Structure of the Earth

When studying plate tectonics, the best starting point is examining the structure of the Earth. The Earth is very similar to a peach in its structure. In the centre is a solid core. Surrounding the core is the inner core, then the lithosphere, the rigid outer layer comprising the crust and the upper mantle.

Figure 1. Cross-section of the earth (source: Wikipedia)

The inner core is the centre of the Earth and is the hottest part of the earth. It is a solid mass of iron and nickel. The temperature of the core is around 5500°C.

The outer core is the layer around the inner core. It is also made up of iron and nickel though it is liquid.

The next layer is the mantle. The mantle surrounding the core is a solid material that can flow very slowly. The upper portion of the mantle is a weak layer called the asthenosphere, which can deform like plastic.

The final layer is the Earth’s crust which is very thin compared to the thickness of the mantle and core. This layer is, on average, between around 15 km (9 mi) to 20 km (12 mi). The rigid shell that forms the crust is called the lithosphere. The lithosphere is broken into large pieces called tectonic plates. These large pieces move slowly over the upper mantle.

Tectonic plates can be described as oceanic, continental, and some consist of both oceanic and continental parts. Oceanic crust is, on average, around 5km thick, whereas continental crust is thicker, reaching 60km in depth. Continental crust is lighter (less dense) than the oceanic crust. Oceanic crust is much younger in geologic age than continental crust. Continental crust is, on average thicker than oceanic crust.