6.6: Volcanism

INTRODUCTION

In 1980, Mount St. Helens blew up in the costliest and deadliest volcanic eruption in United States history. The eruption killed 57 people, destroyed 250 homes and swept away 47 bridges (figure 1).

Figure 1. Mount St. Helens on May 18, 1980.

Mt. St. Helens still has minor earthquakes and eruptions. The volcano now has a horseshoe-shaped crater with a lava dome inside. The dome is formed of viscous lava that oozes into place.

The Pacific Northwest is volcanically active. Nearly every type of volcano exists or has existed in the Pacific Northwest, and every type of volcanic eruption has occurred. The Cascade volcanic arc has been catastrophically active during recent human memory. In the last 10,000 years volcanic eruptions have also occurred in such places as eastern Idaho and central Oregon. In some landscape regions of the Northwest, the entire upper crust is almost entirely volcanic rock.

To understand volcanoes, you have to start with magma, the molten rock that comes up from the interior of the earth to form volcanoes. The composition, gas content, and crystal content of the magma determines the type of eruption that occurs, the kind of volcanic rock that forms, and the type of volcano that forms.

Volcanoes

Volcanoes mark locations where lava rises to the surface. One example is mid ocean ridges, which are long undersea mountain ranges formed by lava rising from Earth’s mantle at plate boundaries. A second major kind of volcanic activity is associated with subduction zones, and volcanoes sometimes also appear in regions where continental plates are colliding. In each case, the volcanic activity gives us a way to sample some of the material from deeper within our planet.

Other volcanic activity occurs above mantle “hot spots”—areas far from plate boundaries where heat is nevertheless rising from the interior of Earth. One of the best-known hot spots is under the island of Hawaii, where it currently supplies the heat to maintain three active volcanoes, two on land and one under the ocean. The Hawaii hot spot has been active for at least 100 million years. As Earth’s plates have moved during that time, the hot spot has generated a 3500-kilometer-long chain of volcanic islands. The tallest Hawaiian volcanoes are among the largest individual mountains on Earth, more than 100 kilometers in diameter and rising 9 kilometers above the ocean floor. One of the Hawaiian volcanic mountains, the now-dormant Maunakea, has become one of the world’s great sites for doing astronomy.

Link to Learning

The US Geological Service provides an interactive map of the famous “ring of fire,” which is the chain of volcanoes surrounding the Pacific Ocean, and shows the Hawaiian “hot spot” enclosed within.

Not all volcanic eruptions produce mountains. If lava flows rapidly from long cracks, it can spread out to form lava plains. The largest known terrestrial eruptions, such as those that produced the Snake River basalts in the northwestern United States or the Deccan plateau in India, are of this type. Similar lava plains are found on the Moon and the other terrestrial planets.

Magma Composition

Volcanoes do not always erupt in the same way. Each volcanic eruption is unique, differing in size, style, and composition of erupted material. One key to what makes the eruption unique is the chemical composition of the magma that feeds a volcano, which determines (1) the eruption style, (2) the type of volcanic cone that forms, and (3) the composition of rocks that are found at the volcano.

Remember from the Rocks chapter that different minerals within a rock melt at different temperatures. The amount of partial melting and the composition of the original rock determine the composition of the magma. Magma collects in magma chambers in the crust at 160 kilometers (100 miles) beneath the surface.

The words that describe composition of igneous rocks also describe magma composition.

• Mafic magmas are low in silica and contain more dark, magnesium and iron rich mafic minerals, such as olivine and pyroxene.
• Felsic magmas are higher in silica and contain lighter colored minerals such as quartz and orthoclase feldspar. The higher the amount of silica in the magma, the higher is its viscosity. Viscosity is a liquid’s resistance to flow (figure 2).

Figure 2. Honey flows slowly. It is more viscous than water.

Viscosity determines what the magma will do. Mafic magma is not viscous and will flow easily to the surface. Felsic magma is viscous and does not flow easily. Most felsic magma will stay deeper in the crust and will cool to form igneous intrusive rocks such as granite and granodiorite. If felsic magma rises into a magma chamber, it may be too viscous to move and so it gets stuck. Dissolved gases become trapped by thick magma. The magma churns in the chamber and the pressure builds.

As noted in the previous section, the types of magma produced in the various volcanic settings can differ significantly. At divergent boundaries and oceanic mantle plumes, where there is little interaction with crustal materials the magma tends to be consistently mafic. At subduction zones, where the magma ascends through significant thicknesses of crust, interaction between the magma and the crustal rock—some of which is quite felsic—leads to increases in the felsic character of the magma.

Figure \(\PageIndex{1}\) The important processes that lead to changes in the composition of magmas stored within magma chambers within relatively felsic rocks of the crust.  © Steven Earle. CC BY. 

Figure \(\PageIndex{1}\) image description: Four processes that can affect the composition of magma stored in chambers.

1. Loss of olivine or pyroxene (by crystal settling) makes the upper magma more felsic.
2. Partial melting of country rock makes the magma more felsic.
3. Partial or complete melting of xenoliths makes the magma more felsic.
4. Possible re-melting of olivine or pyroxene can make the lower magma more mafic.

As illustrated in Figure \(\PageIndex{1}\), several processes can make magma that is stored in a chamber within the crust more felsic than it was to begin with, and can also contribute to development of vertical zonation from more mafic at the bottom to more felsic at the top. Partial melting of country rock and country-rock xenoliths increases the overall felsic character of the magma; first, because the country rocks tends to be more felsic than the magma, and second, because the more felsic components of the country rock melt preferentially. Settling of ferromagnesian crystals from the upper part of the magma, and possible remelting of those crystals in the lower part can both contribute to the vertical zonation from relatively mafic at the bottom to more felsic at the top.

From the perspective of volcanism there are some important differences between felsic and mafic magmas. First, as we’ve already discussed, felsic magmas tend to be more viscous because they have more silica, and hence more polymerization. Second, felsic magmas tend to have higher levels of volatiles; that is, components that behave as gases during volcanic eruptions. The most abundant volatile in magma is water (H₂O), followed typically by carbon dioxide (CO₂), and then by sulfur dioxide (SO₂).

 mafic magmas typically have 1% to 3% volatiles, intermediate magmas have 3% to 4% volatiles, and felsic magmas have 4% to 7% volatiles.

Differences in viscosity and volatile levels have significant implications for the nature of volcanic eruptions. When magma is deep beneath the surface and under high pressure from the surrounding rocks, the gases remain dissolved. As magma approaches the surface, the pressure exerted on it decreases. Gas bubbles start to form, and the more gas there is in the magma, the more bubbles form. If the magma is runny enough for gases to rise up through it and escape to surface, the pressure will not become excessive. Assuming that it can break through to the surface, the magma will flow out relatively gently. An eruption that involves a steady non-violent flow of magma is called effusive.

A good analogy for a magma chamber in the upper crust is a plastic bottle of pop on the supermarket shelf. Go to a supermarket and pick one up off the shelf (something not too dark). You’ll find that the bottle is hard because it was bottled under pressure, and you should be able to see that there are no gas bubbles inside.

Buy a small bottle of pop (you don’t have to drink it!) and open it. The bottle will become soft because the pressure is released, and small bubbles will start forming. If you put the lid back on and shake the bottle (best to do this outside!), you’ll enhance the processes of bubble formation, and when you open the lid, the pop will come gushing out, just like an explosive volcanic eruption.

A pop bottle is a better analogue for a volcano than the old baking soda and vinegar experiment that you did in elementary school, because pop bottles—like volcanoes—come pre-charged with gas pressure. All we need to do is release the confining pressure and the gases come bubbling out, forcing the pop with them.

If the magma is felsic, and therefore too viscous for gases to escape easily, or if it has a particularly high gas content, it is likely to be under high pressure. Viscous magma doesn’t flow easily, so even if there is a conduit for it to move towards surface, it may not flow out. Under these circumstances pressure will continue to build as more magma moves up from beneath and gases continue to exsolve. Eventually some part of the volcano will break and then all of that pent-up pressure will lead to an explosive eruption.

Mantle plume and spreading-ridge magmas tend to be consistently mafic, so effusive eruptions are the norm. At subduction zones, the average magma composition is likely to be close to intermediate, but as we’ve seen, magma chambers can become zoned and so compositions ranging from felsic to mafic are possible—even likely. Eruption styles can be correspondingly variable.

Figure \(\PageIndex{2}\) image description: The following table describes the range of data points adapted from the original scatter plot graph.

Type of Magma	Weight % of gasses (mostly H2O)	Weight % of SiO2
Mafic	1 to 3%	47 to 50%
Intermediate	3 to 4%	57 to 64%
Felsic	4 to 7%	69 to 72%

ERUPTIONS

The type of magma in the chamber determines the type of volcanic eruption. Although the two major kinds of eruptions—explosive and effusive—are described in this section, there is an entire continuum of eruption types. Which magma composition do you think leads to each type?

Explosive Eruptions

A large explosive eruption creates even more devastation than the force of the atom bomb dropped on Nagasaki at the end of World War II in which more than 40,000 people died. A large explosive volcanic eruption is 10,000 times as powerful. Felsic magmas erupt explosively. Hot, gas-rich magma churns within the chamber. The pressure becomes so great that the magma eventually breaks the seal and explodes, just like when a cork is released from a bottle of champagne. Magma, rock, and ash burst upward in an enormous explosion. The erupted material is called tephra (figure 3).

Figure 3. Ash and gases create a mushroom cloud above Mt. Redoubt in Alaska, 1989. The cloud reached 45,000 feet and caught a Boeing 747 in its plume.

Scorching hot tephra, ash, and gas may speed down the volcano’s slopes at 700 km/h (450 mph) as a pyroclastic flow. Pyroclastic flows knock down everything in their path. The temperature inside a pyroclastic flow may be as high as 1,000°C (1,800°F) (figure 4).

Figure 4. (a) An explosive eruption from the Mayon Volcano in the Philippines in 1984. Ash flies upward into the sky and pyroclastic flows pour down the mountainside. (b) The end of a pyroclastic flow at Mount St. Helens.

Watch this video of a pyroclastic flow at Montserrat volcano.

Prior to the Mount St. Helens eruption in 1980, the Lassen Peak eruption on May 22, 1915, was the most recent Cascades eruption. A column of ash and gas shot 30,000 feet into the air. This triggered a high-speed pyroclastic flow, which melted snow and created a volcanic mudflow known as a lahar. Lassen Peak currently has geothermal activity and could erupt explosively again. Mt. Shasta, the other active volcano in California, erupts every 600 to 800 years. An eruption would most likely create a large pyroclastic flow, and probably a lahar. Of course, Mt. Shasta could explode and collapse like Mt. Mazama in Oregon (figure 5).

Figure 5. Crater Lake fills the caldera of the collapsed Mt. Mazama, which erupted with 42 times more power than Mount St. Helens in 1980. The bathymetry of the lake shows volcanic features such as cinder cones.

Volcanic gases can form poisonous and invisible clouds in the atmosphere. These gases may contribute to environmental problems such as acid rain and ozone destruction. Particles of dust and ash may stay in the atmosphere for years, disrupting weather patterns and blocking sunlight (figure 6).

Figure 6. The ash plume from Eyjafjallajökull volcano in Iceland disrupted air travel across Europe for six days in April 2010.

Effusive Eruptions

Mafic magma creates gentler effusive eruptions. Although the pressure builds enough for the magma to erupt, it does not erupt with the same explosive force as felsic magma. People can usually be evacuated before an effusive eruption, so they are much less deadly. Magma pushes toward the surface through fissures. Eventually, the magma reaches the surface and erupts through a vent (figure 7).

Figure 7. In effusive eruptions, lava flows readily, producing rivers of molten rock.

• The Kilauea volcanic eruption in 2008 is seen in this short video.
  http://www.youtube.com/watch?v=BtH79yxBIJI
• Watch this movie with thermal camera of a lava stream within the vent of a Hawaiian volcano.

Low-viscosity lava flows down mountainsides. Differences in composition and where the lavas erupt result in three types of lava flow coming from effusive eruptions (figure 8).

Figure 8. (a) A’a lava forms a thick and brittle crust that is torn into rough and jagged pieces. A’a lava can spread over large areas as the lava continues to flow underneath the crust’s surface. (b) Pāhoehoe lava forms lava tubes where fluid lava flows through the outer cooled rock crust, as can be seen at the Thurston Lava Tube in Hawai’i Volcanoes National Park. (c) Pāhoehoe lava is less viscous than a’a lava so its surface looks is smooth and ropy. (d) Mafic lava that erupts underwater creates pillow lava. The lava cools very quickly to roughly spherical rocks. Pillow lava is common at mid-ocean ridges.

• Watch this videos of an undersea eruption. In this video you can hear the underwater eruption as well.

Although effusive eruptions rarely kill anyone, they can be destructive. Even when people know that a lava flow is approaching, there is not much anyone can do to stop it from destroying a building or road (figure 9).

Figure 9. A road is overrun by an eruption at Kilauea volcano in Hawaii.

 

Types of Volcanoes

INTRODUCTION

A volcano is a vent through which molten rock and gas escape from a magma chamber. Volcanoes differ in many features such as height, shape, and slope steepness. Some volcanoes are tall cones and others are just cracks in the ground (figure 1). As you might expect, the shape of a volcano is related to the composition of its magma.

Figure 1. Mount St. Helens was a beautiful, classic, cone-shaped volcano. The volcano’s 1980 eruption blew more than 400 meters (1,300 feet) off the top of the mountain.

There are numerous types of volcanoes or volcanic sources; some of the more common ones are summarized in Table 4.1.

Table 4.1 A summary of the important types of volcanism

[Skip Table]
Type	Tectonic Setting	Size and Shape	Magma and Eruption Characteristics	Example
Cinder cone	Various; some form on the flanks of larger volcanoes	Small (10s to 100s of meters) and steep (Greater than 20°)	Most are mafic and form from the gas-rich early stages of a shield- or rift-associated eruption	Eve Cone, northern B.C.
Composite volcano	Almost all are at subduction zones	Medium size (1000s of meters high and up to 20 km across) and moderate steepness (10° to 30°)	Magma composition varies from felsic to mafic, and from explosive to effusive	Mount St. Helens
Shield volcano	Most are at mantle plumes; some are on spreading ridges	Large (up to several 1,000 meters high and up to 200 kilometers across), not steep (typically 2° to 10°)	Magma is almost always mafic, and eruptions are typically effusive, although cinder cones are common on the flanks of shield volcanoes	Kilauea, Hawaii
Large igneous provinces	Associated with “super” mantle plumes	Enormous (up to millions of square kiometers) and 100s of meters thick	Magma is always mafic and individual flows can be 10s of meters thick	Columbia River basalts
Sea-floor volcanism	Generally associated with spreading ridges but also with mantle plumes	Large areas of the sea floor associated with spreading ridges	Pillows form at typical eruption rates; lava flows develop if the rare of flow is faster	Juan de Fuca ridge
Kimberlite	Upper-mantle sourced	The remnants are typically 10s to 100s of meters across	Most appear to have had explosive eruptions forming cinder cones; the youngest one is dated at about 10 ka, and all others are at least 30 Ma	Lac de Gras Kimberlite Field, N.W.T.

The sizes and shapes of typical shield, composite, and cinder-cone volcanoes are compared in Figure \(\PageIndex{1}\), although, to be fair, Mauna Loa is the largest shield volcano on Earth; all others are smaller. Mauna Loa rises from the surrounding flat sea floor, and its diameter is in the order of 200 km. Its elevation is 4,169 m above sea level. Mount St. Helens, a composite volcano of average size, rises above the surrounding hills of the Cascade Range. Its diameter is about 6 km, and its height is 2,550 m above sea level. Cinder cones are much smaller. On this drawing, even a large cinder cone is just a dot.

Figure \(\PageIndex{1}\) Profiles of Mauna Loa shield volcano, Mount St. Helens composite volcano, and a large cinder cone.  © Steven Earle. CC BY.

Cinder Cones

Cinder cones are the most common type of volcano. A cinder cone has a cone shape, but is much smaller than a composite volcano. Cinder cones rarely reach 300 meters in height but they have steep sides. Cinder cones grow rapidly, usually from a single eruption cycle (figure 5). Cinder cones are composed of small fragments of rock, such as pumice, piled on top of one another. The rock shoots up in the air and doesn’t fall far from the vent. The exact composition of a cinder cone depends on the composition of the lava ejected from the volcano. Cinder cones usually have a crater at the summit.

Figure 5. In 1943, a Mexican farmer first witnessed a cinder cone erupting in his field. In a year, Paricutín was 336 meters high. By 1952, it reached 424 meters and then stopped erupting.

Cinder cones are often found near larger volcanoes (figure 6).

Figure 6. This Landsat image shows the topography of San Francisco Mountain, an extinct volcano, with many cinder cones near it in northern Arizona. Sunset crater is a cinder cone that erupted about 1,000 years ago.

Cinder cones, like Eve Cone in northern B.C. (Figure \(\PageIndex{2}\)), are typically only a few hundred meters in diameter, and few are more than 200 m high. Most are made up of fragments of vesicular mafic rock (scoria) that were expelled as the magma boiled when it approached the surface, creating fire fountains. In many cases, these later became the sites of effusive lava flows when the gases were depleted. Most cinder cones are monogenetic, meaning that they formed during a single eruptive phase that might have lasted weeks or months. Because cinder cones are made up almost exclusively of loose fragments, they have very little strength. They can be easily, and relatively quickly, eroded away.

Figure \(\PageIndex{2}\) Eve Cone, situated near to Mount Edziza in northern B.C., formed approximately 700 years ago.  Eve Cone © nass5518. CC BY.

Composite Volcanoes

Composite volcanoes are made of felsic to intermediate rock. The viscosity of the lava means that eruptions at these volcanoes are often explosive (figure 2).

Figure 2. Mt. Fuji, the highest mountain in Japan, is a dormant composite volcano.

The viscous lava cannot travel far down the sides of the volcano before it solidifies, which creates the steep slopes of a composite volcano. Viscosity also causes some eruptions to explode as ash and small rocks. The volcano is constructed layer by layer, as ash and lava solidify, one upon the other (figure 3). The result is the classic cone shape of composite volcanoes.

Figure 3. A cross section of a composite volcano reveals alternating layers of rock and ash: (1) magma chamber, (2) bedrock, (3) pipe, (4) ash layers, (5) lava layers, (6) lava flow, (7) vent, (8) lava, (9) ash cloud. Frequently there is a large crater at the top from the last eruption.

Composite volcanoes, like Mount St. Helens in Washington State (Figure \(\PageIndex{3}\)), are almost all associated with subduction at convergent plate boundaries—either ocean-continent or ocean-ocean boundaries (Figure \(\PageIndex{2}\)b). They can extend up to several thousand meters from the surrounding terrain, and, with slopes ranging up to 30˚ They can be up to about 20 km across. At many such volcanoes, magma is stored in a magma chamber in the upper part of the crust. For example, at Mount St. Helens, there is evidence of a magma chamber that is approximately 1 kilometre wide and extends from about 6 km to 14 km below the surface (Figure \(\PageIndex{4}\)). Systematic variations in the composition of volcanism over the past several thousand years at Mount St. Helens imply that the magma chamber is zoned, from more felsic at the top to more mafic at the bottom.

Figure \(\PageIndex{3}\) The north side of Mount St. Helens in southwestern Washington State, 2003. The large 1980 eruption reduced the height of the volcano by 400 m, and a sector collapse removed a large part of the northern flank. Between 1980 and 1986 the slow eruption of more mafic and less viscous lava led to construction of a dome inside the crater.  © Steven Earle. CC BY.

Mafic eruptions (and some intermediate eruptions), on the other hand, produce lava flows; the one shown in Figure \(\PageIndex{5}\)b is thick enough (about 10 m in total) to have cooled in a columnar jointing pattern (Figure \(\PageIndex{7}\)). Lava flows both flatten the profile of the volcano (because the lava typically flows farther than pyroclastic debris falls) and protect the fragmental deposits from erosion. Even so, composite volcanoes tend to erode quickly. Patrick Pringle, a volcanologist with the Washington State Department of Natural Resources, describes Mount St. Helens as a “pile of junk.” The rock that makes up Mount St. Helens ranges in composition from rhyolite (Figure \(\PageIndex{5}\)a) to basalt (Figure \(\PageIndex{5}\)b); this implies that the types of past eruptions have varied widely in character. As already noted, felsic magma doesn’t flow easily and doesn’t allow gases to escape easily. Under these circumstances, pressure builds up until a conduit opens, and then an explosive eruption results from the gas-rich upper part of the magma chamber, producing pyroclastic debris, as shown on Figure \(\PageIndex{5}\)a. This type of eruption can also lead to rapid melting of ice and snow on a volcano, which typically triggers large mudflows known as lahars (Figure \(\PageIndex{5}\)a). Hot, fast-moving pyroclastic flows and lahars are the two main causes of casualties in volcanic eruptions. Pyroclastic flows killed approximately 30,000 people during the 1902 eruption of Mount. Pelée on the Caribbean island of Martinique. Most were incinerated in their homes. In 1985 a massive lahar, triggered by the eruption of Nevado del Ruiz, killed 23,000 people in the Colombian town of Armero, about 50 km from the volcano.

In a geological context, composite volcanoes tend to form relatively quickly and do not last very long. Mount St. Helens, for example, is made up of rock that is all younger than 40,000 years; most of it is younger than 3,000 years. If its volcanic activity ceases, it might erode away within a few tens of thousands of years. This is largely because of the presence of pyroclastic eruptive material, which is not strong.

Figure \(\PageIndex{5}\) Mount St. Helens volcanic deposits: (a) lahar deposits (L) and felsic pyroclastic deposits (P) and (b) a columnar basalt lava flow. The two photos were taken at locations only about 500 m apart.  © Steven Earle. CC BY.   Image (A) shows a cliff wall with grey/brown and orange horizontal layers. The sides look soft like they would be easily worn away. The grey/brown layers are lahar deposits and the orange layers are felsic pyroclastic deposits. Image (B) shows a columnar basalt lava flow which looks like a rocky, stone cliff with vertical layers.

 

Figure \(\PageIndex{7}\) The development of columnar jointing in basalt, here seen from the top looking down. As the rock cools it shrinks, and because it is very homogenous it shrinks in a systematic way. When the rock breaks it does so with approximately 120˚ angles between the fracture planes. The resulting columns tend to be 6-sided but 5- and 7-sided columns also form.  © Steven Earle. CC BY.

 

Shield Volcanoes

Lava flow after lava flow of low-viscosity, mafic lava builds up a basaltic shield volcano. Shield volcanoes are the largest type of volcano on earth, as exemplified by Mauna Loa on the Island of Hawaii. Shield volcanoes in the Pacific Northwest include: Medicine Lake Volcano in northern California; Newberry Volcano (Paulina Peak) in Oregon; and Simco Volcano in south Central Washington, on the Yakima Indian Reservation near the town of Goldendale.

Shield volcanoes get their name from their shape. Although shield volcanoes are not steep, they may be very large. Shield volcanoes are common at spreading centers or intraplate hot spots (figure 4).

Figure 4. Mauna Loa Volcano in Hawaii (in the background) is the largest shield volcano on Earth with a diameter of more than 112 kilometers (70 miles). The volcano forms a significant part of the island of Hawaii.

The lava that creates shield volcanoes is fluid and flows easily. The spreading lava creates the shield shape. Shield volcanoes are built by many layers over time and the layers are usually of very similar composition. The low viscosity also means that shield eruptions are non-explosive.

Most shield volcanoes are associated with mantle plumes, although some form at divergent boundaries, either on land or on the sea floor. Because of their non-viscous mafic magma they tend to have relatively gentle slopes (2 to 10˚) and the larger ones can be over 100 km in diameter. The best-known shield volcanoes are those that make up the Hawaiian Islands, and of these, the only active ones are on the big island of Hawaii. Mauna Loa, the world’s largest volcano and the world’s largest mountain (by volume) last erupted in 1984. Kilauea, arguably the world’s most active volcano, has been erupting, virtually without interruption, since 1983. Loihi is an underwater volcano on the southeastern side of Hawaii. It is last known to have erupted in 1996, but may have erupted since then without being detected.

All of the Hawaiian volcanoes are related to the mantle plume that currently lies beneath Mauna Loa, Kilauea, and Loihi (Figure \(\PageIndex{8}\)). In this area, the Pacific Plate is moving northwest at a rate of about 7 centimeters (cm) per year. This means that the earlier formed — and now extinct — volcanoes have now moved well away from the mantle plume. As shown on Figure \(\PageIndex{8}\), there is evidence of crustal magma chambers beneath all three active Hawaiian volcanoes. At Kilauea, the magma chamber appears to be several kilometers in diameter, and is situated between 8 km and 11 km below surface.( Lin, G, Amelung, F, Lavallee, Y, and Okubo, P, 2014, Seismic evidence for a crustal magma reservoir beneath the upper east rift zone of Kilauea volcano, Hawaii. Geology. V. )

Figure \(\PageIndex{8}\) The mantle plume beneath the volcanoes of the island of Hawaii  “Hawaii hotspot cross-sectional diagram” by USGS. Public domain.

Although it is not a prominent mountain (Figure \(\PageIndex{2}\)), Kilauea volcano has a large caldera in its summit area (Figure \(\PageIndex{9}\)). A caldera is a volcanic crater that is more than 2 km in diameter; this one is 4 km long and 3 km wide. It contains a smaller feature called Halema’uma’u crater, which has a total depth of over 200 m below the surrounding area. Most volcanic craters and calderas are formed above magma chambers, and the level of the crater floor is influenced by the amount of pressure exerted by the magma body. During historical times, the floors of both Kilauea caldera and Halema’uma’u crater have moved up during expansion of the magma chamber and down during deflation of the chamber.

Figure \(\PageIndex{9}\) Aerial view of the Kilauea caldera. The caldera is about 4 km across, and up to 120 m deep. It encloses a smaller and deeper crater known as Halema’uma’u.  “Kilauea ali 2012 01 28” by NASA. Public domain.

One of the conspicuous features of Kilauea caldera is rising water vapor (the white cloud in Figure \(\PageIndex{9}\)) and a strong smell of sulfur (Figure \(\PageIndex{10}\)). As is typical in magmatic regions, water is the main volatile component, followed by carbon dioxide and sulfur dioxide. These, and some minor gases, originate from the magma chamber at depth and rise up through cracks in the overlying rock. This degassing of the magma is critical to the style of eruption at Kilauea, which, for most of the past 35 years, has been effusive, not explosive.

Figure \(\PageIndex{10}\) A gas-composition monitoring station (left) within the Kilauea caldera close to the edge of Halema’uma’u crater. The rising clouds are mostly composed of water vapor, but also include carbon dioxide and sulfur dioxide. Sulphur crystals (right) have formed around a gas vent in the caldera.  © Steven Earle. CC BY.

The Kilauea eruption that began in 1983 started with the formation of a cinder cone at Pu’u ’O’o, approximately 15 km east of the caldera (Figure \(\PageIndex{11}\)). The magma feeding this eruption flowed along a major conduit system known as the East Rift, which extends for about 20 km from the caldera, first southeast and then east. Lava fountaining and construction of the Pu’u ’O’o cinder cone (Figure \(\PageIndex{12}\)a) continued until 1986 at which time the flow became effusive. From 1986 to 2014, lava flowed from a gap in the southern flank of Pu’u ’O’o down the slope of Kilauea through a lava tube (Figure \(\PageIndex{12}\)d), emerging at or near the ocean. During 2014 and 2015, the lava flowed northeast toward the community of Pahoa (see Exercise 4.4). In May of 2018 a new eruption started another 15 km east of the 2014/15 flow in the area known as Leilani Estates. The Lower East Rift Flow was active for 6 months. During that time, 35 km² of existing land was covered in lava and 3.5 km² of new land was created (Figure \(\PageIndex{11}\)), about 48 km of road were covered in lava and 716 dwellings were destroyed (see USGS Overview of Kilauea Volcanoe’s 2018 eruption[PDF]). Volcanic activity on the East Rift ceased in August 2018, and there has been no activity on Kilauea since then. This appears to mark the end of the eruption cycle that lasted—with only a few short interruptions—for 35 years. Kilauea will almost certainly erupt again within years or decades.

Figure \(\PageIndex{11}\) Satellite image of Kilauea volcano showing the East rift and Pu’u ’O’o, the site of the eruption that started in 1983 and continues to the present day. The 2014-2015 lava flow described in Exercise 4.4 is shown with a yellow arrow, and the area of the 2018 lava flow is shown in red. The puffy white blobs are clouds.  Island of Hawai’i – Landsat mosaic by NOAA. Public domain. Modified by Steven Earle.

The two main types of textures created during effusive subaerial eruptions are pahoehoe and aa. Pahoehoe, ropy lava that forms as non-viscous lava, flows gently, forming a skin that gels and then wrinkles because of ongoing flow of the lava below the surface (Figure \(\PageIndex{12}\)b, and “lava flow video”). Aa, or blocky lava, forms when magma is forced to flow faster than it is able to (down a slope for example) (Figure \(\PageIndex{12}\)c). Tephra (lava fragments) is produced during explosive eruptions, and accumulates in the vicinity of cinder cones.

Figure \(\PageIndex{12}\)d is a view into an active lava tube on the southern edge of Kilauea. The red glow is from a stream of very hot lava (~1200°C) that has flowed underground for most of the 8 km from the Pu’u ’O’o vent. Lava tubes form naturally and readily on both shield and composite volcanoes because flowing mafic lava preferentially cools near its margins, forming solid lava levées that eventually close over the top of the flow. The magma within a lava tube is not exposed to the air, so it remains hot and fluid and can flow for tens of km, thus contributing to the large size and low slopes of shield volcanoes. The Hawaiian volcanoes are riddled with thousands of old lava tubes, some as long as 50 km.

Figure \(\PageIndex{12}\) Images of Kilauea volcano. (a) Pu’u’O’o cinder cone in the background with tephra in the foreground and aa lava in the middle, (b) Formation of pahoehoe on the southern edge of Kilauea, (c) Formation of aa on a steep slope on Kilauea, (d) Skylight in an active lava tube, Kilauea. Photos B & C taken in 2002 photos A & D taken in 2007.  © Steven Earle. CC BY.

Kilauea started forming at approximately 300 ka, while neighbouring Mauna Loa dates back to 700 ka and nearby Mauna Kea ito around 1 Ma. If volcanism continues above the Hawaii mantle plume in the same manner that it has since 85 Ma, it is likely that Kilauea will continue to erupt for at least another 500,000 years. By that time, its neighbour, Loihi, will have emerged from the sea floor, and its other neighbours, Mauna Loa and Mauna Kea, will have become significantly eroded, like their cousins, the islands to the northwest (Figure \(\PageIndex{8}\)).

Large Igneous Provinces

Flood Basalts

Flood basalts usually originate from fissure eruptions, cracks in the surface of the earth that rapidly disgorge huge volumes of lava. Flood basalts are high-volume, widespread flows of mafic magma that solidify into basalt flows that in some cases cover thousands of square miles. The Columbia River Basalts comprise one of the world’s great flood basalt provinces. Some of the Columbia River basalt flows originated near the border of Idaho, covered much of eastern Washington and north central Oregon, and flowed down through the Columbia River gorge beyond the Cascade Range to cover parts of the Portland area and reach the Pacific Ocean, all in gigantic individual flows.

While the Hawaii mantle plume has produced a relatively low volume of magma for a very long time (~85 Ma), other mantle plumes are less consistent, and some generate massive volumes of magma over relatively short time periods. Although their origin is still controversial, it is thought that the volcanism leading to large igneous provinces (LIP) is related to very high volume but relatively short duration bursts of magma from mantle plumes. An example of an LIP is the Columbia River Basalt Group (CRGB), which extends across Washington, Oregon, and Idaho (Figure \(\PageIndex{14}\)). This volcanism, which covered an area of about 160,000 square kilometers (km²) with basaltic rock up to several hundred meters thick, took place between 17 and 14 Ma.

Figure \(\PageIndex{14}\) A part of the Columbia River Basalt Group at Frenchman Coulee, eastern Washington. All of the flows visible here have formed large (up to two m in diameter) columnar basalts, a result of relatively slow cooling of flows that are tens of meters thick. The inset map shows the approximate extent of the 17 to 14 Ma Columbia River Basalts, with the location of the photo shown as a star.  © Steven Earle. CC BY.  The Columbia River Basalt Group covers most of south eastern Washington state and stretches along the borders between Washington, Idaho, and Oregon. The columnar basalts shown in the photo are in in eastern Washington. They rise up out of a flat valley as tall cliffs.

Most other LIP eruptions are much bigger. The Siberian Traps (also basalt), which erupted at the end of the Permian period at 250 Ma, are estimated to have produced approximately 40 times as much lava as the CRBG.

The mantle plume that is assumed to be responsible for the CRBG is now situated beneath the Yellowstone area, where it leads to felsic volcanism. Over the past 2 Ma three very large explosive eruptions at Yellowstone have yielded approximately 900 cubic kilometers (km³) of felsic magma, about 900 times the volume of the 1980 eruption of Mount St. Helens, but only 5% of the volume of mafic magma in the CRBG.

Sea-Floor Volcanism

Pillow basalts form when mafic lava erupts underwater. The lava freezes on the outside against the cold water, but keeps flowing on the inside, bulging out into pillow-like shapes which break off from the main flow. These pillows can accumulate into a large volume of pillow basalts. Pillow basalts serve as an indicator that lava was erupted into a body of water. Much of the world’s oceanic crust consists of basalt, most of which erupted onto the seafloor to form pillow basalts. Pillow basalt can also form when mafic lava erupts on land and flows into a lake or into the ocean. In some places, there are layers of pillow basalt in the Columbia River Basalts, showing that the basalt flowed into lakes, forming pillows as it entered the water.

Some LIP eruptions occur on the sea floor, the largest known being the one that created the Ontong Java plateau in the western Pacific Ocean at around 122 Ma. But most sea-floor volcanism originates at divergent boundaries and involves relatively low-volume eruptions. Under these conditions, hot lava that oozes out into the cold seawater quickly cools on the outside and then behaves a little like toothpaste. The resulting blobs of lava are known as pillows, and they tend to form piles around a sea-floor lava vent (Figure \(\PageIndex{15}\)). In terms of area, there is very likely more pillow basalt on the sea floor than any other type of rock on Earth.

Figure \(\PageIndex{15}\) (Left) Modern sea-floor pillows in the south Pacific. (Right) Ancient sea-floor pillow basalts. Eroded 40 to 50 Ma pillows on the shore of Vancouver Island, near to Sooke. The pillows are 30 to 40 cm in diameter.  (Left): Pillow Basalt Crop by NOAA. Public domain. © Steven Earle. CC BY.

SUPERVOLCANOES

Supervolcano eruptions are extremely rare in Earth history. It’s a good thing because they are unimaginably large. A supervolcano must erupt more than 1,000 cubic km (240 cubic miles) of material, compared with 1.2 km³ for Mount St. Helens or 25 km³ for Mount Pinatubo, a large eruption in the Philippines in 1991. Not surprisingly, supervolcanoes are the most dangerous type of volcano.

Figure 7. The caldera at Santorini in Greece is so large that it can only be seen by satellite.

Supervolcanoes are a fairly new idea in volcanology. The exact cause of supervolcano eruptions is still debated. However, scientists think that a very large magma chamber erupts entirely in one catastrophic explosion. This creates a huge hole or caldera into which the surface collapses (figure 7).

The largest supervolcano in North America is beneath Yellowstone National Park in Wyoming. Yellowstone sits above a hotspot that has erupted catastrophically three times: 2.1 million, 1.3 million, and 640,000 years ago. Yellowstone has produced many smaller (but still enormous) eruptions more recently (figure 8). Fortunately, current activity at Yellowstone is limited to the region’s famous geysers.

Figure 8. The Yellowstone hotspot has produced enormous felsic eruptions. The Yellowstone caldera collapsed in the most recent super eruption.

Long Valley Caldera, south of Mono Lake in California, is the second largest supervolcano in North America (figure 9). Long Valley had an extremely hot and explosive rhyolite about 700,000 years ago. An earthquake swarm in 1980 alerted geologists to the possibility of a future eruption, but the quakes have since calmed down.

Figure 9. The hot water that gives Hot Creek, California, its name is heated by hot rock below Long Valley Caldera.

• This site provides and interactive image of the geological features of Long Valley Caldera.

A supervolcano could change life on Earth as we know it. Ash could block sunlight so much that photosynthesis would be reduced and global temperatures would plummet. Volcanic eruptions could have contributed to some of the mass extinctions in our planet’s history. No one knows when the next super eruption will be.

Interesting volcano videos are seen on National Geographic Videos, Environment Video, Natural Disasters, Earthquakes. One interesting one is “Mammoth Mountain,” which explores Hot Creek and the volcanic area it is a part of in California.

 

Kimberlites

While all of the volcanism discussed so far is thought to originate from partial melting in the upper mantle or within the crust, there is a special class of volcanoes called kimberlites that have their origins much deeper in the mantle, at depths of 150 km to 450 km. During a kimberlite eruption, material from this depth may make its way to surface quickly (hours to days) with little interaction with the surrounding rocks. As a result, kimberlite eruptive material is representative of mantle compositions: it is ultramafic.

Kimberlite eruptions that originate at depths greater than 200 km, within areas beneath old thick crust (shields), traverse the region of stability of diamond in the mantle, and in some cases, bring diamond-bearing material to the surface. All of the diamond deposits on Earth are assumed to have formed in this way; an example is the rich Ekati Mine in the Northwest Territories.

The kimberlites at Ekati erupted between 45 and 60 Ma. Many kimberlites are older, some much older. There have been no kimberlite eruptions in historic times. The youngest known kimberlites are in the Igwisi Hills in Tanzania and are only about 10,000 years old. The next youngest known are dated to about 30 Ma.

How frequently do volcanoes erupt?

The Smithsonian Institution maintains a comprehensive catalogue of the world’s volcanoes, with information and eruptive history for nearly 2700 volcanic sites. If you spend some time looking around that site you’ll discover the frequency of eruptions at different volcanoes is enormously variable, although we can make some generalizations. Focusing just on shield volcanoes and composite volcanoes some of the data are as follows:

Table 4.2 Eruptions of composite and shield volcanoes

Composite volcanoes	Shield volcanoes
Avachinsky (Russia): 5 eruptions over the past 7000 years	Fernandina (Galapagos): 31 eruptions over the past 1000 years
Pinatubo (Philippines): 4 eruptions over the past 9000 years	Kilauea (Hawaii): 62 eruptions over the past 250 years
Adams (Oregon, USA): 6 eruptions over the past 7000 years	Nyamuragira (Congo): 48 eruptions over the past 154 years

Based only on these numbers it is evident that, in general, shield volcanoes are much more active than composite volcanoes, but there are many exceptions to this trend. Some composite volcanoes are nearly as active as the shield volcanoes listed here, and some shield volcanoes that are still considered to be “active” are almost as inactive as the composite volcanoes listed here.

 

Attributions

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

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

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

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

https://geo.libretexts.org/Bookshelv...sics-Volcanoes

https://phys.libretexts.org/Bookshel..._Earth's_Crust