Geology Reference
In-Depth Information
orableandirreversiblechange,thegatewaytothemodernlivingworld,yetoncelargelyig-
nored by scholars. Our self-inflicted ignorance may be self-reinforcing as well. Ambitious
students, who seek to establish their academic reputations in the brief temporal window
of graduate school and postdoctoral fellowships, are unlikely to focus on a geological era
when nothing much is thought to have happened.
Butthegeologicalstrataofthatenigmaticagemustcontainsurprisesfortheastuteschol-
ar. Hints of dramatic transformations must lie hidden in rocks whose story is largely un-
read.SomeofEarth'smostvaluableorereserves—vastdepositsofleadandzincandsilver
from Zambia and Botswana in Africa, from Nevada and British Columbia in North Amer-
ica, and from the Czech Republic and southern Australia—seem to cluster in rocks of that
age. Other localities rich in exotic minerals of beryllium, boron, and uranium also appear
to have flourished at about that time. Emerging evidence suggests that Earth's continents
may have clumped together into a single gigantic supercontinent during the boring billi-
on, then broken apart, then clumped together again in the planet's most majestic surface
cycle. And throughout that billion-year interval, abundant microorganisms—beautifully
preserved today as fossils—crowded coastal shallows and offshore environments. Surely
we have much to learn about Earth's dark ages.
A History of Change
Dramatic change has been the one constant in our saga of Earth's evolution up to now, a
couple hundred million years after its two-and-a-half-billionth birthday. The solar nebula
coalesced, and the Sun formed. The dust around it melted into chondrules. Chondrules
clumped into planetesimals, and planetesimals into the proto-Earth and other terrestrial
bodies thousands of miles in diameter. The impact of Theia, the subsequent formation of
the Moon, the incandescent magma ocean hardening to a blackened basaltic crust pock-
marked by thousands of explosive volcanoes, the hot sea that soon covered almost all the
solid surface so that only the tops of the tallest volcanic cones were dry—all these dra-
matic events transpired within a half-billion years. Even in the less tumultuous two billion
yearsthatfollowedtheaccrualofEarth'suniqueocean,ourplanet'ssurfacewasconstantly
in flux, as granite emerged from basalt melts and proto-continents grew on the convection
cells that drove plate tectonics.
It was on such a dynamic, variable world that life emerged, evolved, and eventually
learned to make oxygen. Constant change was Earth's hallmark. Like a precocious artist,
our planet had reinvented itself over and over again, at every stage trying something new.
How, then, could our dynamic planet have found itself mired in an aeon of stasis?
The simple answer is that Earth wasn't static even then. Change was incessant, though
perhaps not as dramatic as a Moon-forming impact or the Great Oxidation Event. The bor-
