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The Crust

It will take less than a minute to fall through the Earth’s crust – its outermost hard shell made of the lighter rocks. The skin of our planet is only 35km thick and a mere 1 per cent of the Earth’s mass. The crust is composed of two types. The continental crust is made of the lightest rock such as granite and it forms the continents that overlay denser rocks. The continents have the oldest rocks found on the surface of the Earth and the record holder is a greenish, finely grained rock 4.4 billion years old and found on the shore of Hudson Bay, Canada. These rocks solidified soon after the Moon was formed, and if you were there at that time you could have looked up from them and seen active volcanoes glowing on the Moon. Studies of the earliest rocks indicate that the early Earth was not hot and hellish for very long but cooled swiftly and had extensive oceans almost from the start. Everything mankind has ever built is made from metals and minerals mined from the crust with the exception of diamonds – they come to us from much deeper.

The so-called oceanic crust is much younger than the continents and is constantly being formed at mid-ocean ridges like that seen bisecting the Atlantic, from where it spreads as slabs of moving basalt. This is the driving force behind the movements of the continents. On the sea floor these slabs often gather sediments that can pile up in a thick layer. But its life at the Earth’s surface is a relatively brief one, no more than 200 million years, for these rocks so recently solidified from the molten Earth below will soon rejoin it, bringing cold from the surface, sediments and great changes. Being heavier than continental crust, oceanic sea-floor slabs are pushed back into the Earth under the continents in a process called subduction.

The Mantle

Like you, the slab is heading through the crust into the mantle – the largest region of our planet, comprising about 82 per cent of its volume and 65 per cent of its mass. The mantle is where the archaeology of our planet is stored as well as newly recognised ancient structures and processes that may be necessary for life to exist on the surface. Indeed, scientists are beginning to realise that the conditions for life are created as much from below as they are by the Sun above.

As the slab descends it comes under extreme pressure and, being colder than the rock it is plunging into, it is brittle. Consequently it fractures and slips, causing the most powerful earthquakes we ever experience. Japan – the world’s most earthquake-prone nation – sits above where the Pacific oceanic plate is subducting beneath the Asian continental crust. As the slab gets deeper the temperature and pressure increase to levels that it cannot withstand. It is heated so that the rock flows like plastic, meaning that below a few hundred km the earthquakes cease. Minerals containing water break down and release fluid. The water seeps out and rises and reaches the mantle rocks above it. There it causes the melting temperature of those rocks to lower, sometimes by as much as 400 degrees. This rock, now much less viscous, makes its way to the surface. That is why a hundred or so kilometres beyond a subduction zone there are arcs of new volcanoes.

The mantle – and this is generally true of the entire Earth – is mainly composed of four elements: oxygen, silicon, magnesium and iron. Their atoms link to form latticework structures that yield under pressure to form ever-closer arrangements. For many years only the mantle’s upper section was considered interesting. Below that the increasing pressure and heat were supposed to have squeezed the rocks so mightily as to erase any structures. The result would be a uniform mass of rock reaching downwards until the dramatic boundary with the outer core is reached.

We now know that the mantle is a more active place than we had thought. Although it’s made of solid rock, it does move slowly with material descending to and rising from its deepest layers. Some scientists believe there are plumes of rock making their way through the mantle heading for the surface and that every few hundred million years vast amounts of lava are disgorged at the surface causing mass extinctions.

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