Scientists reveal why world's highest mountains are at the equator

Ice and glacier coverage at lower altitudes in cold climates more important than collision of tectonic plates, researchers find
Alok Jha
guardian.co.uk, Wednesday 12 August 2009 18.00 BST
Scientists have solved the mystery of why the world's highest mountains sit near the equator - colder climates are better at eroding peaks than had previously been realised.
Mountains are built by the collisions between continental plates that force land upwards. The fastest mountain growth is around 10mm a year in places such as New Zealand and parts of the Himalayas, but more commonly peaks grow at around 2-3mm per year.
In a study published today in Nature, David Egholm of Aarhus University in Denmark showed that mountain height depends more on ice and glacier coverage than tectonic forces. In colder climates, the snowline on mountains starts lower down, and erosion takes place at lower altitudes. At cold locations far from the equator, he found, erosion by snow and ice easily matched any growth due to the Earth's plates crunching together.
Egholm used radar maps of the Earth's surface, created by Nasa in 2001, to examine the height of all the world's mountains at a single point in time. The analysis showed that mountains had a significant land area up to their snowlines, after which it dropped rapidly. In general, mountains only rise to around 1,500m above their snow lines, so it is the altitude of these lines — which depends on climate and latitude — which ultimately decides their height.
At low latitudes, the atmosphere is warm and the snowline is high. "Around the equator, the snowline is about 5,500m at its highest so mountains get up to 7,000m," said Egholm. "There are a few exceptions [that are higher], such as Everest, but extremely few. When you then go to Canada or Chile, the snowline altitude is around 1,000m, so the mountains are around 2.5km."
"What we show is that, once the mountain is pushed up across the snow line, a very effective erosion agent comes into play and that is represented by glaciers," said Egholm. "It's so effective that it can keep pace with any tectonic uplift rate that we have on the Earth today." Below the snowline, rivers and rock falls are the main erosion agents.