Geology Reference
In-Depth Information
The big six elements, each of which is an inescapable consequence of the evolution of ex-
ploding stars and terrestrial planets, are also responsible for Earth's most abundant rocks.
Their distinctive chemical behaviors set our planet on an irreversible course of transforma-
tion into the world we inhabit today. But before rocks could form, Earth had to cool.
Imagine once again the violent years following the Moon-forming impact. For a few
days or weeks, what would become Earth and what would become the Moon were still be-
ing sorted out. Neither Earth nor the Moon in those early post-Theia days had any solid
surface. These coalescing companion globes were both bounded by an encircling magma
ocean, roiling and incandescent red, pelted by an incandescent molten silicate rain at tem-
peratures of thousands of degrees.
AstheairclearedofTheia'sremains,blast-furnace-likeheatradiatedfromEarthintothe
cold vacuum of space, inexorably cooling the planet's outer shell. Even so, cosmic events
conspired to keep Earth's surface molten a while longer. For one thing, big asteroids kept
pounding the planet. Each collision added more thermal energy, superheating the impact
area, thwarting any attempts to form a stable crust. Intense gravity-induced tides from the
nearby Moon also helped to maintain Earth's liquid state, as an equatorial bulge of turbu-
lent magma swept around the planet every five hours, fragmenting the organization of any
thin,solidveneer.Earth'samplestoreofhighlyradioactiveelements—boththeshort-lived,
heat-generating isotopes of aluminum and tungsten and the long-lived radioactive isotopes
of uranium, thorium, and potassium—continued to add even more heat. And a young and
growing atmosphere, fueled by the volcanic release of vapors rich in carbon dioxide and
water, may have amplified these effects by inducing a “super greenhouse” state.
Foranunknownlengthoftime,perhapsahundredyearsorahundredthousandyears—a
geological blink—the molten state prevailed. Butcoolingandhardeningwerepreordained.
The second law of thermodynamics demands that hot objects, lacking significant new en-
ergy inputs, must cool, and the hotter the object, the faster the rate of cooling.
Threefamiliarmechanismsfacilitatethistransferofheat.Firstthere'sconduction.When
a hotter object touches a cooler object, heat energy must flow from hot to cool. This pro-
cess,painfullyobviousifyourfeethaveeverbeenburnedfromwalkingonsunbakedpave-
ment or your hand has ever blistered from touching a stove burner, results from the con-
stant twitching of atoms. Atoms in hotter objects experience more violent motions. When
a cooler object, with more slowly wiggling atoms, contacts a hotter object, with more fren-
etic atoms, some of that violent motion is transferred by atom-to-atom collisions. If the hot
objectyoutouchishotenough,itcandisruptthemoleculesinyourskin,killingcells,caus-
ing a burn. Conduction is a fine way to transfer heat locally, from one object to an adjacent
object, but it is a poor choice for heat transfer on a planetary scale. It just takes too long to
move heat from one wiggling atom to the next.
