Geology Reference
In-Depth Information
along the suture zone; each range provides compelling evidence for the ancient assembly
of larger landmasses.
The supercontinents, in turn, rifted apart and fragmented into separate, ocean-bounded
island continents. Each time a continent rifted, a widening ocean formed between the di-
verging pieces and a telltale suite of sediments was laid down: shallow-water sandstone
andlimestone atfirst,thendeeper water mudandblack shale. Suchsedimentary sequences
point to episodes of continental fragmentation. Over and over, supercontinents have been
forged and then ripped asunder. It's an immense jigsaw puzzle with an unknown picture,
where pieces constantly change their positions and their shapes.
What does all this have to do with the boring billion? Everything. For a period devoid of
flashy signs of activity—a thwack-free, treeless time before elaborate flora and fauna ar-
rived in the geological record—we must turn to paleogeographers to comprehend what it
lookedlike.Todecipherthedetailsofthecratons'multibillion-yeardanceacrosstheglobe,
these geologists trek to the most remote places on Earth, map the rocks, collect samples,
and subject them to a battery of laboratory tests.
At the core of every craton are really old rocks, typically three billion years old or more.
These fragmentary parcels of Earth's most ancient crust represent, in total, only a small
proportion of the planet's continental mass. They have invariably been baked by heat and
pressure, altered by the dissolving powers of subsurface waters, and contorted by crustal
stresses. Even so, the nature of the original rocks, whether granitelike intrusions or sedi-
mentary layerings, can often be deduced. Moreover, it's fortunate that cratons aren't static.
Throughout their histories, new pulses of magma penetrate the old, forming igneous rock
bodiesinveinsandpods.Newsedimentarydepositsforminlandinlakesandrivers,aswell
as along shallow, sandy coastlines. Distinctive rock types and characteristic structures also
form whenever cratons smack together or rip apart—events that suggest the relative mo-
tions of two landmasses. Careful studies of these varied younger formations can discern a
suite of rock types that spans the entire history of a craton. Then the fun begins.
The younger rocks provide hints about the chronology of craton movements. Igneous
rocksholdtinymagneticmineralsthat,whentheysolidify,lockintheorientationofEarth's
magnetic field. Careful paleomagnetic studies can identify not only the orientation of the
former north and south poles but also the rocks' approximate latitude when they cooled.
Those data, while not exactly GPS coordinates, do record the relative positions of cratons
through time. Sedimentary rocks complement these data, for they can host telltale clues
about climate and ecology. Sediments deposited in rapidly weathering tropical zones differ
markedly from those in temperate lakes or the glacial deposits of higher latitudes. Some
sedimentaryrocksalsoincorporatetinygrainsofmagneticmineralsthatholdcluestopolar
positions.
