as black smokers, producing thick deposits of minerals on the ocean floor around the vents.

Burial Metamorphism

Burial metamorphism occurs to rocks buried beneath sediments to depths that exceed the conditions in which sedimentary rocks form. Because rocks undergoing burial metamorphism encounter the uniform stress of lithostatic pressure, not differential pressure, they do not develop foliation. Burial metamorphism is the lowest grade of metamorphism. The main type of mineral that usually grows during burial metamorphism is zeolite, a group of low-density silicate minerals. It usually requires a strong microscope see the small grains of zeolite minerals that form during burial metamorphism.

Dynamic Metamorphism

Dynamic metamorphism is caused mainly by high shear stress along fault zones or shear zones in the crust. The minerals of the protolith may be pulverized and crushed by the high rate of shear strain that occurs in these zones. Rocks metamorphosed in these zones usually exhibit a combination of fractured, partly disintegrated, partly recrystallized versions of the original minerals, along with new mineral growth that occured during the metamorphism. Because fault zones and shear zones are highly localized, rocks that have undergone dynamic metamorphism are not a widespread type of metamorphic rock; they are much less abundant, than regional or contact metamorphic rocks.

Subduction Zone Metamorphism

During subduction, a tectonic plate, consisting of oceanic crust and lithospheric mantle, is recycled back into the deeper mantle. In most subduction zones the subducting plate is relatively cold compared with the high temperature it had when first formed at a mid-ocean spreading ridge. Subduction takes the rocks to great depth in the Earth relatively quickly. This produces a characteristic type of metamorphism, sometimes called high-pressure, low-temperature (high-P, low-T) metamorphism, which only occurs deep in a subduction zone. In oceanic basalts that are part of a subducting plate, the high-P, low-T conditions create a distinctive set of metamorphic minerals including a type of amphibole, called glaucophane, that has a blue color. Blueschist is the name given to this type of metamorphic rock. Blueschist is generally interpreted as having been produced within a subduction zone, even if the plate boundaries have subsequently shifted and that location is no longer at a subduction zone.

Metamorphic Facies

Much as the minerals and textures of sedimentary rocks can be used as windows to see into the environment in which the sediments were deposited on the Earth’s surface, the minerals and textures of metamorphic rocks provide windows through which we view the conditions of pressure, temperature, fluids, and stress that occurred inside the Earth during metamorphism. The pressure and temperature conditions under which specific types of metamorphic rocks form has been determined by a combination labratory experiments, physics-based theoretical calculations, along with evidence in the textures of the rocks and their field relations as recorded on geologic maps. The knowledge of temperatures and pressures at which particular types of metamorphic rocks form led to the concept of metamorphic facies. Each metamorphic facies is represented by a specific type of metamorphic rock that forms under a specific pressure and temperature conditions.

Even though the name of the each metamorphic facies is taken from a type of rock that forms under those conditions, that is not the only type of rock that will form in those conditions. For example, if the protolith is basalt, it will turn into greenschist under greenschist facies conditions, and that is what facies is named for. However, if the protolith is shale, a muscovite-biotite schist, which is not green, will form instead. If it can be determined that a muscovite-biotite schist formed at around 350ºC temperature and 400 MPa pressure, it can be stated that the rock formed in the greenschist facies, even though the rock is not itself a greenschist.

The diagram below shows metamorphic facies in terms of pressure and temperature condiditons inside the Earth. Earth’s surface conditions are near the top left corner of the graph at about 15ºC which is the average temperature at Earth’s surface and 0.1 MPa (megapascals), which is about the average atmospheric pressure on the Earth’s surface. Just as atmospheric pressure comes from the weight of all the air above a point on the Earth’s surface, pressure inside the Earth comes from the weight of all the rock above a given depth. Rocks are much denser than air and MPa is the unit most commonly uses to express pressures inside the Earth. One MPa equals nearly 10 atmospheres. A pressure of 1000 MPa corresponds to a depth of about 35 km inside the Earth. Although pressure inside the Earth is determined by the depth, temperature depends on more than depth. Temperature depends on the heat flow, which varies from location to location. The way temperature changes with depth inside the Earth is called the geothermal gradient, geotherm for short. In the diagram below, three different geotherms are marked with dashed lines. The three geotherms represent different geological settings in the Earth.

High-pressure, low-temperature geotherms occurs in subduction zones. As the diagram shows, rocks undergoing prograde metamorphism in subduction zones will be subjected to zeolite, blueschist, and ultimately eclogite facies conditions.

High-temperature, low-pressure geotherms occur in the vicinity of igneous intrusions in the shallow crust, underlying a volcanically active area. Rocks that have their pressure and temperature conditions increased along such a geotherm will metamorphose in the hornfels facies and, if it gets hot enough, in the granulite facies.

Blueschist facies and hornfels facies are associated with unusual geothermal gradients. The most common conditions in the Earth are found along geotherms between those two extremes. Most regional metamorphic rocks are formed in conditions within this range of geothermal gradients, passing through the greenschist facies to the amphibolites facies. At the maximum pressures and temperatures the rocks may encounter within the Earth in this range of geotherms, they will enter either the granulite or eclogite facies. Regionally metamorphosed rocks that contain hydrous fluids will begin to melt before they pass beyond the amphibolite facies.

Protoliths

The minerals in metamorphic rock are often a completely different set of minerals than in the protolith. But, because the mineral assemblage in the metamorphic rock reflects the overall chemical composition of the rock, the set of minerals found in the rock can give us a good idea of the type of protolith, even if the metamorphic rock no longer looks anything like its protolith.

The following terms are used to describe protoliths, and the types of metamorphic rocks they turn into, in terms of their general chemical compositions.

• pelitic—pelitic rocks are high in alumina and, as protoliths, were usually shales or mudstones. Pelitic metamorphic rocks contain alumina-rich minerals such as the micas, garnet, andalusite, kyanite, silliminate, or staurolite. Pelitic metamorphic rocks formed during regional metamorphism include many varieties of slate, phyllite, schist, and gneiss.
• mafic—mafic protoliths and the metamorphic rocks they become are high in magnesium and iron relative to silicon. Basalt is the most common mafic protolith. It can turn into mafic metamorphic rocks such as greenschist and amphibolites with chlorite, actinolite, biotite, hornblende, or plagioclase in them, depending on metamorphic grade.
• calcareous—Calcareous rocks are calcium-rich rocks. Typically, as protoliths, calcareous rocks were either limestone or dolostone, which most commonly turn into marble as metamorphic rocks.
• quartzofeldspathic—Protoliths such as granite, rhyolite, and arkose are rocks that consist mostly of a combination of quartz and feldspar are quartzofeldspathic. High-grade regional metamorphism of quartzofeldspathic rocks produces gneisses containing feldspar, quartz, biotite, and possibly hornblende.

Types of Metamorphic Rocks

Metamorphic rock fall into two categories, foliated and unfoliated. Most foliated metamorphic rocks originate from regional metamorphism. Some unfoliated metamorphic rocks, such as hornfels, originate only by contact metamorphism, but others can originate either by contact metamorphism or by regional metamorphism. Quartz and marble are prime examples of unfoliated that can be produced by either regional or contact metamorphism. Both rock types consist of metamorphic minerals that do not have flat or elongate shapes and thus cannot become layered even if they are produced under differential stress.

A geologist working with metamorphic rocks collects the rocks in the field and looks for the patterns the rocks form in outcrops as well as how those outcrops are related to other types of rock with which they are in contact. Field evidence is often required to know for sure whether rocks are products of regional metamorphism, contact metamorphism, or some other type of metamorphism. If only looking at rock samples in a laboratory, one can be sure of the type of metamorphism that produced a foliated metamorphic rock such as schist or gneiss, or a hornfels, which is unfoliated, but one cannot be sure of the type of metamorphism that produced an unfoliated marble or quartzite.

Foliated Metamorphic Rocks

Foliated metamorphic rocks are named for their style of foliation. However, a more complete name of each particular type of foliated metamorphic rock includes the main minerals that the rock comprises, such as biotite-garnet schist rather than just schist.

• slate—slates form at low metamorphic grade by the growth of fine-grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break along parallel planes, giving the rock a slaty cleavage. Some slate breaks into such extensively flat sheets of rock that it is used as the base of pool tables, beneath a layer of rubber and felt. Roof tiles are also sometimes made of slate.
• phyllite—phyllite is a low-medium grade regional metamorphic rock in which the clay minerals and chlorite have been at least partly replaced by mica mica minerals, muscovite and biotite. This gives the surfaces of phyllite a satiny luster, much brighter than the surface of a piece of slate. It is also common for the differential stresses under which phyllite forms to have produced a set of folds in the rock, making the foliation surfaces wavy or irregular, in contrast to the often perfectly flat surfaces of slaty cleavage.
• schist—the size of mineral crystals tends to grow larger with increasing metamorphic grade. Schist is a product of medium grades of metamorphism and is characterized by visibly prominent, parallel sheets of mica or similar sheet silicates, usually either muscovite or biotite, or both. In schist, the sheets of mica are usually arranged in irregular planes rather than perfectly flat planes, giving the rock a schistose foliation (or simply schistosity). Schist often contains more than just micas among its minerals, such as quartz, feldspars, and garnet.
• amphibolite—a poorly foliated to unfoliated mafic metamorphic rock, usually consisting largely of the common black amphibole known as hornblende, plus plagioclase, plus or minus biotite and possibly other minerals; it usually does not contain any quartz. Amphibolite forms at medium-high metamorphic grades. Amphibolite is also listed below in the section on unfoliated metamorphic rocks.
• gneiss—like the word schist, the word gneiss is originated from the German language; it is pronounced “nice.” As metamorphic grade continue to increase, sheet silicates become unstable and dark minerals such as hornblende or pyroxene start to grow. The dark-colored minerals tend to form separate bands or stripes in the rock, giving it a gneissic foliation of dark and light streaks. Gneiss is a high-grade metamorphic rock. Many types of gneiss look somewhat like granite, except that the gneiss has dark and light stripes whereas in granite randomly oriented and distributed minerals with no stripes or layers.
• migmatite—a combination of high-grade regional metamorphic rock – usually gneiss or schist – and granitic igneous rock. The granitic rock in migmatite probably originated from partial melting of some of the metamorphic rock, though in some migmatites the granite may have intruded the rock from deeper in the crust. In migmatite you can see metamorphic rock that has reached the limits of metamorphism and begun transitioning into the igneous stage of the rock cycle by melting to form magma.

Names of different styles of foliation come from the common rocks that exhibit such foliation:

• slate has slaty foliation
• phyllite has phyllitic foliation
• schist has schistose foliation
• gneiss has gneissic foliation (also called gneissose foliation)

Unfoliated Metamorphic Rocks

Unfoliated metamorphic rocks lack a planar (oriented) fabric, either because the minerals did not grow under differential stress, or because the minerals that grew during metamorphism are not minerals that have elongate or flat shapes. Because they lack foliation, these rocks are named entirely on the basis of their mineralogy.

• hornfels—hornfels are very hard rocks formed by contact metamorphism of shale, siltstone, or sandstone. The heat from the nearby magma “bakes” the sedimentary rocks and recrystallizes the minerals in them into a new texture that no longer breaks easily along the original sedimentary bedding planes. Depending on the composition of the rock and the temperature reached, minerals indicative of high metamorphic grade such as pyroxene may occur in some hornfels, though many hornfels have minerals indicating medium grade metamorphism.
• amphibolite—amphibolites are dark-colored rocks with amphibole, usually the common black amphibole known as hornblende, as their most abundant mineral, along with plagioclase and possibly other minerals, though usually no quartz. Amphibolites are poorly foliated to unfoliated and form at medium to medium-high grades of metamorphism from basalt or gabbro.
• quartzite—quartzite is a metamorphic rock made almost entirely of quartz, for which the protolith was quartz arenite. Because quartz is stable over a wide range of pressure and temperature, little or no new minerals form in quartzite during metamorphism. Instead, the quartz grains recrystallize into a denser, harder rock than the original sandstone. If struck by a rock hammer, quartzite will commonly break right through the quartz grains, rather than around them as when quartz arenite is broken.
• marble—marble is a metamorphic rock made up almost entirely of either calcite or dolomite, for which the protolith was either limestone or dolostone, respectively. Marbles may have bands of different colors which were deformed into convoluted folds while the rock was ductile. Such marble is often used as decorative stone in buildings. Some marble, which is considered better quality stone for carving into statues, lacks color bands.

Metamorphic rocks form when the minerals in an existing rock are changed by heat or pressure within the Earth. See figure 6 for an example of a metamorphic rock.

Figure 6. Quartzite is a metamorphic rock formed when quartz sandstone is exposed to heat and pressure within the Earth.

In the large outcrop of metamorphic rocks in figure 16, the rocks’ platy appearance is a result of the process metamorphism. Metamorphism is the addition of heat and/or pressure to existing rocks, which causes them to change physically and/or chemically so that they become a new rock. Metamorphic rocks may change so much that they may not resemble the original rock.

Figure 16. The platy layers in this large outcrop of metamorphic rock show the effects of pressure on rocks during metamorphism.

Metamorphism

Any type of rock—igneous, sedimentary, or metamorphic—can become a metamorphic rock. All that is needed is enough heat and/or pressure to alter the existing rock’s physical or chemical makeup without melting the rock entirely. Rocks change during metamorphism because the minerals need to be stable under the new temperature and pressure conditions. The need for stability may cause the structure of minerals to rearrange and form new minerals. Ions may move between minerals to create minerals of different chemical composition. Hornfels, with its alternating bands of dark and light crystals, is a good example of how minerals rearrange themselves during metamorphism. Hornfels is shown in table 6.

Figure 17. A foliated metamorphic rock.

Extreme pressure may also lead to foliation, the flat layers that form in rocks as the rocks are squeezed by pressure (figure 17). Foliation normally forms when pressure is exerted in only one direction. Metamorphic rocks may also be non-foliated. Quartzite and limestone, shown in table 6, are nonfoliated.

The two main types of metamorphism are both related to heat within Earth:

1. Regional metamorphism: Changes in enormous quantities of rock over a wide area caused by the extreme pressure from overlying rock or from compression caused by geologic processes. Deep burial exposes the rock to high temperatures.
2. Contact metamorphism: Changes in a rock that is in contact with magma because of the magma’s extreme heat.

The Rock Cycle

Rocks change as a result of natural processes that are taking place all the time. Most changes happen very slowly; many take place below the Earth’s surface, so we may not even notice the changes. Although we may not see the changes, the physical and chemical properties of rocks are constantly changing in a natural, never-ending cycle called the rock cycle.

Figure 7. James Hutton is considered the Father of Geology

The concept of the rock cycle was first developed by James Hutton, an eighteenth century scientist often called the “Father of Geology” (shown in figure 7). Hutton recognized that geologic processes have “no [sign] of a beginning, and no prospect of an end.” The processes involved in the rock cycle often take place over millions of years. So on the scale of a human lifetime, rocks appear to be “rock solid” and unchanging, but in the longer term, change is always taking place.

In the rock cycle, illustrated in figure 8, the three main rock types—igneous, sedimentary, and metamorphic—are shown. Arrows connecting the three rock types show the processes that change one rock type into another. The cycle has no beginning and no end. Rocks deep within the Earth are right now becoming other types of rocks. Rocks at the surface are lying in place before they are next exposed to a process that will change them.

Figure 8. The Rock Cycle.

Processes of the Rock Cycle

Several processes can turn one type of rock into another type of rock. The key processes of the rock cycle are crystallization, erosion and sedimentation, and metamorphism.

Crystallization

Magma cools either underground or on the surface and hardens into an igneous rock. As the magma cools, different crystals form at different temperatures, undergoing crystallization. For example, the mineral olivine crystallizes out of magma at much higher temperatures than quartz. The rate of cooling determines how much time the crystals will have to form. Slow cooling produces larger crystals.

Erosion and Sedimentation

Weathering wears rocks at the Earth’s surface down into smaller pieces. The small fragments are called sediments. Running water, ice, and gravity all transport these sediments from one place to another by erosion. During sedimentation, the sediments are laid down or deposited. In order to form a sedimentary rock, the accumulated sediment must become compacted and cemented together.

Metamorphism

When a rock is exposed to extreme heat and pressure within the Earth but does not melt, the rock becomes metamorphosed. Metamorphism may change the mineral composition and the texture of the rock. For that reason, a metamorphic rock may have a new mineral composition and/or texture.

The rock components of the crust are slowly but constantly being changed from one form to another and the processes involved are summarized in the rock cycle (Figure \(\PageIndex{1}\)). The rock cycle is driven by two forces: (1) Earth’s internal heat engine, which moves material around in the core and the mantle and leads to slow but significant changes within the crust, and (2) the hydrological cycle, which is the movement of water, ice, and air at the surface, and is powered by the sun.

The rock cycle is still active on Earth because our core is hot enough to keep the mantle moving, our atmosphere is relatively thick, and we have liquid water. On some other planets or their satellites, such as the Moon, the rock cycle is virtually dead because the core is no longer hot enough to drive mantle convection and there is no atmosphere or liquid water.

Figure \(\PageIndex{1}\) A schematic view of the rock cycle. The rock cycle takes place both above and below the Earth’s surface. The rock deepest beneath the earth’s surface, and under extreme heat and pressure, is metamorphic rock. This metamorphic rock can melt and become magma. When magma cools below the earth’s surface, it becomes “intrusive igneous rock.” If magma cools above the earth’s surface, it is “extrusive igneous rock” and becomes part of the outcrop. The outcrop is subject to weathering and erosion, and can be moved and redeposited around the earth by forces such as water and wind. As the outcrop is eroded, it becomes sediment which can be buried, compacted, and cemented beneath the Earth’s surface to become sedimentary rock. As sedimentary rock gets buried deeper and comes under increased heat and pressure, it returns to its original state as metamorphic rock. Rocks in the rock cycle do not always make a complete loop. It is possible for sedimentary rock to be uplifted back above the Earth’s surface and for intrusive and extrusive igneous rock to be reburied and become metamorphic rock.  © Steven Earle. CC BY.

In describing the rock cycle, we can start anywhere we like, although it’s convenient to start with magma. As we’ll see in more detail below, magma is rock that is hot to the point of being entirely molten, with a temperature of between about 800° and 1300°C, depending on the composition and the pressure.

Figure \(\PageIndex{2}\) Magma forming pahoehoe basalt at Kilauea Volcano, Hawaii.  © Steven Earle. CC BY.

Magma can either cool slowly within the crust (over centuries to millions of years)—forming intrusive igneous rock, or erupt onto the surface and cool quickly (within seconds to years)—forming extrusive igneous rock (volcanic rock) (Figure \(\PageIndex{2}\)). Intrusive igneous rock typically crystallizes at depths of hundreds of meters to tens of kilometers below the surface. To change its position in the rock cycle, intrusive igneous rock has to be uplifted and then exposed by the erosion of the overlying rocks.

Through the various plate-tectonics-related processes of mountain building, all types of rocks are uplifted and exposed at the surface. Once exposed, they are weathered, both physically (by mechanical breaking of the rock) and chemically (by weathering of the minerals), and the weathering products—mostly small rock and mineral fragments—are eroded, transported, and then deposited as sediments. Transportation and deposition occur through the action of glaciers, streams, waves, wind, and other agents, and sediments are deposited in rivers, lakes, deserts, and the ocean.

Figure \(\PageIndex{3}\) Cretaceous-aged marine sandstone overlying marine mudstone, Gabriola Island, B.C.  © Steven Earle. CC BY.

Unless they are re-eroded and moved along, sediments will eventually be buried by more sediments. At depths of hundreds of meters or more, they become compressed and cemented into sedimentary rock (See Figure \(\PageIndex{3}\) for example). Again through various means, largely resulting from plate-tectonic forces, different kinds of rocks are either uplifted, to be re-eroded, or buried deeper within the crust where they are heated up, squeezed, and changed into metamorphic rock (Figure \(\PageIndex{4}\))

Figure \(\PageIndex{4}\) Metamorphosed and folded Triassic-aged limestone, Quadra Island, B.C.  © Steven Earle. CC BY.

EarthPublic Domain | Image courtesy of NASA.

Earth is the largest of the Solar System’s rocky planets and the only known planet with liquid water. Water makes up about 75% of Earth’s surface.

Daily changes in the level of the water – known as tides – are due to the Moon and Sun’s gravitational influences. These gravitational effects raise tidal bulges in the oceans. The size of a specific tide primarily depends on the Sun-Moon orientations relative to Earth. Tidal interaction between Earth and Moon causes the Earth’s rotation to slow and the Moon to be locked in a synchronous orbit – revolution is equal to rotation. Some “rocking” of the Moon back and forth occurs, called librations.

Forces that Shape Earth: Earth’s Geological Processes

Impact Cratering was very intense during the early period of the Solar System; this also shaped Earth into its characteristics.

Other forces that shape Earth include erosion, weathering, plate tectonics, and volcanism.

• Erosion, which is a continuous process,is the wearing away of the surface by water, atmosphere, mechanical, and chemical processes.
• Weathering is a gradual physical and chemical wearing away of rocks and surface material.
• Plate tectonics is the motion of a body’s plates driven by internal stresses.
• Volcanism is the eruption of molten rock from a body’s interior onto its surface.

Earth has a dynamic and active atmosphere. Overall, Earth has the most dynamic weather in our Solar System. Our atmosphere is not as thick as the Gas Giants or even Venus (Rocky Planet), yet it is like looking through 30 feet of water.

Earth is the only known planet with life. Earth’s orbit is within the circumstellar habitable zone – area in a star’s orbit where ideal conditions exist for life. If Earth was 5% closer to the Sun, it would be Venus-like, and 20% farther out from the Sun, Earth would be Mars-like. Earth also exists in a system that has the right type of star, only one star, good location within the Milky Way Galaxy, the right type of galaxy, etc. Astronomers, biologists, and physicists have identified over 800 identified factors necessary for intelligent life.

Planet Earth at a Glance

Characteristic — Current State

• Impact Craters — Yes
• Tectonic Craters — Yes
• Volcanoes — Yes
• Atmosphere — Nitrogen (N), Oxygen (O ₂ ), Clouds, Rain, Snow
• Water — Vapor, Liquid, and Ice → Primarily a water planet
• Erosion — Yes
• Dunes — Yes
• Polar Caps — Yes
• Satellites — One, Moon
• Life — Yes

Geological Process in the Solar System

During the time all the planets have been subject to such impacts, internal forces on the terrestrial planets have buckled and twisted their crusts, built up mountain ranges, erupted as volcanoes, and generally reshaped the surfaces in what we call geological activity. (The prefix geo means “Earth,” so this is a bit of an “Earth-chauvinist” term, but it is so widely used that we bow to tradition.) Among the terrestrial planets, Earth and Venus have experienced the most geological activity over their histories, although some of the moons in the outer solar system are also surprisingly active. In contrast, our own Moon is a dead world where geological activity ceased billions of years ago.

Geological activity on a planet is the result of a hot interior. The forces of volcanism and mountain building are driven by heat escaping from the interiors of planets. As we will see, each of the planets was heated at the time of its birth, and this primordial heat initially powered extensive volcanic activity, even on our Moon. But, small objects such as the Moon soon cooled off. The larger the planet or moon, the longer it retains its internal heat, and therefore the more we expect to see surface evidence of continuing geological activity. The effect is similar to our own experience with a hot baked potato: the larger the potato, the more slowly it cools. If we want a potato to cool quickly, we cut it into small pieces.

For the most part, the history of volcanic activity on the terrestrial planets conforms to the predictions of this simple theory. The Moon, the smallest of these objects, is a geologically dead world. Although we know less about Mercury, it seems likely that this planet, too, ceased most volcanic activity about the same time the Moon did. Mars represents an intermediate case. It has been much more active than the Moon, but less so than Earth. Earth and Venus, the largest terrestrial planets, still have molten interiors even today, some 4.5 billion years after their birth.

Attributions

https://geo.libretexts.org/Bookshelv...nce_of_Geology

https://geo.libretexts.org/Bookshelv...hat_is_Geology

https://geo.libretexts.org/Bookshelv..._Geologists_Do

https://geo.libretexts.org/Bookshelv...rals_and_Rocks

https://geo.libretexts.org/Bookshelv...The_Rock_Cycle

https://geo.libretexts.org/Bookshelv...The_Rock_Cycle

https://geo.libretexts.org/Bookshelv...Types_of_Rocks

https://geo.libretexts.org/Bookshelv..._Igneous_Rocks

https://geo.libretexts.org/Bookshelv...imentary_Rocks

https://geo.libretexts.org/Bookshelv...amorphic_Rocks

https://phys.libretexts.org/Bookshel...A_Planet_Earth

https://phys.libretexts.org/Bookshel...ure_of_Planets