Geology Reference
In-Depth Information
Loose Ends
I don't pretend to understand the universe -
it is a great deal bigger than I am.
Thomas Carlyle
The Earth is very old - present estimates put it at 4.54 billion
years, ±45 million. Most of the rocks we see today have been
recycled many, many times. They have been down to the bottom
of the deepest oceans, buried kilometres below the surface of
the Earth, before being uplifted once again to form the very
highest peaks of the Himalayas, the Andes, the Rockies or the
Alps, where erosion starts them on their weary way again, back
down to the sea. The cycle goes on unceasingly. It has done so
for billions of years in the past and will continue for billions of
years to the future. At the same time the continents have moved
around the globe, e¬ortlessly, like so many birds on migration
- once buried under glaciers at the poles they soon find them-
selves passing the equator
en route
to another destination.
Given all this mobility, it is hardly surprising that initially
Holmes did not find very ancient rocks on Earth. Most of the
evidence has disappeared long ago - but not all. In 1915 he pre-
dicted
'It was in zircon that the hope of the future lay, for that
mineral was widespread in time and place, and stable and
resistant to external forces.'
Indeed, he was right; crystals of
the mineral zircon have been found in Western Australia
