Making mountains

The mountain ranges we see today include some still under construction, others where the process has long ended and whose erosion reveals the innards of the mountain-making process, and yet others at every stage in between. The one thing we never see is a stable mountain range. Once it is built, a mountain is doomed to be removed, however long it takes.

Sometimes the force involved in raising mountain ranges is a strange one. For example, remember the idea of continents as ships floating in the mantle rock below. It the ship throws its cargo overboard, it will rise in the water. By the same logic, Scandinavia has been becoming more mountainous at a rate of a few centimetres a year since the end of the last ice age. It is springing upwards because of the weight of ice removed from it, a process called isostatic rebound. Nor is the effect confined to Scandinavia. Around Hudsons Bay in Canada, the isostatic rebound has totalled about 350m in the past 8000 years.

However, isostatic rebound is not a major means of mountain manufacture – or orogeny as the purists call it. The main method by which big mountain ranges such as the Alps, the Rockies or the Himalayas are formed is collision. The Himalayan orogeny – which has given us Everest (Chomolungma), the highest mountain of todays world at 8850m – is the result of the northward movement of the Indian plate as it crunches into the Eurasian plate. Satellite images of the Himalayas shows the astounding way in which they rise almost vertically from the plains of India.

The Himalayan orogeny is still going on. However, many past orogenies have now been analysed in detail. The first major orogeny to be described – in the nineteenth century – was the formation of the Alps. The detail in which it is known has made it the model for other mountain-making episodes. The Alps were where the key concept of nappes, folded-over bodies of rock formed by tectonic collisions, was developed. The word comes from the French for a tablecloth and to envisage one, spread a tablecloth out and push on it from one side. Then imagine the folds you see as kilometres-thick zones of rock. And note that the bottom half of the fold will be completely upside down by comparison with the original flat strata. There have been Alpine folding episodes from about 40 million to 5 million years ago, exposing deeper rocks such as granites in the more southerly Alps. The Alps as we see them today are about twice as high as the Urals in Russia (Mont Blanc 4810m, Mount Narodnaya 1895m) and are less than 200km from south to north, the direction of most of the so-called overthrusting that forms the nappes. But measuring the nappes shows that the folding has used up about 200km of crust. Look at the Alps today and you also see vast areas of newer sediment caused by the erosion of these big new mountains. Come back in a few hundred million years and you may well find something like the Urals, or even the mountains of Scotland, as we see them today, with these rocks gone and new ones exposed that today are many kilometres below the surface.

The worlds biggest mountains have been formed either by such collision processes or at subduction zones, where material is vanishing into the mantle. This process produces deep ocean trenches such as the Marianas in the Pacific or the trench running along the western side of South America. But the area behind the trench also becomes compressed as all that ocean floor slides beneath it The Andes are the result of this compression, enhanced by the new molten rock formed from crust material dragged deep into the Earth. Hence all the volcanoes and earthquakes in that region.

A good example of this process is the formation of the Urals. Politically the Urals are regarded as the boundary between Europe and Asia. But there is geological reality underlying this convenient line on the map. They were formed as a long chain of mountains over 2500km long but only 150km wide at most during the late Palaeozoic era, when plate tectonic movement closed the ocean between Asia and the predecessors of Europe.

The detailed mapping of the processes responsible for the formation of the Urals shows that it began with a third continent roughly equivalent to present-day Kazakhstan. The western edge of this continent was active – picture volcanic islands offshore – while the edge of Europe was passive, and the ocean between them closed as the ocean floor was sucked below Kazakhstan and crust material piled up to mountain height. Only later did the Asian plate arrive to complete the story.

All this happened so long ago that the Urals we see now are essentially the roots of the huge range that was first thrown up. This means that we can see igneous rocks such as granite which were produced by melting and whose large crystals show that they solidified deep in the Earth. The chemical composition of granite is the same as the lava andesite but, having cooled at or very near the surface, andesite has far smaller crystals. The presence of metamorphic rocks – sedimentary rocks altered by the heat and pressure of the deep Earth – are another sign that the Urals as we see them today are the remains of a range of mountains that must have been of Alpine proportions when new. These revealed depths also house many valuable mineral veins, including ores of metals such as gold, silver and platinum.

Don't forget that these orogenies are not just local events. The Downs, a chain of beautiful hills across south-east England, were created by the Alpine orogeny, but at a distance where its power was far less than at its centre. The same applies to many of the gentler but still interesting parts of the Earths topography.

Stretch and bend

Not all mountains are produced by processes on a global scale. More localized stretching or compressional forces in the Earths crust can make new mountains, or lowland areas such as the Rhine Valley in Germany, which is more than 200m deep in places. The names here are German too. A dropped-down area bounded by faults – essentially where stretching has made the crust give way – is a graben, as with the Rhine Valley, and the opposite, where compression has raised an area, is a horst.

Despite its placid and stable appearance, the Rhine Valley is an active area of the Earths crust. Towards its southern end, the city of Basel was shaken by a large earthquake in 1356. Recent research suggests that the whole southern end of the Rhine Valley has not ceased moving and could be subject to another major shock. In the meantime, the steep-sided valleys terraces are one of the great wine-producing areas of the world.

In addition, isostatic effects like those that caused land to rebound after the last ice age can kick both ways. Thus a large area of the Earths crust that is subject to significant amounts of deposition of sediment can subside as the weight builds up. There can be faulting, where all or part of the structure slips en masse, but it is also possible for the whole lot to bend so that folding of the rocks is seen rather than faulting.

Another handy concept is epeirogeny, the opposite of orogeny. It involves the shifting of large amounts of crustal matter up or down with- out significant horizontal movement like the folding that makes major mountain chains. The resulting terrain is largely free of faulting except at the edges. This large stable area is known as a craton. Epeirogenic episodes seem to be associated with changes in the underlying mantle. The term is only applied to large areas of land or of the ocean floor.

The rolling hills of South Africa are a classic epeirogenic landscape. Little changed in millions of years, they have been created by successive episodes of uplilt separated by long periods of erosion. The result
is that the surface has been gradually scoured away by wind and water, and about half a dozen successive land surfaces of differing ages, known as erosion surfaces, can be identified. The process has had big economic effects. Millions of years of erosion have eaten away so much material that diamonds, formed deep inside the Earth, can now be mined there.