Plate tectonics tends to dominate the common view of mountain formation: where two plates meet, rock is pushed up. However, massive shifts in the Earth’s crust can’t take full credit for this awe-inspiring process.
As Alan Collins, a geologist at Australia’s University of Adelaide puts it: “Mountains act like lungs do in humans. They are the place where material is transferred from the deep earth to the Earth’s surface through rapid and efficient erosion.”
This transfer of material depends on the ongoing processes on the surface, meaning that mountains can also be created, shaped and destroyed by climate and erosion.
But there is an ongoing debate about how exactly climate, tectonics and mountain ranges interact, due to an imperfect understanding of the balance between the removal of rock through erosion and the replacement of rock by uplift.
Now, research published in the journal Science Advances has precisely captured the dramatic erosional effect of rainfall, showing how it sculpts peaks and valleys over millions of years.
“It may seem intuitive that more rain can shape mountains by making rivers cut down into rocks faster,” says lead author Byron Adams, from the University of Bristol, UK. “But scientists have also believed rain can erode a landscape quickly enough to essentially ‘suck’ the rocks out of the Earth, effectively pulling mountains up very quickly.”
By removing an enormous amount of material from the surface, water can create a pressure gradient in the Earth’s crust, causing rocks to “flow” and eventually push mountains upwards.
The measurements needed to determine which is the most accurate model are complex and painstaking to make. But Adams and his team were up to the challenge.
Focusing on the dynamic region of the central and eastern Himalayas, they took a series of samples of sand grains with the aim of finding out how quickly rivers erode rocks – by looking at “cosmic clocks” within the grains.
“When a cosmic particle from outer space reaches Earth, it is likely to hit sand grains on hillslopes as they are transported toward rivers,” Adams explains.
“When this happens, some atoms within each grain of sand can transform into a rare element. By counting how many atoms of this element are present in a bag of sand, we can calculate how long the sand has been there, and therefore how quickly the landscape has been eroding.”
After taking samples from across the ranges, the team tested a series of numerical models to find one able to accurately predict the erosion rates they measured. This allowed them to quantify – for the first time – how rainfall affects erosion in rugged terrain.
Adams believes the discovery is an exciting breakthrough, because “it strongly supports the notion that atmospheric and solid earth processes are intimately connected”. This could have real implications for understanding the history of the Earth, and even life itself.
Collins also sees the technique as a major advance. “If it proves to work generally – and their study is the largest of its type done so far – it can be used to quantify how elements are transferred into the earth surface systems from the deep planet.”
“Life needs nutrients,” Collins adds. “Most of these, from phosphorous that makes up the framework of DNA to essential elements such as molybdenum or selenium, come from the deep earth and from erosion of rocks in mountain belts.
“If we better understand how this transfer is facilitated in these mountain belts, we are better able to understand how this feedback occurs and how our earth evolved.”
For Adams and his team, the next step is to address whether the erosion rates they measured are big enough to drive the “crustal flow” of rocks.
Intriguingly, Adams is also working on a project that uses this type of research to help forecast the impact of the changing climate on mountain landscapes.
Lauren Fuge is a science journalist at The Royal Institution of Australia.
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