The topmost layer is the lithosphere, which is comprised of the crust and solid portion of the upper mantle. The lithosphere is divided into many plates that move in relation to each other due to tectonic forces. The lithosphere essentially floats atop a semi-liquid layer known as the asthenosphere. This layer allows the solid lithosphere to move around since the asthenosphere is much weaker than the lithosphere.
Everything in Earth's system can be placed into one of four major subsystems: land, water, living things, or air. These four subsystems are called spheres. They are the lithosphere, hydrosphere, biosphere, and atmosphere. Each of these four spheres can be further divided into sub-spheres. To keep things simple in this course, there will be no distinction among the sub-spheres of any of the four major spheres. The lithosphere contains all of the cold, hard solid land of the planet's crust, the semi-solid land underneath the crust, and the liquid land near the center of the planet. The surface of the lithosphere is very uneven. The solid, semi-solid, and liquid land of the lithosphere form layers that are physically and chemically different. If someone were to cut through Earth to its center, these layers would be revealed like the layers of an onion. The outermost layer of the lithosphere consists of loose soil rich in nutrients, oxygen, and silicon. Beneath that layer lies a very thin, solid crust of oxygen and silicon. Next is a thick, semi-solid mantle of oxygen, silicon, iron, and magnesium. Below that is a liquid outer core of nickel and iron. At the center of Earth is a solid inner core of nickel and iron. The hydrosphere contains all the solid, liquid, and gaseous water of the planet. It ranges from 10 to 20 kilometers in thickness. The hydrosphere extends from Earth's surface downward several kilometers into the lithosphere and upward about 12 kilometers into the atmosphere.
