The Earth's Crust
The lithosphere is the solid, outer part of the Earth. The Earth consists of three main layers: the core, or the inner layer; the mantle, in the middle; and the crust, which includes the continents and ocean floor. The lithosphere, which is about 100 kilometers (60 miles) deep in most places, includes the brittle upper portion of the mantle and the crust.
The lithosphere is always moving, but very slowly. It is broken into huge sections called tectonic plates. The extreme heat from the mantle part of the lithosphere makes it easier for the plates to move; this is similar to how iron is bendable once it's heated. The movement of the lithosphere, called plate tectonics, is the reason behind a lot of Earth's most dramatic geologic events. When one plate moves beneath another, or when two plates rub together, they can create earthquakes and volcanoes.