contain chemical fingerprints of complex cells as much as 3.9 billion years
old.Around this time, a swarm of debris left over from the creation of the solar
system bombarded Earth and the Moon.The bombardment might have deliv-
ered heat and organic compounds to Earth, sparking the rapid formation of
primitive life. Alternatively, the pummeling could have wiped out existing life
in a colossal mass extinction.
The continental crust was perhaps only about 10 percent of its present
size and contained slivers of granite that drifted freely over Earth’s watery face.
These slices of ancient crust called Archean terranes are older than 3.3 billion
years and are found notably in Canada, Greenland, southern Africa, and West-
ern Australia.The 2.5-billion-year-old Kaapvaal, comprising a large portion of
South Africa, is one of the oldest of Earth’s cratons. A wedge of 3.5-billion-
year-old continental crust called the Pilbara Craton, upward of thousands of
square miles in length, lies in northwestern Australia and has barely been dis-
turbed down through the ages.The area is also known for a famous formation
called the Warrawoona Group that contains the world’s oldest fossil cells, the
earliest solid evidence of life on Earth.
Cherts are dense, extremely hard sedimentary rocks composed of micro-
scopic grains of silica. Archean chert deposits more than 2.5 billion years old
suggest that most of the crust was below the sea during this time. Most Pre-
cambrian cherts are thought to be chemical precipitates derived from silica-
rich water in the deep ocean. The seas contained abundant dissolved silica,
which leached out of volcanic rock pouring onto the ocean floor. Modern
seawater is deficient in silica because organisms such as sponges and diatoms
extract it to build their skeletons. When the organisms die, their skeletons
build massive deposits of diatomaceous earth.
Few rocks date beyond 3.7 billion years, suggesting that little continen-
tal crust was being formed before this time or that the crust was recycled into
the mantle. Slices of granitic crust combined into stable bodies of basement
rock called cratons (Fig.28),upon which all other rocks were deposited.They
are composed of highly altered granite and metamorphosed marine sediments
and lava flows.The rocks originated from intrusions of magma into the prim-
itive ocean crust. Only three sites in the world, located in Canada, Australia,
and Africa, contain rocks exposed on the surface during Earth’s early history
that have remained essentially unchanged throughout geologic time.
Eventually, the slices of crust began to slow their erratic wanderings and
combined into larger landmasses. Constant bumps and grinds from vigorous
tectonic activity built the crust inside and out. The continents continued
growing rapidly up to the end of the Archean 2.5 billion years ago, when they
occupied up to a quarter of Earth’s surface or about 80 percent of the present
continental landmass. During this time, plate tectonics began to operate
extensively. Much of the world as we know it began to take shape.Historical Geology