Geology Reference
In-Depth Information
low-pressure environments, but it was a rather minor mineral. Instead, magnesium-rich
pyroxene, the commonest of the chain silicate minerals, appeared in abundance, to com-
mingle with olivine in a thick crystal slush. Earth's earliest rocks thus predominantly fea-
tured olivine and pyroxene in a hard, greenish-black rock called peridotite. Varieties of
peridotite began to crystallize throughout Earth's outer fifty miles, probably commencing
more than 4.5 billion years ago and continuing for many hundreds of millions of years.
In spite of its early abundance, peridotite, too, is relatively rare at Earth's surface today.
By one persuasive scenario, rafts of peridotite hardened and cooled to form Earth's first
transient rigid surface. But cooling peridotite, like its dunite predecessor, is significantly
denser than the hot magma ocean in which it formed. The peridotite surface layer thus
cracked, buckled, and sank back into the mantle, to displace more magma that cooled to
formmoreperidotite.Overaspanofhundredsofmillionsofyears,themantleitselfslowly
solidified,ridingakindofperidotiteconveyorbeltthatoperatedinEarth'souterfiftymiles.
The ratio of dense solid peridotite to magma increased, until the upper mantle was mostly
solid olivine-pyroxene rock.
Core Truths
Deeper in the mantle, fifty to two hundred miles beneath the crust, cooling and crystalliz-
ation must have proceeded in a similar fashion, albeit more slowly. Details of the process
remain uncertain—the next generation of high-pressure, high-temperature apparatus must
be brought to bear to sort out the complexities—but separation of crystals from melts by
sinkingandfloating probablyplayedasignificant role,astheydidinthenearersurface en-
vironment.
Much of what we know of those hidden, deep domains comes from the science of seis-
mology, the study of sound waves speeding through Earth's deep interior. Earth is con-
stantly ringing like a bell: crashing tides, rumbling trucks, and earthquakes both big and
small all conspire to shake Earth and propagate seismic waves. And like sound waves in a
steep-walled canyon, seismic waves echo when they bump into a surface. Seismic waves
reveal that Earth's interior is a complexly layered place.
At the most basic anatomical level, Earth is triply layered—it has a thin, lower-density
crust at the surface, a thick, higher-density mantle in the middle, and a thicker,really dense
metallic core in the center. Each of those three domains contains further layering. The
mantle, for example, is divided into three sublayers—upper mantle, transition zone, and
lower mantle. The peridotite-dominated upper mantle extends down perhaps 250 miles, at
which depth pressure forces the atoms in olivine to pack into a denser silicate crystal form
called wadsleyite, the dominant mineral of the mantle's transition zone. The lower mantle,
150 miles farther down, features an even denser assemblage of magnesium silicates. The
