What is the structure of the Earth?

To understand plate tectonics, we must start by understanding the structure of the Earth.

Cross section of the Earth illustrating oceanic and continental crust.

The Earth has a series of distinct layers:

1. Inner Core: The innermost part of the Earth is its inner core. It’s solid, consisting primarily of iron and nickel, and is incredibly hot, with temperatures soaring up to around 5500°C. This heat is due to the decay of radioactive particles and the immense pressure at such depth.
2. Outer Core: Enveloping the inner core is the outer core. Also rich in iron and nickel, this layer is different as it exists in a liquid state. The movement within this liquid layer generates Earth’s magnetic field.
3. Mantle: Positioned above the outer core, the mantle is largely solid but behaves like a viscous fluid over long periods. The upper part of the mantle called the asthenosphere, is softer and can deform, similar to a soft plastic.
4. Crust: Earth’s outer shell is the crust, which is relatively thin compared to the deeper layers. It averages between 15 km to 20 km in thickness, though this can vary. The lithosphere is a rigid layer consisting of the crust and the very top part of the mantle. This lithosphere is fragmented into massive pieces known as tectonic plates. These plates aren’t static; they move.

When discussing tectonic plates, there are some key distinctions:

• Oceanic crust: Primarily found beneath the oceans, the crust has an average thickness of about 5km. Oceanic crust is denser than continental crust and is mainly composed of basalt.
• Continental crust: This crust is considerably thicker, sometimes reaching depths of up to 60km. Continental crust is mainly made up of granitic rock, which is less dense than the basalt found in oceanic crust. Another point of interest is that the continental crust is much older geologically than oceanic crust.

Some tectonic plates possess both oceanic and continental crust. Due to the differences in density, when plates consisting of oceanic and continental crust converge, the denser oceanic plate often gets pushed beneath the continental plate, a process known as subduction.