Geology Reference
In-Depth Information
Gray Earth
The First Granite Crust
Earth's Age: 200 to 500 million years
Earth today is a world of contrasts—one-third land, two-thirds water; seen from space,
a mélange of blue, brown, green, and swirling white. Not so 4.4 billion years ago, when
widely scattered, symmetrical volcanic cones of black basalt were the only meager parcels
of dry land to poke above the blue monotony of the shallow seas. All that was about to
change with the invention of granite—the rugged foundation stone of the continents.
Earth's story is a saga of differentiation—of the separation and concentration of elements
into new rocks and minerals, into continents and seas, and ultimately into life. Time and
time again this theme has played out. The inner rocky planets—Mercury, Venus, Earth, and
Mars—formed when intense pulses of solar wind separated hydrogen and helium from the
heavier big six elements, sweeping the lighter gaseous elements outward to the domain of
the giant planets Jupiter, Saturn, Uranus, and Neptune. On Earth, dense molten iron settled
to the center, as the metal core separated from the peridotite-rich mantle. Partial melting of
peridotite produced basalt, a rock rich in silicon, calcium, and aluminum, which separated
from peridotite to form Earth's first thin, black crust. As basalt erupted explosively onto the
surface,waterandothervolatilesseparatedfromthebasalticmagmatoformthefirstoceans
and atmosphere. Each heat-driven step separated and concentrated elements; each step led
to an increasingly layered, differentiated planet.
The rise of continents was yet another important step in the differentiation of Earth. As
the outer crustal layers of basalt cooled and hardened, they formed a lidlike, heat-trapping
cover to the roiling mantle beneath. Basalt, reheated from below, began to melt at relatively
low temperatures, especially in the presence of water—as chilly as 1,200 degrees Fahren-
heit.Asthetemperatureincreased,sodidthepercentofbasaltmelting—first5percent,then
10 percent, eventually up to 25 percent melt. In an echo of peridotite melting, the resulting
magma was sharply different in composition from its host basaltic rock. Most notably, this
new melt was much richer in silicon, with a significantly enhanced component of sodium
and potassium as well. Water, too, concentrated in this hot fluid, as did dozens of rare trace
elements—beryllium, lithium, uranium, zircon, tantalum, and many more. This new silicon-
rich magma was much less dense than its parent basalt, so it inevitably pushed its way to-
ward the surface, forming the first granite.
