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Six Things to Know About the Earth's Mantle

The rocks and continents of the world belong to the Earth's crust. Beneath that crust is the Earth's mantle.

The mantle is the thick layer of hot, solid rock between the crust and the molten iron core. The mantle makes up the bulk of the Earth, accounting for two-thirds of its 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 of these six items 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.

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 erup

tions, from as deep as about 300 kilometers. 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 uppermost 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 commotion 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, but not the centers of eruptive action called hotspots. Hotspots may be a clue to the rise and fall of material much deeper in the mantle, even from its very bottom. Or they may not. There is a vigorousscientific discussion about hotspots these days.

3. Exploring the Mantle with Earthquake Waves

Our most powerful tool for exploring the mantle is to monitor 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 makesome impressive maps of the mantle.

4. Modeling the Mantle in the Laboratory

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 olivinechanges to different crystal forms at depths of around 410 and 660 kilometers.

We study the behavior of minerals under conditions in the mantle with two methods: calculations based on the equations of mineral physics and laboratory experiments. Thus 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

Over the last century, seismic data 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 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 chemically different. 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 learnedabout 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.

The Earth Core

A century ago, science barely knew that the Earth even has a core. Today we are tantalized by the core and its connections with the rest of the planet. Indeed, we're at the start of a golden age of core studies.

The Core's Gross Shape

We knew by the 1890s, from the Earth's response to the gravity of the Sun and Moon, that the planet has a dense core, probably iron. In 1906 Richard Dixon Oldham found that earthquake waves move through the Earth's center much slower than through the mantle around it—because the center is liquid.

In 1936 Inge Lehmann reported that something reflects seismic waves from within the core. It became clear that the core consists of a thick shell of liquid iron—the outer core—with a smaller, solid inner core at its center. It's solid because at that depth the high pressure overcomes the effect of high temperature.

In 2002 Miaki Ishii and Adam Dziewonski of Harvard University published evidence of an "innermost inner core" some 600 kilometers across. In 2008 Xiadong Song and Xinlei Sunproposed a different inner inner core about 1200 km across (here's a condensed version in scientific jargon). Not much can be made of these ideas until others confirm the work.

Whatever we learn raises new questions. The liquid iron must be the source of Earth's geomagnetic field—the geodynamo—but how does it work? Why does the geodynamo flip, switching magnetic north and south, over geologic time? What happens at the top of the core, where molten metal meets the rocky mantle? Answers began to emerge during the 1990s.

Studying the Core

Our main tool for core research has been earthquake waves, especially those from large events like the 2004 Sumatra quake. The ringing "normal modes," which make the planet pulsate with the sort of motions you see in a large soap bubble, are useful for examining large-scale deep structure.

But a big problem is nonuniqueness—any given piece of seismic evidence can be interpreted more than one way. A wave that penetrates the core also traverses the crust at least once and the mantle at least twice, so a feature in a seismogram may originate in several possible places. Many different pieces of data must be cross-checked.

The barrier of nonuniqueness faded some as we began to simulate the deep Earth in computers with realistic numbers, and as we reproduced high temperatures and pressures in the laboratory with the diamond-anvil cell. These tools have let us peer through the layers of the Earth until at last we can contemplate the core.

What the Core Is Made Of

Considering that the whole Earth on average consists of the same mixture of stuff we see elsewhere in the solar system, the core must be iron metal along with some nickel. But it's less dense than pure iron, so about 10 percent of the core must be something lighter. Sulfur and oxygen are the two best candidates, though hydrogen has also been considered. But the money is still on one of the other two, with oxygen looking slightly more probable, or maybe both.

Seismologists continue to probe the inner core. The core's eastern hemisphere appears to differ from the western hemisphere in the way the iron crystals are aligned. The problem is hard to attack because seismic waves have to go pretty much straight from an earthquake, right through the Earth's center, to a seismograph. Events and machines that happen to be lined up just right are rare. And the effects are subtle.

Core Dynamics

In 1996, Xiadong Song and Paul Richards confirmed a prediction that the inner core rotates slightly faster than the rest of the Earth. The magnetic forces of the geodynamo seem to be responsible.

Over geologic time, the inner core grows as the whole Earth cools. Iron crystals freeze out at the top of the outer core and rain onto the inner core. At the base of the outer core, the iron freezes under pressure taking much of the nickel with it. The remaining liquid iron is lighter and rises. These rising and falling motions, interacting with geomagnetic forces, stir the whole outer core at a speed of 20 kilometers a year or so.

The planet Mercury also has a large iron core and a magnetic field, though much weaker than Earth's. Recent research hints that Mercury's core is rich in sulfur and that a similar freezing process stirs it, with "iron snow" falling and sulfur-enriched liquid rising.

Core studies surged in 1996 when computer models by Gary Glatzmaier and Paul Roberts first reproduced the behavior of the geodynamo, including spontaneous reversals. Hollywood gave Glatzmaier an unexpected audience when it used his animations in the action movie The Core.

Recent high-pressure lab work by Raymond Jeanloz, Ho-Kwang (David) Mao and others has given us hints about the core-mantle boundary, where liquid iron interacts with silicate rock. The experiments show that core and mantle materials undergo strong chemical reactions. This is the region where many think mantle plumes originate, rising to form places like the Hawaiian Islands chain, Yellowstone, Iceland, and other surface features. The more we learn about the core, the closer it becomes.