Geology Reference
In-Depth Information
their melting temperatures and turned them into a superheated soup that rose toward the
surface. Close to the surface, the gases dissolved in that magmatic soup transformed from
liquid to violently expanding gas in massive volcanic explosions, much as a shaken soda
will blast its way out of a confining can. Water-rich fluids also dissolved and concentrated
rare elements—beryllium, zirconium, silver, chlorine, boron, uranium, lithium, selenium,
gold,andmanymore—thatwouldeventually becomethegreatorebodiesofEarth'sdiver-
sifying crust. At the chaotic surface, roaring rivers and crashing waves became principal
agents for the erosion of rock, the formation of Earth's first sandy beaches, and the accu-
mulation of thickening near-shore wedges of sediments. In short, water became the chief
architect of Earth's solid surface.
Anyfocusonoceansandatmospherereflectsasomewhatanthropocentricview,forthese
fluid bodies are trivial components of the planet as a whole. Oceans today represent only
about 0.02 percent of Earth's total mass, while the atmosphere is no more than one part per
million of its bulk. Nevertheless, oceans and atmosphere have exerted, and continue to ex-
ert, disproportionately large influences in making Earth the unique world that it is.
Four principal players—nitrogen, carbon, sulfur, and water—take the leading roles as
Earth's mobile, gaseous components. All of these ingredients are produced abundantly in
large stars, all are widely dispersed in supernova explosions, and all became concentrated
in the most primitive carbon-rich chondrite meteorites more than 4.56 billion years ago.
In many respects, the average composition of chondrite meteorites closely matches that
of Earth today. The big six elements discussed in chapter 3 (oxygen, silicon, aluminum,
magnesium,calcium,andiron)areremarkablysimilarintheirproportions,asarenumerous
less common elements. But even a cursory study of those fascinating ancient objects re-
veals that much of Earth's original inventory of volatiles is missing from today's planet.
The most primitive chondrites average more than 3 percent carbon, but all the known car-
bon reservoirs on Earth now add up to less than 0.1 percent. Similarly, the water content of
chondrites is much greater than Earth's modern average—perhaps a hundred times more.
Such gross compositional disparities point to a chaotic and violent past. Most of Earth's
volatiles must have been lost to space or deeply buried, far beyond our ability to sample.
ThekeytounderstandingEarth'searlytransformationfromablasted,inhospitableblack
planet to a cooler, habitable blue planet lies in the story of its peripatetic volatiles. But no
volatiles survived in an unaltered state from the Earth's first half-billion years. Almost all
thenitrogenandcarbon,allthesulfurandwater,havebeenrecycledcountlessthousandsof
times,asthesameatomshavebeenusedoverandoveragain.Chondritemeteoritesprovide
a quantitative starting point for our guesswork; the few known rock and mineral samples
from Earth's first billion years, coupled with data from the Moon and other objects in the
Solar System, further delineate our speculations. And as was the case with understanding
the evolution of mantle and crust during Earth's first hundred million years, as well as
Search WWH ::




Custom Search