THE discovery of rocks in Nepal that are nearly 10 times as old as the Himalayas themselves suggests that today鈥檚 mountains formed on the site of a much more ancient mountain range.
The present-day mountains started to form 55 million years ago when India collided with Asia. But when George Gehrels at the University of Arizona in Tucson analysed rocks from the Himalayas in Nepal, he found that a significant proportion were ancient metamorphic rocks, dating back 500 million years (GSA Today, vol 13, issue 9, p 4). Metamorphic rocks need high temperatures and pressures to form and are often created during mountain building.
Geologists found ancient sediments in the Himalayas in the 1960s that suggested an older mountain range. 鈥淏ut that was as far as they could take it,鈥� says Gehrels. 鈥淲e are adding an earlier chapter in the Himalayas鈥� history.鈥�
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He thinks erosion pared down the ancestral mountains around 450 million years ago and the site was blanketed by sediment until 55 million years ago. Then the collision between the Indian and Asian plates pushed the remnants of the mountains upwards, forming the present-day Greater Himalaya. The Lesser Himalaya formed from new rock as India continued to push north.
Earth scientist John Platt at Imperial College London says he would be interested to see the work extended to other locations along the Himalayas, to discover whether ancient mountains existed along the whole range. Other mountain ranges, such as the Alps, are thought to have formed above ancestral mountains, but the older range is not usually exactly on top of the younger one.