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5.15 Geological Processes

Terrestrial surfaces are shaped through 3 basic factors: impacts with asteroids and comets, erosion from liquid (water on earth) and wind, and geologic processes. While impact events are perhaps the most dramatic, it is the geologic process that on active worlds do the most to shape the large scale features of a moon or planet. The driving forces for all such processes, including volcanism and plate tectonics, are gravity and energy released from the decay of radioactive elements (and to a lesser degree the energy left over in the form of heat from a planet's formation). Heat flows from hotter to cooler regions. So the energy from a planet's core must move outwards and eventually be radiated into space. Some of this heat is carried out by conduction within solid rock. However, there is too much heat to be transferred by conduction alone. Convection also carries heat from the hot interior toward the cooler surface. This form of thermal transfer is exactly what is seen in lava lamps, and in heated oil. Due to convection, huge, sluggish masses of hot rock in a molten or plastic state float slowly upward until they hit the bottom of the lithosphere. Then they spread laterally, radiate their heat to the lithosphere, and eventually sink again. In the Earth's viscous mantle, this rolling cycle takes about 200 million years. (Compare this to the few-minute cycle of fluid in a lava lamp!)

 


Convection processes bring mantle material toward the lithosphere where it dissipates heat and then sinks back toward the core. Click here for original source URL
 

The churning of the semi-liquid layers underneath the lithosphere creates tremendous stresses on the brittle rock layers of the lithosphere. Depending on the nature of the stresses, the lithosphere may compress or it may stretch. If the stresses are compressive, folding and buckling of the lithosphere may create mountain ranges, like the Earth's Rockies or Himalayas. A traveler through such ranges can see the huge curved folds in the rock layers. If the stresses stretch the lithosphere, it may thin like stretched taffy or even pull apart in a series of fractures called faults. An example of this occurs along Baja California and the California coast, where stretching forces have pulled Baja California away from the mainland in the last few million years. Similarly, Saudi Arabia is being pulled away from Africa. Stretching also causes fractures like the San Andreas fault in California and the Alpine Fault in New Zealand.

If compressive or stretching stresses become too great, the lithosphere will be deformed beyond its limit, like taffy or putty pulled to fast or too hard. As the lithosphere fractures, it sends off seismic waves that travel through the crust and mantle. This is the phenomenon we perceive as an earthquake. The faults are usually underground, but sometimes they can be seen at the surface. Once an earthquake occurs, it will relieve the stresses, but if the sluggish convection currents continue, the forces will build up again. This is why a given fault line can have continued sporadic earthquakes for hundreds of years.

Under certain conditions, such as along diverging plates, fractures may allow molten lava to gain access toward the surface from the interior. This results in volcanism, including hot springs and geysers such as are seen in Yellowstone Park in the U.S., and even the eruption of molten lava onto the surface. (Usually this material is called magma while it is underground and lava once it reaches the surface.) This explains why volcanoes are often concentrated in regions of fractures and earthquakes, where the lithosphere is being deformed most rapidly by underlying mantle currents. Alternatively, volcanoes can occur where the lithosphere happens to be thin and the magma can penetrate through. The volcanoes of the Hawaiian Island chain and the Indonesian Island chain were created when thin lithosphere plates moved over a hot spot in the mantle, and the magma punched a series of holes through the thin crust. The volcanoes seen on Jupiter's moon Io, and the dormant volcanoes on Mars are likely hot spot volcanoes.

Geological processes occur all over the planet Earth. All of the continents have mountain ranges, and mild earthquakes can be felt even in unlikely places like England and Kansas. However, the big picture of geological activity is given by the dozen or so lithosphere plates that fit across the Earth's surface like the pieces of a jigsaw puzzle. Most of the world's earthquakes and volcanoes occur where these plates meet. Mountain ranges are found where these plates collide (for example, the Andes and the Himalayas) or separate (the mid-Atlantic range). Unfortunately, nearly a billion people live near the "ring of fire" that traces the plate boundaries at the rim of the Pacific Ocean. Radio astronomy has been used to directly measure the motion of continents. Europe and North America are separating at a rate of about 5 centimeters per year. About 150-200 million years ago they were together, and all the continents formed a primeval landmass called Pangaea.


The San Andreas fault, caused by stretching in the lithosphere. Click here for original source URL.


The Himalayas, caused by compression of land mass in the lithosphere. Click here for original source URL.

All the processes that govern the geology of the Earth — gravity, internal heating, volcanism — are encountered with other planets and their satellites. The laws that govern how rocky worlds behave apply everywhere in the universe. Earth is just one of the worlds we can find in space. Those other worlds have their own rich geology and fascinating landscapes to explore. Just as gravity and energy transport cause geological processes, they also place limits on them. The heights of mountains are limited by gravity, with larger mountains occurring on Mars, where the gravity is less, than we find on Earth. We also find more active volcanoes on Io, where the heat transport is greater, than we find here, on the less energetic Earth.

Earth's surface remains in a process of constant renewal. All of the major geological processes – the moving of continents, the churning of the mantle, the raising and erosion of mountains — take place on a time-scale of 100 to 200 million years. This amounts to only a few percent of the age of the Earth. It is hard to find surface features that are more about 800 million years old. This is why the search for the oldest rocks on Earth was a major piece of detective work. In order to sample the conditions of the early Solar System we must instead look for unweathered rocks from space — asteroids turned meteorites — that survive their journey to the surface of the Earth.