Geology Reference
In-Depth Information
Mostgraniteshostasimplemineralogyoffourdifferentspecies.Clear,colorlesscrystals
of quartz—pure silicon oxide—abound in granite; their tough grains would erode to pro-
duce Earth's first white sandy beaches. Two kinds of feldspar, one rich in potassium and
the other in sodium, gave Earth's earliest granites their monotonous grayish-white color.
Andsprinkledineverygraniteisafourth,darker,iron-bearingmineral—sometimesblocky
pyroxene, sometimes sheetlike mica, sometimes elongate amphibole. The next time you
see a polished granite countertop or bathroom fixture, take a look for this simple suite of
four minerals.
The presence of rarer elements often leads to scattered tinier grains of additional miner-
als,suchaszircon,forexample,whichconcentrateszirconium.Recallfromthelastchapter
that minute, gemmy red zircon crystals extracted from the remote Jack Hills of Australia
provide hints of an early ocean 4.4 billion years ago. Those same crystals, which appear
to have formed under relatively cool, wet conditions, may also point to the beginnings of
granite formation at that early stage. Not only do the Jack Hills zircons bear the distinctive
heavy oxygen isotope signature of a cool and wet origin, but a few 4-billion-year-old crys-
tals also hold inclusions of quartz—a mineral rarely produced before the advent of granite.
Someexpertssuggestthattheseold,cool,quartz-bearingzirconcrystalsarethelastsurviv-
ing remnants of the earliest granite crust.
With the origin of granite, we see for the first time a significant divergence of Earth's
mineral evolution from that of some of its planetary neighbors. Granite formation requires
abundant basalt near the planetary surface as well as intense internal heat to remelt it. The
smaller planets Mars and Mercury, as well as Earth's Moon, are girdled by the necessary
basaltic veneer, but they are too small to make much granite. They lack the necessary in-
ternal heat. Small volumes of granite were undoubtedly generated on these worlds, but
nothing like the deeply rooted granite continents of Earth.
Buoyancy
Earth's primordial crust of black basalt, softened by heat from below and with a uniform
density about three times that of water, never could support much topography. A few vol-
canic edifices may have soared a mile or two above the mean, enough for scattered black
islands to rise above the sea, but there were no great mountain ranges, nor any deep ocean
basins before the rise of continents. Granite, with a significantly lower average density
(about 2.7 times that of water), changed that dynamic. Granite inevitably floats on basalt
and peridotite; it piles up in great mounds, rising miles above the surface like an iceberg
floating on water.
Ice, which is about 10 percent less dense than water, provides a familiar analogue. Be-
cause of this density difference, about 10 percent of an iceberg's volume sticks out of
