Chapter 5: The Earth-Moon System

Oceanic plates are subducted creating oceanic trenches.

Terrestrial surfaces are shaped by three 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 does 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!)

Galileo spacecraft image of Io. The dark spot just left of center is the erupting volcano Prometheus. Whitish plains on either side of it are coated with volcanically emplaced sulfur dioxide frost, while yellower regions are encrusted with a higher proportion of sulfur.

Under certain conditions, such as along diverging plates, fractures may allow molten lava to gain access to 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 on Jupiter's moon Io and the dormant volcanoes on Mars are likely hot spot volcanoes.

The location of the Ring of Fire.

All the processes that govern the geology of the Earth — gravity, internal heating, volcanism — are encountered by 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. 

Simplified animation of the break-up of Pangaea and the formation of modern continents.

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 than 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.