Geology Reference
In-Depth Information
Imagine yourselfbackmorethan4.4billion yearsintime tothenewlyminted blacksur-
face of that Hadean Earth. You could not have survived long in the harsh, alien landscape.
Meteors incessantly bombard the surface, cracking the thin brittle black crust, showering
shatteredrockandgobsofmagmaacrosstheplains.Countlessvolcanicconesrise,growing
steadily to heights of many thousands of feet, their immense magma fountains powered by
the explosive release of steam and other volatiles that will, some fine day, cool enough to
become the oceans and atmosphere. No trace of life-supporting oxygen is to be found. On
this unforgiving young Earth, your nostrils are assaulted by foul-smelling sulfurous com-
pounds, your skin scalded by venting steam, your eyes burned by the noxious hot gases.
Your excruciating death agonies will be brief on such a hostile world.
The receding Moon continued to play a major role in shaping the crust. Globe-spanning
tides of rock and magma, though less extreme than in the first centuries following Theia's
demise, repeatedly cracked and buckled the surface, opening fissures that oozed red-hot
rockymushandthwartedtheformationofasolidsurface.TheMoon'suncomfortableprox-
imity also perpetuated Earth's insanely rapid rotation—five-hour days persisted, accom-
panied by megastorms and ultratornadoes far more severe than anything hyped on today's
Weather Channel.
Butbeneaththatwretchedsurface,Earth'sinexorableevolutiontoalivingworldhadbe-
gun.Thewell-mixed,molteninteriorbegantoseparateintovolumesofdistinctivecompos-
itions—matter that would become the continents and deep-sea crust, the atmospheres and
oceans, plants and animals. Heating and cooling and crystallization, crystal separation by
settling and floating, accumulation of peridotite, partial melting—these processes shaped
Earth through its infancy 4.5 billion years ago, and they persist even unto the present day.
Earth'svastreservoirofinternalheat,thecentralthemeofthischapter,continuestoplay
the dominant, transformative role in shaping our planetary home. Today the most obvious
manifestations of this deep, hot realm are intermittent volcanoes, with their fiery magma
fountainsandred-hotriversofmoltenrock.Eruptinggeysersandsulfuroushotspringsalso
hint at a hellish hidden subsurface realm. Throughout Earth's 4.567-billion-year history,
as heat inexorably worked its way outward from the incandescent center to the fractured
crust and thence into the coldness of space, the surface has borne the brunt. Buffeted by
the swirling convection of the mantle and stressed by the Moon's incessant pull, the crust
has bent and buckled, cracked and twisted. Continents have constantly shuttled across the
globe, ripping apart, colliding, and scraping past one another in the ongoing heat-driven
dance ofthetectonic plates. Everydayofourlives, Earth'sinnerheat reworkstherockson
which we live, recycles the water we drink, and alters the air we breathe.
Because of heat, Earth was destined for a brief time to be a black world, glazed with a
thin basaltic veneer. But that brief juvenile phase could not last long. A new volcano-born
layer of brilliant blue was about to gird the globe.
Search WWH ::




Custom Search