Why might erosion make mountain ranges higher? (Geography, Cambridge)

On every continent, mountains tower above us somewhere, vast, solid expressions of the Earth’s geological power. ‘As old as the hills’, the expression goes, yet the world’s biggest mountain ranges are all quite young in geological terms. The rugged ridges of the Rockies, the snowy summits of the Alps, the majestic heights of the Himalayas have all risen from the plains within the last 50 million years. That means the dinosaurs died out long before they ever got a chance to scale even the foothills.

There would have been other great mountain ranges for the flying dinosaurs to soar over, though, mountains now long much reduced by the power of erosion, such as the Scottish Caledonians, the American Appalachians and the Asian Urals, perhaps the world’s oldest mountains. However solid and immoveable they seem, mountain ranges are not permanent features of the Earth’s surface. They are continually being raised up and brought low in endless sequences of geological upheaval and long exposure to the wind and rain.

In the 19th century, geologists came to believe that mountain belts are raised up in periods they called ‘orogens’ that lasted tens of millions of years or so, then stopped. And when these mountain-building phases cease, the rocks, newly exposed to the elements – wind, rain, frost, running water, moving ice – can be worn back down to sea level in only a little more time. In 1899, one of the great pioneers of geology, William Morris Davis, developed the idea of cycles of erosion, in which mountain ranges were raised up, worn down, then uplifted for the cycle to begin again. It seemed so beautifully simple that it was widely accepted.

But since the 1960s, a rather different picture has emerged. First of all, the discovery of plate tectonics has shown how mountain ranges are actually raised up. Plate tectonics show that the Earth’s surface is far from being fixed. Instead it is made from 40–50 vast, continent-size slabs of rock – the tectonic plates – that continually shift this way and that, form and reform.

The world’s longest range is actually the mid-ocean ridge that rises up where plates are moving apart under the sea, allowing new material to well up from the Earth’s interior. But all the high ranges on land occur where plates are moving together, crumpling up the rocks in between like a rug pushed against a wall.

It’s the plates beneath the oceans that do the most moving, and these ‘fold’ mountains are mostly thrown up along the edges of continents, where the ocean plate crunches against the continental plate. The ocean plate is dense compared to the buoyant continental rocks which float like rafts on the semi-molten interior of the Earth, so as it drives against the continent, the ocean plate gets thrust underneath, back into the Earth’s interior.

As it does, a wedge of debris called a ‘terrane’ piles up between the opposing plates – and as the plates go on pushing together, these terranes are piled higher and higher in mountain belts like the American Rockies. Eventually, the ocean plate may vanish entirely into the interior leaving two continents to crunch together headon, and throwing up the biggest ranges of all. That happened with the Appalachians and the Caledonians, and is now happening with the Himalayas as India ploughs relentlessly north into Asia.

In recent years, though, geologists have begun to realise that this is only half the story. For a start, over geological time, rocks are not rigid and brittle. Instead, they slowly flow. So the Himalayas are more like a giant bow wave in front of India than a rumpled carpet or a fractured rubble heap of rock.

Indeed, the whole picture of continents banging together and crumpling rocks up between them is looking rather too simplistic. When British scientist George Airy was surveying the Himalayas in the 19th century, he was surprised to find his plumb line deviating from the perpendicular, revealing that the mass of the mountains extends in deep roots far below the surface. We now know this is true of all great mountain ranges. In fact, mountains are like icebergs floating on the Earth’s interior.

As mountains get worn away by the weather and other forces of erosion, they float up, like a raft losing some of its load, in a process called isostasy. So the ancient Appalachians, mountains geologists once thought were doomed to dwindling away for ever and ever, are actually getting higher by a few centimetres every century. Even though they are far from any mountain-building continental collisions, erosion has lightened the burden of rock and is allowing the Appalachians to rise jauntily above the plains.

Of course, erosion doesn’t just begin the moment mountain-building begins; it’s there right from the start. And the whole process is complicated by the effect of these geological shifts on climate and the forces of erosion. The uprise of the Himalayas, for instance, created a barrier to the steady flow of air over Asia, and kick-started the to and fro of monsoons that bless India with its alternating seasons of drought and torrential rain. The heavy monsoon rains intensify erosion, and wear away Himalayan rock so that, like the Appalachians, they drift on upwards isostatically.

This, combined with the crunch of folding rocks, make the Himalayas the world’s fastest growing mountains, rising at over a centimetre a year. That might not sound so much, but it would add a kilometre in just a hundred thousand years. But the Himalayas’ ascent may be slowing down, even as erosion is lightening the burden, because the Eurasian plate is stretching out and subsiding rather than only thrusting up as India drives on.

It’s clear that the whole business of mountain-making is very far from a simple cycle of building up then laying low. Indeed, it’s a complex and dynamic process involving a host of interacting factors, and erosion allows mountains to rise higher by lightening the burden of rock – but that’s far from the end of the story. However it ends, though, the mountains will be with us for a long time.