Lesson #13 of 18 in the Brain Makeover collaboration with Professor James Trefil/GMU, the 76ers Cheerleaders and the Science Cheerleader. See Brain Makeover Series.
The surface of the earth is constantly changing.

The Earth can be thought of as being separated into three layers. The core, at the center, consists of heavy materials like iron and nickel. At the very center the core is solid, but farther out it is liquid. The next layer is the mantle, composed of heavy minerals, and the outermost layer is the crust. The surface of the Earth is separated into tectonic plates, some 30-50 miles thick. These plates move around in response to convection in the Earth’s mantle. The continent are the uppermost layer of the tectonic plates. The constant motion of the plates causes a constant change in the surface features of the planet. Only the Earth among  planets in the solar system has this kind of variability in its surface.
Where plates are moving away from each other, hot magma from the mantle comes to the surface to form mountain chains and deep sea vents. Where plates are moving together, one plate will slip beneath the other, forming mountain chains or deep ocean trenches, depending on whether or not there is a continent on the plate. If plates slide by each, as they do in the San Andreas Fault, their motion will cause frequent earthquakes.

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