Geology Reference
In-Depth Information
As in any new scientific endeavor, it took a while for these experiments to catch on, but
the more scientists looked, the more minerals emerged as likely hosts for deep water. Oliv-
ine and pyroxene in the lower crust are fairly dry, holding no more than a hundredth of
a percent water. But raise the pressure to mantle conditions of 100,000 atmospheres and
the temperature to 2,000 degrees Fahrenheit, and olivine transforms to wadsleyite, which
can incorporate a whopping 3 percent water. The corresponding Earth layer, the mantle's
transition zone from about 255 to 410 miles deep, is one of the wettest places in the planet
and may hold nine times all the water in the oceans. Minerals of the lower mantle are less
waterlogged, but they make up for that in their huge volume—half of Earth's total—so the
lower mantle is estimated to hold another sixteen times the water in Earth's oceans. Given
the likelihood of other water-rich minerals and that Earth's iron core probably holds a lot
of hydrogen as well, the deep interior may store more than eighty oceans' worth of water.
First Ocean
Conservative estimates place proto-Earth's original budget of volatiles at more than a hun-
dred times modern levels. Indeed, one of the principal challenges in modeling the history
of Earth's volatiles is figuring out how much was lost—and how it escaped.
Ofsomethingswecanbesure.Fromdayone,volatiles werereleased prodigiouslyfrom
the deep interior, as megavolcanoes pumped huge quantities of steam into a rapidly thick-
ening atmosphere. In the first few million years of proto-Earth's existence, that first atmo-
sphere may have been many times denser than that of the modern world. Water may have
poured out onto the surface in liquid form, cooling the first rocks and forming wide, shal-
low seas within a few tens of millions of years.
AndthentheBigThwackblasteditallaway.Almosteverymoleculethathadworkedits
way to the surface was lost to space, in what amounted to the pushing of a giant reset but-
ton. We have no reasonable estimates as to how much of Earth's store of nitrogen, water,
and other volatiles was lost in that single event, but it was a lot. Dozens of smaller-scale
impacts of hundred-mile-diameter rocks caused unimaginable disruptions for another five
hundred million years, each one vaporizing a significant fraction of the oceans and further
diminishing the volatile inventory.
Nonetheless, within a few million years following the Big Thwack, water vapor had
again become a principal component of the primordial atmosphere, forming a global tem-
pest of turbulent dark clouds, howling winds, shattering lightning, and unceasing torrential
rain. The surface of the storm-lashed basalt crust cooled and hardened, as low-lying basins
graduallyfilled,slowlyformingtheoceans.Foratimetheencroachingseascreatedaglob-
al sauna, as the thin veneer of surface water penetrated cracks and fissures, contacted the
hot rocks below, and returned to the surface as gargantuan geysers of roaring steam and
