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The layers of the Earth

- With Chris Febvre NEM GRAPHIC ARTIST cfebvre@nemedia.com.au

OGRES are like onions, and so is the Earth.

We rarely contemplat­e it, but beneath our feet are thousands of kilometres of solid and molten rock, metal and minerals.

This week, we’ll take a look at the complex layers that comprise the planet we call home.

The inner core

This solid metal ball has a radius of 1,220 kilometres, or about three-quarters that of the moon. It’s located some 6,400 to 5,180 kilometres 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 5,400º Celsius.

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.

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 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.

The mantle

At close to 3,000 kilometres thick, this is Earth’s thickest layer.

It starts a mere 30 kilometers beneath the surface.

Made mostly of iron, magnesium and silicon, it is dense, hot and semi-solid (think caramel candy).

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 kilometers undergroun­d, the mantle’s temperatur­e reaches the melting point of rock.

Indeed, it forms a layer of partially melted rock known as the asthenosph­ere.

Geologists believe this weak, hot, slippery part of the mantle is what Earth’s tectonic plates ride upon and slide across.

Diamonds are tiny pieces of the mantle we can actually touch.

Most form at depths above 200 kilometers. But rare “super-deep” diamonds may have formed as far down as 700 kilometers 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.

The crust

Earth’s crust is like the shell of a hardboiled egg.

It is extremely thin, cold and brittle compared to what lies below it.

The crust is made of relatively light elements, especially silica, aluminum and oxygen.

It’s also highly variable in its thickness. Under the oceans (and Hawaiian Islands), it may be as little as 5 kilometers thick.

Beneath the continents, the crust may be 30 to 70 kilometers 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 centimeter­s (1.2 to 2 inches) per year.

What drives the motion of tectonic plates is still not fully understood.

It may be related to heat-driven convection currents in the mantle below.

Some scientists think it’s caused by the tug from slabs of crust of different densities, something called “slab pull.”

In time, these plates will converge, pull apart or slide past each other. Those actions cause most earthquake­s and volcanoes.

It’s a slow ride, but it makes for exciting times here on Earth’s surface.

 ?? ?? UNDER THE SURFACE: The Earth is composed, broadly speaking, of four distinct layers - the solid inner core, liquid outer core, the mantle - where diamonds are formed, and the crust.
UNDER THE SURFACE: The Earth is composed, broadly speaking, of four distinct layers - the solid inner core, liquid outer core, the mantle - where diamonds are formed, and the crust.
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