Geology Reference
In-Depth Information
Earth'smantleinamajesticcyclehiddenfromview.Thepaceoftheseplanetaryshufflings
is equally vast—it may take a hundred million years or more for a single revolution of the
convection cell to complete itself.
At first, perhaps for more than a billion years, mantle convection beneath Earth's uni-
form basalt crust must have been a chaotic, swirling hodgepodge. Here and there, hotter
melts of lower-density granite rose in disorganized pulses and plumes toward the surface,
where they accumulated, disrupting the colder, denser basalt. Isolated dense chunks of that
colder crust slowly sank into the interior, in a global-scale exchange of heat.
Over the next half-billion years, mantle churning became more organized. Dozens of
smaller convection cells, each with rising plumes and sheets of magma and descending
blocksofcrust,consolidatedintoahandfulofmajesticcycles,eachhundredsofmilesdeep
andthousands ofmiles across. New,hotbasaltic crust formed where these convection cells
rose upward along growing seafloor ridges, while old, cold basaltic crust plunged into the
mantle at a steep angle—subduction zones, in an Earth increasingly dominated by the new,
transformative processes of plate tectonics. In cross section, Earth's turbulent outer layers
mighthavelookedlikeacollection ofsidewayswhirlpools,eachrotationlastingahundred
million years or more.
Then as now, Earth's evolving surface reflected the epic processes occurring far below.
Greatridgesofbasalticvolcanoesgrewaboveconvectionzonesofrisingmagma.Gashlike
trenches formed where old subducted crust plunged downward into the mantle, bending
and buckling the adjacent ocean floor. Subduction also accelerated the all-important pro-
duction of granite. As cold, wet, subducted basalt crust plunged deeper, swallowed back
into Earth, it heated up and began to melt—not completely, but perhaps 20 or 30 percent.
Those growing volumes of granitic magma rose to the surface, producing chains of gray
volcanic islands hundreds of miles long. The stage was set to build the continents.
Revolution
Granite floats, basalt sinks: that's the key to the origins of continents. Magmas of granitic
compositionaremuchlessdensethantheirparentbasalticrock,sothesefreshmeltsslowly,
inevitably rise to crystallize as near-surface rock masses or to erupt from volcanoes that
spew layers of cinders and ash onto the surface. Over billions of years of Earth history,
countless granite islands have formed by this continuous process.
Plate tectonics not only produced these granite-rooted island chains; it also assembled
them into continents. The key lies in the simple fact that granite cannot subduct. The dense
basalt on which it floats easily sinks into the mantle, but granite is like a buoyant cork.
Once formed, it remains at the surface, conserved. As subduction produces more islands,
the total area of granite irreversibly increases.
