Geology Reference
In-Depth Information
neat solution to that niggling missing-element problem: just stick all the incompatible ele-
ments in the inaccessible D´´ layer, where they are forever sequestered in that enigmatic,
heterogeneous zone of mineralogical junk.
Andwhatofthecoreitself?WhenEarthwasveryyoung,thedense,iron-richcore,more
than 2,000 miles in diameter, had fully formed and was probably entirely molten (unlike
today, when the inner core appears to be a growing ball of solid iron crystals 750 miles in
diameter). Temperatures at that sharp core-mantle dividing line may have exceeded 10,000
degrees Fahrenheit, while pressures exceeded a million times that of our modern atmo-
sphere.
Thehotcorewasfromtheoutset,andremainstothisday,adynamicplaceofswirlingli-
quid metal currents. One important consequence of these currents was the early generation
of Earth's magnetic field—the magnetosphere, which is like an immense electromagnet.
Magnetic fields bend the paths of electrically charged particles, so Earth's magnetosphere
provides a kind of invisible deflector shield to the intense bombardment of solar wind and
cosmic rays—a barrier that is perhaps a prerequisite for the origins and survival of life.
The core is also an important source of heat energy that helps to drive convection in the
mantle. Even today plumes of hot magma from the core-mantle boundary rise almost two
thousand miles to the surface in volcanic hot spots such as Hawaii and Yellowstone. Re-
markably, the fixed locations of these plumes on the surface may be dictated by deep topo-
graphy.Thethree-hundred-mile-tallmountainsoftheD´´layermayactasthermalblankets
lying on the hot core, so it's possible that hot spots originate at the deepest, heat-releasing
valleys between those epic, hidden mountains.
Basalt
At heart, the mineral evolution story rests on a preordained succession of rock types, each
mineral-forming stage following logically from the previous stage. Earth's first peridotite
crust was a critical but fleeting juvenile phase, born of the primordial magma sea. When it
ultimately cooled and hardened, it proved too dense to remain anywhere near the surface
and thus sank back into Earth's depths. Another, less dense rock was required to girdle the
globe. That rock was basalt.
Black basalt dominates the near-surface rocks of every terrestrial planet. The asteroid-
scarred exterior of Mercury is mostly basalt. So are the scorched, mountainous surface of
Venus and the weathered red surface of Mars. The Moon's darkly splotched mares (seas),
which contrast so vividly with the paler gray anorthosite highlands, are the hardened re-
mains of immense black basaltic lakes. And on Earth, 70 percent of the surface, including
all the floors of all the oceans, is underlain by basalt crust.
