The Core:
This region is divided into two parts. The outer part is called the Outer Core. It is about 2100km thick and made of liquid nickel and iron. The inner part is called the Inner Core and it is the real centre of the Earth. This part is about 2800km in diameter and is made of solid iron and nickel.
Plate tectonics
Plate tectonics are a reasonably new theory that has revolutionized the way geologists thinks about the Earth. According to the theory, the surface of the Earth is broken into large plates. The size and position of these plates change over time. The edges of these plates, where they move against each other, this causes earthquakes, volcanoes, and mountain building. Plate tectonics is a combination of two earlier ideas, continental drift and sea-floor spreading.
The Earth is made up of several of these different layers. The outer layer is called the Crust. It is made of the lighter, less dense, rocks that floated to the surface when the Earth was formed. The deepest mine in the world is only 3.3km deep, and nobody has ever been able to drill down further than 15km. Everything that exists below this is worked out by scientists using evidence from earthquakes, volcanoes and meteorites. Being relatively cold, the crust is rocky and brittle, so it can fracture during earthquakes. The crust is not a continuous layer of rocks but consists of large masses, called plates, which are free to drift slowly across the surface of the planet. There are two types of crust - Continental and Oceanic. Continental crust is less dense, much older and much thicker than Oceanic crust. Continental crust floats on the mantle rather like an iceberg floats in water.
There is a balance that exists between the level of the crust and the mantle. If the crust builds up in one area, it will push down on the mantle with more force causing some of the material to move away... this results in another area of the crust being pushed up by the material from that mantle that has been moved away If material is moved off the crust, the mantle pushes up harder causing that area of land to rise and another area (where the material of the mantle came from) to sink. This process goes on constantly and always maintains a balance of downward forces from the crust with the upward forces from the mantle.
The proof that they think the earth is made of plates is shown on the picture below.
