Ain't no mountain high enough

Geologists are using raindrops to gauge the changing size of the world's peaks and unlock the secrets of climate change, says Kate Ravilious

Is Everest now at its highest point, or did it once reach even loftier altitudes? What was the greatest height ever reached by Ben Nevis? And was Scafell Pike once taller than it is today?

Is Everest now at its highest point, or did it once reach even loftier altitudes? What was the greatest height ever reached by Ben Nevis? And was Scafell Pike once taller than it is today?

Strangely, the answer could lie inside ancient raindrops. Andreas Mulch, a geologist from Stanford University in California, and colleagues from the universities of Minnesota and Lausanne have developed a technique to work out the changing heights of mountain ranges. They have done it by looking at the chemistry of the rain that has lashed against the mountain peaks over many millions of years.

Being able to estimate the peak height of mountain ranges in the past is important for understanding the climate. "The presence or absence of mountain belts strongly controls rainfall and climate," Mulch says. For example, the monsoon in central Asia only started after the formation of the Himalayan mountain chain, while countries such as Argentina would be much wetter were it not for the existence of the Andes, because they would avoid the rain shadow of the mountains. Plotting how high mountains have been at different times will enable scientists to understand better how mountains interact with the weather and control both local and global climates.

The secret behind Mulch's technique lies in the fact that the make-up of raindrops varies with temperature. Delving inside a raindrop or snowflake reveals hydrogen and oxygen molecules. But the character of these molecules depends upon the temperature at which the raindrop or snowflake was made. Both hydrogen and oxygen have heavy and light versions of their atoms, known as isotopes, and raindrops are preferentially made from the heavier isotopes.

As anyone who has ever walked up a mountain will know, the higher you go, the cooler it gets. Cold air cannot hold as much moisture as warm air, so when damp air travels up a mountain slope it loses much of its moisture. At lower altitudes the heavier isotopes are stripped out to make raindrops; and by the time the air reaches the mountain's top, only the lighter isotopes are left. This means that the rain and snow that falls on the higher slopes is made of lighter isotopes than the rain and snow that falls lower down.

Theoretically, a handful of raindrops from the top of a mountain is all you need to calculate the height of the mountain. Leave a bucket at the top of Ben Nevis for a year and, if it doesn't blow away, the isotopes in the rain and snow should tell you that Ben Nevis is about 1,300 metres above sea level.

Mulch and his colleagues have tried to take this theory a step further to work out how high mountains have been in the past. But going back in time is a bit more difficult. For a start, how do you capture raindrops that might have fallen on mountain peaks millions of years ago?

Most of the rain that falls on a mountain dribbles down into the ground. And some of this water gets incorporated into rocks that lie deep underneath the mountain. As the water trickles past areas where fresh rocks are being crystallised, some of it is sucked in and incorporated into the minerals. Rocks that form underneath mountains absorb some of the mountain rainwater, and so contain a fingerprint of the raindrops that have run down the mountain.

The oxygen part of the fingerprint is not very useful because oxygen is a very common element in most rocks, and so the rainwater signature becomes swamped. It is the hydrogen isotopes that tell the story. Almost all hydrogen in the rocks found beneath the mountains is likely to have come from rainwater. By examining the hydrogen isotopes it is possible to work out what the raindrops were like, and whether they contained heavy or light isotopes.

To test this theory, the scientists have gathered rocks from the Shuswap complex in south-western Canada, which is part of the Rocky Mountains. In particular, they looked for rocks containing the mineral "muscovite", a water-absorbing silicate formed deep (about 10 kilometres) beneath mountains when they start to collapse.

Young mountains are held up by being squeezed together from either side. But once mountains reach a certain height and age, the squeezing force eases and they begin to sag. Finally, the mountains get pulled apart and they sink downwards, splurging out sideways as they go. As the mountains get pulled outwards, lots of cracks and faults are created, making it easier for water to trickle down into the earth. Water-loving minerals, like muscovite, are more commonly formed during this period of extension.

Many millions of years later, erosion brings some of these rocks to the surface. It is these extensional rocks that Mulch and his team are most interested in. Using isotopic dating techniques, they have been able to pick out the oldest extensional rocks in the Shuswap complex, probably formed when the mountains above were at their tallest (about 45 to 50 million years ago). The water that these rocks guzzled when they were being made would have been a mixture of all the raindrops and snowflakes that fell on the mountain range above at that time.

Analysing the hydrogen isotopes from these ancient muscovites suggests that much of the rainfall 50 million years ago occurred at cool temperatures compatible with an average height of about 4,000m. Today, the highest mountains in that area reach about 3,000m, at least 1,000m lower. "Our results show that 50 million years ago in south-western Canada, there must have been a high mountain-range, with peaks higher than 5,000m, or a high plateau of around 4,000m, like we see in present-day Tibet," Mulch says.

Other indicators of "palaeoelevation" back this up. Leaves from plants and trees are known to change their shape according to the moisture content and temperature of the air, therefore fossil leaves can give some clues to the climate they were growing in, and hence the elevation they were growing at. Fossil-leaf assemblages collected from the same area as the Shuswap complex indicate that the mountains were between 1,000m and 2,000m higher then than they are today.

But, compared to fossil leaves, the ancient raindrops are more precise at recording the height of the mountain range they fell on. Leaf shape can depend on many variables, such as soil type, latitude and climate, whereas rain tends to fall just about everywhere. What is more, Mulch's method is directly linked to when the mountains were at their highest, because it analyses the rocks that formed when the mountains started to fall down.

For now, Mulch and his colleagues are working their way along the Rockies to get an idea of the shape of the entire mountain range when it was at its highest. They are also looking at different time periods to try to get a picture of how quickly the Rocky Mountains collapsed. They believe their method can be applied to any mountain range, and are keen to test it on the Himalayas. Indeed, the application of the new technique might settle once and for all the debate among geologists as to whether the Himalayas are still rising, or whether the Tibetan plateau was previously even closer to the sky.

As for Ben Nevis and Scafell Pike, who knows how high they could have been? Ancient raindrops could help to remove the British inferiority complex over our mountains by showing that Britain once had a Himalayan range all of its own.

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