Geology Reference
In-Depth Information
that, at more than four
billion
years old, are only a few hundred
million years younger than the age of the Earth. They are the
oldest things so far found on this planet. Today, the oldest rocks
in the world are found in the Slave Province of Canada. They are
just less than four billion years old, but who knows what might
turn up tomorrow?
Moon rocks turned out to be much the same age as Earth
rocks, the oldest of these being just over four billion years, pro-
viding strong evidence that the Earth and moon were formed at
the same time from the same material, although even now the
origin of the moon is not entirely clear. In Kelvin's time George
Darwin, older son of Charles Darwin and a great supporter of
Kelvin's theories of a young Earth (much to his father's annoy-
ance), proposed that the rapid rotation of Kelvin's 'molten
globe' had resulted in a huge tidal bulge which eventually spun
o¬ from the Earth to form the moon. Back in 1913 Holmes
described this theory as
'one of the most fascinating romances
in the domain of cosmogony'
.
More recently it was thought that the moon was 'captured' by
the Earth from a different part of the Solar System, but the
very latest idea is that early in the Earth's formation it colli-
ded with another body of similar composition, but a third of
its size. The huge impact caused the iron core of the two bod-
ies to coalesce while lighter material was projected into space
where it later aggregated to form the moon. In particular, this
idea explains why the Earth has an unusually dense core,
something Holmes speculated upon in 1911 while in
Mozambique, and that the moon has no core at all. But
whichever way the moon was formed, the fact that the Earth,
moon and meteorites all have very similar ages strongly sup-
ports the hypothesis that the whole Solar System formed at
the same time from the same material.
