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Six Things to Know About the Earth's Mantle

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The mantle is the thick layer of hot, solid rock between the Earth's crust and the molten iron core. It makes up the bulk of the Earth, accounting for two-thirds of the planet's mass. The mantle starts about 30 kilometers down and is about 2900 kilometers thick.

Let's take a look at six different aspects of the mantle. Each item links to an article with more detail.

1. Samples from the Mantle

Earth has the same recipe of elements as the Sun and the other planets (ignoring hydrogen and helium, which have escaped Earth's gravity). Subtracting the iron in the core, we can calculate that the mantle is a mix of magnesium, silicon, iron, and oxygen that roughly matches the composition of garnet.

But exactly what mix of minerals is present at a given depth is an intricate question that is not firmly settled. It helps that we have samples from the mantle, chunks of rock carried up in certain volcanic eruptions, from as deep as about 300 kilometers and sometimes much deeper. These show that the uppermost part of the mantle consists of the rock types peridotite and eclogite. But the most exciting thing we get from the mantle is diamonds.

2. Activity in the Mantle

The top part of the mantle is slowly stirred by the plate motions going on above it. The main activities are the downward motion of subducting plates and the upward motion of mantle rock at spreading centers. All this convection does not mix the upper mantle thoroughly, however, and geochemists think of the upper mantle as a rocky version of marble cake.

The world's patterns of volcanism faithfully reflect plate tectonics, except for the centers of eruptive action called hotspots. Hotspots may be a clue to the rise and fall of material much deeper in the mantle, possibly from its very bottom. Or they may not. There is a vigorous scientific discussion about hotspots these days.

3. Exploring the Mantle with Earthquake Waves

Our most powerful technique for exploring the mantle is monitoring seismic waves from the world's earthquakes. The two different kinds of seismic wave, P waves (analogous to sound waves) and S waves (like the waves in a shaken rope), respond to the physical properties of the rocks they go through. Like light waves, they reflect off density boundaries and refract in rocks of different density. We use these effects to map the Earth's insides.

Our tools are good enough to treat the Earth's mantle the way doctors make ultrasound pictures of their patients. After a century of collecting earthquakes, we're able to make some impressive maps of the mantle.

4. Modeling the Mantle in the Lab

With the human body, ultrasound images are just shadows unless we have hands-on knowledge of what is beneath the skin. The same is true of seismic mantle maps. Minerals and rocks change under high pressure. For instance, the common mantle mineral olivine changes to different crystal forms at depths around 410 kilometers and again at 660 kilometers.

We study the behavior of minerals under mantle conditions with two methods: computer models based on the equations of mineral physics and laboratory experiments. Thus modern mantle studies are a three-way conversation of seismologists, computer programmers and lab researchers who can now reproduce conditions anywhere in the mantle with high-pressure laboratory equipment like the diamond-anvil cell.

5. The Mantle's Layers and Internal Boundaries

A century of research has let us fill some of the blanks in the mantle. It has three main layers. The upper mantle extends from the base of the crust (the Moho) down to 660 kilometers depth. Many workers distinguish the transition zone between 410 and 660 kilometers, two depths at which major physical changes occur to minerals.

The lower mantle extends from 660 down to about 2700 kilometers, a point where seismic waves are affected so strongly that most researchers believe the rocks beneath are different in their chemistry, not just in their crystallography. This controversial layer at the bottom of the mantle, about 200 kilometers thick, has the odd name "D-double-prime." Read more of what we've learned about these layers and the crucial boundaries between them.

6. Why Earth's Mantle Is Special

Because the mantle is the bulk of the Earth, its story is fundamental to geology. The mantle began, during Earth's birth, as an ocean of magma atop the iron core. As it solidified, elements that didn't fit into the major minerals collected as a scum on top—the crust. After that the mantle began the slow circulation it has had for the last 4 billion years, with at least the upper part being cooled, stirred and hydrated by the tectonic motions of the surface plates.

At the same time, we have learned a great deal about the structure of Earth's sister planets Mercury, Venus and Mars. Compared to them, Earth has an active, lubricated mantle that is very special thanks to the same ingredient that distinguishes its surface: water.

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