Geology Reference
In-Depth Information
White Earth
The Snowball-Hothouse Cycle
Earth's Age: 3.7 to 4.0 billion years
The Proterozoic Eon, spanning almost half of Earth's history from 2.5 billion to 542
million years ago, was a long period of sharp contrasts. Its newsworthy first 500 million
or so years witnessed the great flourishing of photosynthetic algae and the consequent rise
of atmospheric oxygen, the transformation of once iron-rich oceans by the deposition of
massive banded iron formations, and the biological innovation of eukaryote cells with their
DNA sequestered in a nucleus—cells that were the precursors of all plants and animals.
The middle billion years of the Proterozoic—the so-called boring billion—were a much
more plodding, steadily changing time that was also very smelly.
By contrast, the final three hundred million years were perhaps most dynamic of all, with
continental breakup and assembly, radical climate swings, epic shifts in ocean and atmo-
spheric chemistry, and the rise of animal life.
I hope I've established that Earth systems are complexly interconnected. Air, water, and
land appear to us as separate spheres, which change over very different scales of time.
Weather varies daily; oceans change over millennia; rocks cycle over millions of years; su-
percontinents take hundreds of millions of years to assemble and break apart. Yet every
Earth system affects every other in ways both obvious and hidden from view.
Ahouseservesasauseful,ifimperfect,metaphorforourhomeplanet.Whenyouarecon-
sidering buyingahouse,youwant toknowmany things—when it was built, forexample, as
wellastheageandgeometryofitsvariousadditionsandrenovations.Youwantdetailsabout
your house's building materials and their installation, from the foundation to the roof. You
need to learn about the plumbing system and its source of water, as well as the air handling
system—the furnace and air conditioner and their sources of energy. The smart home buyer
also asks about potential risks: from fire and carbon monoxide, termites and carpenter ants,
radon and asbestos, leaks and mold. Geologists, likewise, study Earth's origins and major
transitions, the nature of rocks and minerals, the movement of water and air, the sources of
energy, and the risk from geological hazards.
A house also mimics some of Earth's complex behaviors, as different systems are inter-
connectedinsometimessurprisingandunexpectedwaysthroughnegativeandpositivefeed-
backloops.Onacoldwinterday,asthetemperatureinsidethehousedropsbelowyourcom-
