In terms of the composition of the Earth, there are only 3 layers.
The innermost layer is known as the Core. Its composition is about 95% Fe (iron) and about 5% Ni (nickel).
How do we know that? Partly from the fact that one of the most common meteorite types are "iron-nickel" meteorites (you can see some of them in the lobby to the X building). Partly from the fact that the density is correct and that the Earth has a magnetic field.
The next layer is the Mantle. The Mantle is composed of an igneous rock composed of silicate minerals called Peridotite. These particular minerals happen to have comparatively small amounts of silica in them, so this rock is called a "very low silica, silicate rock" (another name for that composition is "ultramafic"). Pieces of this break off and come up in volcanic eruptions, and are also a common kind of meteorite, called a "stony" meteorite.
The last layer, the Crust, is the most variable in composition, but on average, it is a "low to high silica, silicate rock". As you might guess from the drawing, it varies in thickness as well. That variation will be discussed soon.
(NOT TO SCALE)
Draw this yourself. Click here to download some blank drawings you can practice on. Label each with name of the layer, compositions or physical properties, and where given, depths.
(Once you have finished understanding this page, click on your "back" button to return to the main Plate Tectonics page.)
The physical structure of the Earth is somewhat more complicated. For starters, we know that temperature rises as we go deeper into the Earth. At its center, the Earth's temperature is about 6000oC, about the same as the surface of the Sun.
The Core is divided into two parts, an Inner Core and an Outer Core. The difference is that one is liquid and the other is solid. From temperature considerations, you might think the hotter inner core would be liquid, but remember that the pressure also rises as we go deeper. And the pressure on the Inner Core is so high that even at these incredibly high temperatures, the atoms of Fe and Ni are forced into a rigid structural framework: a solid. As we go outwards, the temperature decreases, but so does the pressure, and the Core's atoms are able to break free and become a liquid in the Outer Core. [Currents in this liquid iron are what generate the Earth's magnetic field.]
When we get to the Mantle, it turns out that this rock has a higher melting temperature than Iron and Nickel, so we enter the solid Mesosphere ("middle sphere") in the lowest part of the Mantle (light orange color). This part of the mantle is rigid - it does not tend to deform in any way.
As we rise through the Mantle, pressure decreases. You might think (considering what happened to the Core) that the Mantle would melt. That doesn't QUITE happen. But as the pressure decreases, the Mantle softens and (over long periods of time) attains the ability to flow. A solid material that can flow slowly like this is called "plastic" or "ductile", like silly putty. This transition occurs at a depth of about 700 km. This layer that can flow over millions of years, is known as the Asthenosphere ("weak sphere") (darker orange color). This ability to flow is indicated by the circular arrows.
As we continue to rise through the Mantle, the temperature gets low enough that the mantle again becomes rigid and unable to flow. This uppermost Mantle (light brownish-grey) locks to the Crust, and together becomes the Lithosphere ("rocky sphere"). The base of the Lithosphere varies from about 100 km under the oceans to about 200 km under the continents.
This is the "plate" in Plate Tectonics.