Geology Reference
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the water. A jagged two-hundred-foot-tall iceberg will typically expose thirty or more feet
above the surface; hence “the tip of the iceberg.” By the same token, granite is 10 percent
lessdensethanthebasaltonwhichitfloats.AstheEarth'spartiallymeltedbasaltcrustgen-
eratedlayeruponlayerofgranite,iceberglikeprotrusionsbegantoform.Amile-thickgran-
ite body might have produced a small mound that projected almost a thousand feet above
the average level of basaltic crust. But over time, accumulating masses of granite crust
reachedthicknessesofmanymiles;accordingly,deeplyrootedcontinentallandmassesrose
higher and higher above the oceans, with some mountain ranges soaring miles above the
water'ssurface.Today'sRockyMountainchainintheAmericanWest,withgraniterootsas
much as forty miles deep, boasts numerous peaks over fourteen thousand feet. This grand
spine of the North American continent stands tall as testimony to the buoyancy of granite.
In 1970, when I took my first geology course at MIT, the power of buoyancy in driving
geologicalchangewasstilltextbookorthodoxy.(Weusedthe1965editionofBritishgeolo-
gist Arthur Holmes's richly illustrated classic, Principles of Physical Geology. ) “Isostasy,”
it was called. The driving force of “vertical tectonics” was “isostatic readjustment.” A neat
woodcut,virtuallyunchangedfromnineteenth-centurygeologytexts,showedalineofrect-
angularwoodenblocksofdifferentheightsfloatinginwater.Tallerblocksprojectedhigher
out of the water, just like a mountain. We learned how ocean basins had filled with thick
layers of sediments, and how those sediments had melted to form more granite bodies. We
learned how mountains subsequently rose from those buoyant granitic cores. It all made
perfect sense at the time, and it's still the leading hypothesis for how Earth's earliest crust
formed more than four billion years ago.
Early in Earth's history, perhaps even within the first two hundred million years, modest
landmasses of buoyant gray granite must have begun to form above hot spots, as deep
accumulations of basalt were partially melted. In those early times vertical tectonics and
isostasy must have prevailed, just asArthurHolmes taught. Those first isolated continental
bits of granite were utterly barren and windswept and battered by intense waves. Eroded
rock fragments of quartz slowly accumulated to form meager sandy beaches, while feld-
spars weathered to thin layers of clay-rich soils. The first granite islands were isolated,
modest in size, and low in profile; they gave no hint to the scale of the continents to come.
Impact Redux?
So how did early Earth shift from a volcano-dotted basaltic world to a planet with wide,
gray granitic continents? How did the first few lonely granite islands expand to the
hemisphere-spanninglandmassesweseetoday?Earthscientistshaveneverbeenshyinde-
vising hypotheses. One of the more intriguing ideas posits a continent-forming sequence
triggered by that familiar agent of chance: the careening asteroid.
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