Download The Earth`s Crust

The Earth's Crust
An introduction
By Andrew Alden
The Earth's crust is an extremely thin layer of rock, like the skin of an apple in relative terms. It amounts to less than half of 1 percent
of the planet. But the crust is exceptionally important, and not just because we live on it.
The crust can be thicker than 80 kilometers in some spots, less than one kilometer in others. Underneath it is the mantle, a layer of rock
some 2700 kilometers thick that accounts for the bulk of the Earth. The crust is primarily made of granite and basalt while the mantle
beneath is made of peridotite. More about all that below.
How We Know the Earth Has a Crust
Just a century ago, we didn't know the Earth has a crust. Until the 1900s all we knew was that our planet wobbles in relation to the sky
as if it had alarge, dense core. Astronomical observations told us so. Then along came seismology, which brought us a new type of
evidence from below: seismic velocity, or the speed of sound in rock as measured using seismic waves from earthquakes.
In 1909 a paper by the seismologist Andrija Mohorovicic established that about 50 kilometers deep in the Earth there is a sudden
change in seismic velocity—a discontinuity of some sort. Seismic waves bounce off it (reflect) and bend (refract) as they go through it,
the same way that light behaves at the discontinuity between water and air. That discontinuity, named the Mohorovicic discontinuity or
"Moho," is the accepted boundary between the crust and mantle.
Crusts and Plates
The crust is not the same thing as the plates of plate tectonics. Plates are thicker than the crust and consist of the crust and the shallow
mantle just beneath it; the two-layered combination is stiff and brittle and is called the lithosphere ("stony layer" in scientific Latin). The
lithospheric plates lie on a layer of softer, more plastic mantle rock (the asthenosphere or "weak layer") that allows the plates to move
slowly over it like a raft in thick mud.
We know that the Earth's outer layer is made of two grand categories of rocks: basaltic and granitic. Basaltic rocks underlie the
seafloors and granitic rocks make up the continents. We know that the seismic velocities of these rock types, as measured in the lab,
match those seen in the crust down as far as the Moho, so we're pretty sure that the Moho marks a real change in rock chemistry. The
Moho isn't a perfect boundary, because some crustal rocks and mantle rocks can masquerade as the other, but even so everyone who
talks about the crust, whether in seismological or petrological terms, fortunately means the same thing.
In general, then, there are two kinds of crust, oceanic crust (basaltic) and continental crust (granitic).
Oceanic Crust
Oceanic crust covers about 60 percent of the Earth's surface. Oceanic crust is thin and young—no more than about 20 km thick and no
older than about 180 million years. Everything older has been pulled underneath the continents by subduction. Oceanic crust is born at
the midocean ridges, where plates are pulled apart. As that happens, the pressure upon the underlying mantle is released and
the peridotite there responds by starting to melt. The fraction that melts becomes basaltic lava, which rises and erupts while the
remaining peridotite becomes depleted.
The midocean ridges migrate over the Earth like Roombas, extracting this basaltic component from the peridotite of the mantle as they
go. This works like a chemical refining process. Basaltic rocks contain more silicon and aluminum than the peridotite left behind, which
has more iron and magnesium. Basaltic rocks are also less dense. In terms of minerals, basalt has more feldspar and amphibole, less
olivine and pyroxene, than peridotite. In geologist's shorthand, oceanic crust is mafic while oceanic mantle is ultramafic.
Oceanic crust, being so thin, is a very small fraction of the Earth—about 0.1 percent—but its life cycle serves to separate the stuff of the
upper mantle into a heavy residue and a lighter set of basaltic rocks. It also extracts the so-called incompatible elements, which don't fit
into mantle minerals and move into the liquid melt. These in turn move into the continental crust as plate tectonics proceeds.
Meanwhile, the oceanic crust reacts with seawater and carries some of it down into the mantle.
Continental Crust
Continental crust is thick and old—on average about 50 km thick and about 2 billion years old—and it covers about 40 percent of the
planet. Whereas almost all of the oceanic crust is underwater, most of the continental crust is exposed to the air.
The continents slowly grow over geologic time as oceanic crust and seafloor sediments are pulled beneath them by subduction. The
descending basalts have the water and incompatible elements squeezed out of them, and this material rises to trigger more melting in
the so-called subduction factory.
The continental crust is made of granitic rocks, which have even more silicon and aluminum than the basaltic oceanic crust; they also
have more oxygen thanks to the atmosphere. Granitic rocks are even less dense than basalt. In terms of minerals, granite has even
more feldspar, less amphibole than basalt and almost no pyroxene or olivine, plus it has abundant quartz. In geologist's shorthand,
continental crust is felsic.
Continental crust makes up less than 0.4 percent of the Earth, but it represents the product of a double refining process, first at
midocean ridges and second at subduction zones. The total amount of continental crust is slowly growing.
The incompatible elements that end up in the continents are important because they include the major radioactive elements uranium,
thorium and potassium. These create heat, which makes the continents act like electric blankets on top of the mantle. The heat also
softens thick places in the crust, like the Tibetan Plateau, and makes them spread sideways.
Continental crust is too buoyant to return to the mantle. That's why it is, on average, so old. When continents collide, the crust can
thicken to almost 100 km, but that is temporary because it soon spreads out again. The relatively thin skin of limestones and other
sedimentary rocks tend to stay on the continents, or in the ocean, rather than return to the mantle. Even the sand and clay that is
washed off into the sea returns to the continents on the conveyor belt of the oceanic crust. Continents are truly permanent, selfsustaining features of the Earth's surface.
What the Crust Means
The crust is a thin but important zone where dry, hot rock from the deep Earth reacts with the water and oxygen of the surface, making
new kinds of minerals and rocks. It's also where plate-tectonic activity mixes and scrambles these new rocks and injects them with
chemically active fluids. Finally, the crust is the home of life, which exerts strong effects on rock chemistry and has its own systems of
mineral recycling. All the interesting and valuable variety in geology, from metal ores to thick beds of clay and stone, finds its home in
the crust and nowhere else.
Six Things to Know About the Earth's Mantle
By Andrew Alden
The mantle is the thick layer of hot, solid rock between the Earth's crust and the molten ironcore. 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.
About the Earth's Core
How we study the Earth's core and what it may be made of
By Andrew Alden
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 way Earth responds 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 they do 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 Sun proposed 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 somewhat 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 (and length-of-day studies) 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 has to
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.
Ideas about what that light ingredient is have been evolving. Sulfur and oxygen have been candidates for a long time, and even
hydrogen has been considered. Lately there has been a rise of interest in silicon, as high-pressure experiments and simulations suggest
that it may dissolve in molten iron better than we thought. Maybe more than one of these is down there. It takes a lot of ingenious
reasoning and uncertain assumptions to propose any particular recipe—but the subject is not beyond all conjecture.
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. At the top of the outer core, iron crystals freeze out and rain into 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.
PS: The small, close-knit group of core specialists all belong to the SEDI (Study of the Earth's Deep Interior) group and read its Deep
Earth Dialog newsletter. And they use the Special Bureau for the Core's Web site as a central repository for geophysical and
bibliographic data.
Updated January 2011