Geology Reference
In-Depth Information
Imagine a subducting plate of ocean crust, dotted with unsinkable granite islands. The
basalt subducts, but the islands don't. They must remain at the surface, to form a strip of
land right above the subduction zone. As tens of millions of years pass, more and more
granite islands pile up to form a wider and wider strip, even as new volumes of granite
meltrisefromthesubductingslabtothickenandexpandthegrowingcontinent.Islandsac-
crete toformproto-continents, whichaccrete toformcontinents, justasourSolarSystem's
chondrites once accreted to form planetesimals, planetesimals to form planets.
The epic cycle of plate tectonics transforms our world. Earth's thin, cold, brittle surface
cracks and shifts like scum on a pot of boiling soup. New basaltic crust pours out of vol-
canic ridges that reveal where deep convection cells rise. Old crust is swallowed up at sub-
duction zones that reveal where convection cells descend. Earth's most violent surface dis-
ruptions—themostintenseearthquakes,themightiestvolcanoes—arebuttrivial,incidental
blipscomparedwiththevastlymoreenergeticglobal-scalemovementsofthedeepinterior.
Plate tectonics also revolutionized the Earth sciences. In the prior dark ages of vertical
tectonics,eachgeologicaldisciplinewasseparate,seeminglyunrelatedtoanyother.Before
the revolution, paleontologists had no need to talk to oceanographers; the study of volca-
noes had little to do with ore geology; geophysicists were unconcerned with life's origins
and evolution; and the rocks of one nation were of no obvious relevance to rocks of anoth-
er, much less to rocks on the distant ocean floor.
Plate tectonics unified everything about Earth. Now locations of rare fossil organisms
can be matched precisely across the vast spreading oceans. Extinct volcanic terrains lead
miners to valuable ore deposits hidden in their corresponding subduction zones, long since
solidified into continental rock. Geophysical studies of shifting continents point to key in-
fluences on the evolution of plants and animals. Plate tectonics reveals Earth as an integ-
rated planetary system, from crust to core, at scales from nano to global, with a single uni-
fying principle through space and time.
It took time for the production of granite to shift from the disorderly, vertical tectonic
patchwork of plume-driven islands to the coordinated assembly of subduction-driven con-
tinents. But by the time Earth was 1.5 billion years old, the convecting mantle—the
eighteen-hundred-mile-thick zone that holds most of Earth's mass and heat energy—had
irrevocably transformed the surface of our planet. Unlike black basalt, the growing, barren
granite landmasses appeared whitish gray, the typical blended color of quartz and feldspar.
And so if you were a time traveler to that ancient world three billion years past, you would
findsomefamiliarfeatures.Youcouldstandonproto-continentsdevoidofvegetation,with
jagged hills and steep-walled valleys, not unlike some rugged coastlines of the high arctic.
You would experience periods of violent weather, punctuated by days of sunny blue skies
and puffy white clouds. You would find the ocean saturated with dissolved minerals, in-
cluding calcium and magnesium carbonates, which were occasionally deposited as crystal
