Geoscience Reference
In-Depth Information
14. The changing map of the Earth's continents over the last 200 million years.
The further back in time you go, into the Pre-Cambrian, the less clear the picture becomes
and the harder it is to pick out the land masses we know today. For example, in the Ordovi-
cian period, around 450 million years ago, Siberia was near the equator and most of the
land masses were gathered in the southern hemisphere, with what is now the Sahara Desert
near the South Pole. Late in the Pre-Cambrian, Greenland and Siberia were far south of the
equator, Amazonia was almost at the South Pole, whilst Australia was well into the north-
ern hemisphere.
One of the current record-holders for long-distance drifting must be the land known as
the Alexander Terrane. It now forms a large portion of the Pan Handle of Alaska. About
500 million years ago, it was part of eastern Australia. The palaeomagnetic evidence in the
rocks includes an inclination from the horizontal, dipping down into the Earth, which re-
veals the latitude at which the rock formed. The steeper the inclination, the higher the latit-
