Mountain Answers

by Deb on May 18, 2012

Mt Kilimanjaro

What do you know about mountains? For a look at the questions first, have a look here.

1.     The world’s tallest mountain is a little more complicated than it sounds. Mt Everest is the official highest peak, at 8,848m above sea level. But don’t forget it gets a significant leg up by being part of the massive Himalayan range, and the mountain peak is about 4000m above its base. For the largest mountain base to peak, you are looking at Mauna Kea on the main Hawaiian island. It is only 4,205m above sea level, however because it is a volcano it started right from the sea bed, making it around 10,200m all up.

Mt Everest

Photo of Mt Everest by Pavel Novak

But why choose something arbitrary like sea level? How about looking at the thickness of the crust, the entire unit of rock? In that case the Himalayas win hands down. Continents have different crust to oceans. Because of plate tectonics ocean crust is constantly being recycled from long volcanoes called mid ocean ridges. It is relatively thin and slides under continents when they collide. Continents are the light, massive granite floating on top. They are a bit like foam on a pond – over billions of years they have been moved around and reformed, but because they are so thick it is very difficult to get them to go down. They are more likely to crumple up and build mountain ranges. They are also like the apocryphal iceberg – there is more of them below than on the surface.

The crust under the Himalayas is 50-70km thick. This is where the Indian plate has pushed everything north of it underneath Asia and now is still going. The Himalayas themselves, including Everest, are only the very top creases on these massive mountain building folds. Mauna Kea, on the other hand, sits on a measly 8-16km of ocean crust. In fact, there are two layers of crust under it, and it appears that the deeper layer is the original crust and the upper layer was formed from lava from the volcano, so you could argue that only the top layer is part of the mountain. Even at it’s maximum, this makes Mauna Kea about 26km high – half of the minimum of the Himalayas.

And shall we throw in another contender? The Andes were formed by a plate diving beneath a continent, and as it gets down into the mantle it melts and feeds a line of volcanoes. One of these is Mt Chimborazo in Ecuador. It is 6,268m above sea level, with a very respectable 70km crust underneath it. But it has one more thing going for it – spin. When things spin they tend to flatten out, think of twirling your pizza dough. The same thing happens to the earth, which means it has a slightly larger diameter at the equator than it does from pole to pole.

This means when you stand at the equator your head is around 21km further from the centre of the earth than if you were standing at the poles. For a mountain that’s practically on the equator that’s a large advantage over something as far north as the Himalayas. It makes the top of Mt Chimborazo the point furthest away from the centre of the earth, so in that sense it is the highest point on earth.

Take your pick. Everest, Mauna Kea and Chimborazo can all legitimately argue that they are the world’s tallest mountain.

2.    The Great Dividing Range is actually a series of ranges that dominates the east coast of Australia. Historically the search for a usable pass through the mountains was important in European settlement. It isn’t particularly tall, Australia’s highest mountain Mt Kosciuszko is only 2,228m above sea level, but it is home to many unique species that are remnants of Gondwana.

The Great Dividing Range was built during the Carboniferous about 300 million years ago, when the supercontinent of Pangaea was forming. There were a lot of mountains built at this time, because when continents collide the leading edge of the plates slow down, but the far edge is still being pushed so they have to go somewhere and crumple up. Eastern Australia was on the edge of the supercontinent and had a major subduction zone along the whole coast, with oceanic crust diving under the land. It was the equivalent of the modern Andes, where Pacific crust is diving underneath South America.

Great Dividing Range

When they were forming the Great Dividing Range would have been as high as the modern Andes and Himalayas, but they have been eroded for 280 million years.

3.     Transhumance is a method of pastoralism where people move up and down mountains in different seasons. Their permanent homes and agriculture are lower down the mountains, but in summer they can take their flocks above the tree line onto the high meadows to feed on the grass.

The name ‘Alp’ actually comes from the word for these seasonal pastures – ‘the Alps’ is the mountain range that has lots of high meadows for cattle. Traditionally (and to a certain extent today) there was little competition between animals and crops – crops are grown where the climate and geography is right, animals live around the edges. In Australia grazing happens in mostly arid zones, in other countries it is on steep slopes. In the Alps crops can be farmed in the valleys where it is warmer and the soil is richer. Above the tree line it is too cold and dry for crops but the grasses can support cattle, goats, sheep and pigs during the summer. In winter they come down and are stabled in the valleys.

alp pasture

Photo by Alex1011

Animals are extremely rich sources of high quality food, especially protein and fats. They can also be renewable sources through dairy, and sheep have other uses such as wool. They are land intensive but not labour intensive because a few herders can care for large herds for most of the year. Transhumance, vertical migration with the seasons, allowed far more people to inhabit the Alps by making use of the areas that were not suitable for agriculture.

4.     Pyramidal peaks or horns are tall, often triangular peaks that are shaped by glacial erosion. The permanent ice caps on mountains compress to form glaciers with complex, rotational pressure on them. These can drill out a cirque, a bowl shaped depression with a flatter lip the glacier flows over but a very steep headwall against the mountain. Three or four of these around a rise can erode a distinct peak with three relatively flat faces, the classic mountain peak most of us probably think of.

Matterhorn

The Matterhorn by Stan Shebs

5.     The photo above is a satellite image of Mt Kilimanjaro, a volcano in the Rift Valley zone of East Africa. Unlike mountain ranges such as the Himalayas, Andes and Great Dividing Range this is a zone where a tectonic plate is splitting apart, rather than two pushing together. A large hot spot, like the one under Hawaii, pushes up continental crust and makes a dome. This splits into three rifts where the plate is being torn apart. There is a large dome under the Afar triangle in Ethiopia, the rifts are the Red Sea, Gulf of Aden and East African Rift Valley.

Rift Valley System

As the plates are pushed apart magma comes up from below, forming new crust and fuelling volcanoes. Mt Kilimanjaro has three peaks, two are extinct and one dormant. It is also high enough to have a snow cap with glaciers that are around 11,700 years old. However since 1912 over 80% of the ice has melted, and at current rates it is expected to be ice free in the next 20 years.

And a quick general video on plate tectonics.

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{ 2 comments… read them below or add one }

kelley @ magnetoboldtoo May 18, 2012 at 6:51 pm

I love coming here cause it is always so fascinating and I LEARN something.

Or RElearn as the case may be.

Reply

Deb May 18, 2012 at 9:10 pm

Mwaah! Thank you gorgeous, that makes it worth writing them.

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