Anatomy of the Earth

 

Earth’s surface is an amazing place to behold. Yet even the deepest canyon is but a tiny scratch on the planet. To really understand Earth, you need to travel 6,400 kilometres (3,977 miles) beneath our feet. Starting at the centre, Earth is composed of four distinct layers. They are, from deepest to shallowest, the inner core, the outer core, the mantle and the crust. Except for the crust, no one has ever explored these layers in person. In fact, the deepest humans have ever drilled is just over 12 kilometres (7.6 miles).

Here’s a primer on Earth’s layers, starting with a journey to the centre of the planet.

1. The inner core

This solid metal ball has a radius of 1,220 kilometres (758 miles), or about three-quarters that of the moon. It’s located some 6,400 to 5,180 kilometres (4,000 to 3,220 miles) beneath Earth’s surface. Extremely dense, it’s made mostly of iron and nickel. The inner core spins a bit faster than the rest of the planet. It’s also intensely hot: Temperatures sizzle at 5,400° Celsius (9,800° Fahrenheit). That’s almost as hot as the surface of the sun. Pressures here are immense: well over 3 million times greater than on Earth’s surface. Some research suggests there may also be an inner, inner core. It would likely consist almost entirely of iron.

2. The outer core

This part of the core is also made from iron and nickel, just in liquid form. It sits some 5,180 to 2,880 kilometres (3,220 to 1,790 miles) below the surface. Heated largely by the radioactive decay of the elements uranium and thorium, this liquid churns in huge, turbulent currents. That motion generates electrical currents. They, in turn, generate Earth’s magnetic field. For reasons somehow related to the outer core, Earth’s magnetic field reverses about every 200,000 to 300,000 years.

3. The mantle

At close to 3,000 kilometres (1,865 miles) thick, this is Earth’s thickest layer. It starts a mere 30 kilometres (18.6 miles) beneath the surface. Made mostly of iron, magnesium and silicon, it is dense, hot and semi-solid. Like the layer below it, this one also circulates. It just does so far more slowly.

Near its upper edges, somewhere between about 100 and 200 kilometres (62 to 124 miles) underground, the mantle’s temperature reaches the melting point of rock. Indeed, it forms a layer of partially melted rock known as the asthenosphere.

Diamonds are tiny pieces of the mantle we can actually touch. Most form at depths above 200 kilometres (124 miles). But rare “super-deep” diamonds may have formed as far down as 700 kilometres (435 miles) below the surface. These crystals are then brought to the surface in volcanic rock known as kimberlite.

The mantle’s outermost zone is relatively cool and rigid. It behaves more like the crust above it. Together, this uppermost part of the mantle layer and the crust are known as the lithosphere.

4. The crust

Earth’s crust is like the shell of a hard-boiled egg. It is extremely thin, cold and brittle compared to what lies below it. The crust is made of relatively light elements, especially silica, aluminium and oxygen. It’s also highly variable in its thickness. Under the oceans (and Hawaiian Islands), it may be as little as 5 kilometres (3.1 miles) thick. Beneath the continents, the crust may be 30 to 70 kilometres (18.6 to 43.5 miles) thick.

Along with the upper zone of the mantle, the crust is broken into big pieces, like a gigantic jigsaw puzzle. These are known as tectonic plates. These move slowly—at just 3 to 5 centimetres (1.2 to 2 inches) per year. What drives the motion of tectonic plates is still not fully understood.