Geology Reference
In-Depth Information
orialAfricareinforcessuchfindings.Thesedimentaryrockrecordaddsvitaldata.Different
kindsofsedimentsaccumulateindifferentenvironments:shallowseas,continentalshelves,
tundra, glacial lakes, tidal lagoons, and swamps each host a distinct rock type.
Bolstered with these clues, experts in paleogeography have managed to craft a coherent
and defensible picture of Earth back at least 1.6 billion years, well into the boring billion,
withinformedspeculationsextendingtoevendeepertime,totheformationofthefirstcon-
tinents.Ittookalongtimeforplatetectonicstomaketheoriginalcontinents.Atthetipping
pointofsubduction,attheveryfaultlinewheredenseslabsofEarth'searliestbasalticcrust
plunged down into the mantle depths, unsinkable bits of low-density granite islands piled
up one after the next to make larger and larger stable, long-lasting landmasses. These an-
cient chunks of what are now the continents go by the name
craton,
a term derived from
the Greek word for “strength.”
Cratons are strong; once formed, they last a long time. Earth today preserves perhaps
three dozen more or less intact cratons, some as old as 3.8 billion years and ranging in size
from a hundred to more than a thousand miles across. These diverse pieces, each evocat-
ively named—Slave and Superior in North America, Kaapvaal and Zimbabwe in Africa,
Pilbara and Yilgarn in Australia—have experienced billions of years of migration across
the globe. Jumbled together and ripped apart, along with lots of smaller ancient fragments,
they survive as the continents' foundation stones. Three such cratons form most of Green-
land, while much of central Canada and the northern parts of Michigan and Minnesota
comprise a cluster of a half-dozen others. Large portions of Brazil and Argentina are un-
derlain by several cratons, as are big chunks of northern, western, and southern Australia,
Siberia, Scandinavia, a large piece of Antarctica, separate regions of eastern and southern
China, most of India, and several swaths of western, southern, and central Africa. All of
these cratons began to form more than three billion years ago—a time before modern-style
plate tectonics, when only a tiny portion of Earth's surface was dry land. Consequently, all
cratons carry a precious, if somewhat warped and scrambled, record of Earth's vibrant ad-
olescence.
Cratonsarethekeys—theRosettastonesofearly-Earthhistory.Theoceanscan'thelpus
decipher early Earth. Thanks to the incessant conveyor belt of plate tectonics, which pro-
duces new basaltic crust at the ocean ridges and swallows it up again at convergent bound-
aries,theoldestoceancrustisnotmuchmorethantwohundredmillionyearsold.Anything
older than that must be preserved on the continents or not at all.
The peripatetic cratons have an astonishingly complex history. Propelled by the motions
of tectonic plates, they have been shuttled about, colliding with one another to form com-
posite cratons and supercratons, which in turn occasionally clumped into single giant land-
masses—continents or supercontinents. Each collision produced a new mountain range
