Structure of the Earth

When studying plate tectonics, the best starting point is examining the structure of the Earth. The Earth is divided into a series of layers: the inner core, outer core, mantle and crust.  Each of these layers has its own chemical and mechanical properties.

Cross section of the Earth illustrating oceanic and continental crust.

The outermost layer of the Earth is the crust. The crust is the thinest layer.  There are two types of crust: oceanic and continental.

Continental crust, varying in thickness from 20 to 200 kilometres, is not as dense as its oceanic counterpart. Primarily composed of granite, this crust is indestructible and significantly older than oceanic crust, with an age reaching up to 3.8 billion years.

In contrast, the oceanic crust, with a thickness ranging from 5 to 10 kilometres, consists predominantly of dense basalt. When oceanic crust converges with continental crust at a destructive plate boundary, it descends into the mantle. This is known as subduction. Notably, the oceanic crust is much younger than the continental crust, being less than 200 million years old.

The Structure of the Earth

The crust and the upper mantle form the lithosphere. The lithosphere is around 100km thick and is solid and brittle.

The lithosphere is divided into large sections called tectonic plates. Tectonic plates consist of sections of the lithosphere. The oceanic lithosphere is topped by oceanic crust, and the continental lithosphere is topped by continental crust.

The mantle is not made up of liquid magma. This is a common misconception. The mantle is made up of solid rock.

The section of the Earth’s mantle beneath the lithosphere, extending from a depth of approximately 100 kilometres to 400 kilometres, is known as the asthenosphere.  The lithosphere and asthenosphere have a similar chemical composition. However, they have different mechanical properties.  The lithosphere is brittle, whereas the asthenosphere is solid but ductile (plastic-like, it will flow over a very long timescale). In the asthenosphere, as depth increases, so does the temperature, reaching at least 1300°C. This temperature is sufficient to melt peridotite (the material that makes up the mantle) under surface conditions. However, the high pressure at such depths prevents the peridotite from melting. As a result, it stays solid yet is ductile, allowing it to flow.

The lower mantle is still ductile but denser due to increased pressure.

The outer core is made up of liquid iron and nickel. The inner core, a solid mass over iron and nickel, forms the Earth’s centre. The temperature of the core is around 5500°C.