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, made up of 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 in liquid form.

The next layer is the mantle. The mantle that surrounds the core is made of 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 oceanic crust. Oceanic crust is much younger in geologic age than continental crust. Continental crust is, on average thicker than oceanic crust.

Cross-section of the Earth illustrating oceanic and continental crust.

Characteristics of oceanic crust:

• Thin, 5-10km thick
• Dense
• Formed from basaltic rock
• Sinks when it meets a continental plate
• Recycled at destructive margins
• Young, usually less than 200 million years old

Characteristics of continental crust:

• Thick, 20-60km
• Less dense
• Made up mainly of granite
• Is not destroyed
• Old, up to 3.8 billion years old