Geoscience Reference
In-Depth Information
continent lies in the heart of northern Canada. In the uninhabited, barren lands about 250
kilometres north of Yellowknife, close to the Acasta River, there stands a lonely shed filled
with geological hammers and camping equipment. Above the door is a rough sign, 'Acasta
City Hall, founded 4 billion years ago'. Some of the rocks from around there have yielded
dates a fraction over 4 billion years old.
They have given up their secrets thanks to grains of the mineral zircon, which traps within
its lattice uranium atoms, which decay into lead. The grains can be disturbed by re-melting,
later growth, and cosmic ray damage, but an instrument developed in Australia known as
a SHRIMP (a Sensitive High Resolution Ion Micro Probe) uses a narrow beam of oxygen
ions to blast atoms off tiny portions of the zircon so that different zones of the grain can
be analysed individually. The centres of some of the grains have given ages of 4.055 bil-
lion years, making them the oldest rocks on Earth, and evidence of continents less than 500
million years from the Earth's formation.
Eternity in a grain of sand
But there is tantalizing evidence for something even older. About 800 kilometres north of
Perth in western Australia, in the Jack Hills, there are rocks of conglomerate, a mixture of
rounded grains and pebbles bound into rock about 3 billion years ago. Among the grains
within that rock are zircons that must have eroded out from even earlier rock. One of these
has given an age of 4.4 billion years, and an analysis of oxygen atoms in the crystals sug-
gests that the Earth's surface at that time must have been cool enough for liquid water to
condense. This research suggests that there were continents far earlier than anyone had ex-
pected, within a hundred million years of the Earth's accretion, and seems to run contrary
to the concept of a partially molten, inhospitable world at that time.
Super-continents of the future
We have spent most of this chapter looking back in time at the continental waltz of the past.
But the continents are still on the move, so what will the world map look like in another 50,
100, or more million years? At first, it is reasonable to assume that things will continue in
their present directions. The Atlantic will continue to widen, the Pacific will contract. The
process which closed the Tethys Ocean will continue, with more earthquakes and mountain
uplift in the hazardous country between the Alps and the Himalayas. Australia will con-
tinue north, catching on Borneo and twisting round to collide with China. Further into the
future, some motions may reverse. We know that a predecessor of the Atlantic opened and
closed in the past, and it is probably inevitable that the Atlantic Ocean crust will eventu-
ally cool, contract, and start to sink again, perhaps subducting under the east coast of the
Americas. Then the continents will bunch up again. Christopher Scotese of the University
