Geoscience Reference
In-Depth Information
Deeper time
The list of geological periods that is often shown in topics goes back only about 600 million
years to the start of the Cambrian period. But that ignores 4 billion years of our planet's his-
tory. The trouble with most Pre-Cambrian rocks is that they are, as Professor Bill Schopf of
the University of California puts it, fubaritic - fouled up beyond all recognition. The con-
stant tectonic reprocessing of the Earth from within, and the relentless pounding of weath-
er and erosion from above, mean that most of the Pre-Cambrian rocks that survive at all
are deeply folded and metamorphosed. But on most clear nights you can see rocks that are
more than 4 billion years old - by looking up at the Moon rather than down at the Earth.
The Moon is a cold, dead world with no volcanoes and earthquakes, water or weather to
resurface it. Its surface is covered with impact craters, but most of those happened early in
its history when the solar system was still full of flying debris.
The Pre-Cambrian rocks that do survive on Earth tell a long and fascinating story. They are
not, as Darwin had supposed, devoid of the traces of life. Indeed, the end of the Pre-Cam-
brian, from about 650 to 544 million years ago, has yielded a rich array of strange fossils,
particularly from localities in southern Australia, Namibia, and Russia. Prior to that there
seems to have been a particularly severe period of glaciation. The phrase 'snowball Earth'
has been used, conveying the possibility that all the world's oceans froze over. Inevitably,
that would have been a major setback for life, and there is scant evidence for multicellu-
lar life forms before this. But there is abundant evidence for microorganisms - bacteria,
cyanobacteria, and filamentous algae. There are filamentous microfossils from Australia
and South Africa that are around 3,500 million years old, and there is what looks like the
chemical signature of life in carbon isotopes in rocks from Greenland that are 3,800 million
years old.
During the first 700 million years of its history, the Earth must have been particularly
inhospitable. There were numerous major impacts far bigger than that which may have
killed the dinosaurs. The scars of this late heavy bombardment can still be seen in the great
Maria basins on the Moon, which are themselves giant impact craters filled with basalt lava
melted by the impacts. Such impacts would have melted much of the Earth's surface and
certainly vaporized any primitive oceans. It is possible that the water on our planet today
came from a subsequent rain of comets as well as from volcanic gases.
Dawn of life
The early atmosphere of Earth was once thought to have been a mixture of gases such
as methane, ammonia, water, and hydrogen, a potential source of carbon to primitive life
forms. But it is now believed that strong ultraviolet radiation from the young Sun would
