Deep underneath newly-forming mountains, the rocks can heat up to over 1000C. Geologists have suspected this happens for some time. Now a study of ancient rocks reveals that they really were once heated to these unimaginable temperatures.

Mountains form when rocks are forced upwards. But before this happens, the rocks that will make up the mountains first melt, over many millions of years.

In effect, these forming mountains have roots, "rather like icebergs floating in the sea", says Simon Redfern at the University of Cambridge in the UK, who was not involved in the new study. "If you build up a mountain it needs to have an equivalent amount of mountain root at its base."

Now a new study, published in Earth and Planetary Science Letters, gives the strongest evidence yet that the temperature in these deep parts of the Earth's crust can exceed 1000C.

A team of geologists studied samples of mountainous rock from the Eastern Ghats in east India. They tried to figure out how hot the rocks had once been.

When the Eastern Ghats were forming, the rocks would have been about 30km down. There they would have been subjected to such intense heat and pressure that they were partially liquid.

The higher the temperatures got, the more liquid rock leaked out by rising to shallower levels of the crust. These rocks then cooled and solidified into the mountains.

This would have happened many times with continuing rises in temperature, says co-author Michael Brown of the University of Maryland in College Park.

This process of melting and cooling changed the minerals in the rocks, leaving clues to how hot they were.

According to Redfern, knowing the temperatures of rocks will help us understand how planets work.

That could prove useful. Some rocks are more likely to break and fracture like glass, leading to violent earthquakes. Knowing the temperatures rocks have been exposed to should help us understand this, says Redfern.

Understanding mountain formation could also help us find deposits of valuable minerals, such as those containing rare metals, which ultimately come from deep inside the Earth.

"Most of the economically important elements we have become concentrated in the crust," says Brown. "The more we understand how everything works, the more likely we are able to make predictions of where to find these resources in the future."

"Ultimately, this influences the distribution of resources as well as the distribution of natural hazards," says Redfern.