In a new study from the University of Bristol, scientists have solved a long-standing mystery of how rainfall shapes mountains. While it was not a question that rainfall has a powerful effect on the evolution of mountains, no one has been able to describe exactly how this phenomenon takes place.
The researchers focused their study on the Himalayan mountain range to show how peaks and valleys have developed over millions of years. The pioneering new approach developed by the experts may ultimately be used to predict how climate change will alter landscapes.
“It may seem intuitive that more rain can shape mountains by making rivers cut down into rocks faster. 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,” said study lead author Dr. Byron Adams.
“Both these theories have been debated for decades because the measurements required to prove them are so painstakingly complicated. That’s what makes this discovery such an exciting breakthrough, as it strongly supports the notion that atmospheric and solid earth processes are intimately connected.”
There are an abundance of scientific models that are designed to explain how the Earth works, but it can be difficult to obtain enough quality observations to determine which are the most accurate. The region that includes central and eastern Himalaya of Bhutan and Nepal is one of the most sampled landscapes for erosion rate studies. Using these samples, the scientists analyzed cosmic clocks within sand grains to measure the speed at which rivers erode the rocks beneath them.
“When a cosmic particle from outer space reaches Earth, it is likely to hit sand grains on hillslopes as they are transported toward rivers,” said Dr. Adams. “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.”
“Once we have erosion rates from all over the mountain range, we can compare them with variations in river steepness and rainfall. However, such a comparison is hugely problematic because each data point is very difficult to produce and the statistical interpretation of all the data together is complicated.”
The team addressed this challenge by combining regression techniques with numerical models that represent how rivers erode.
“We tested a wide variety of numerical models to reproduce the observed erosion rate pattern across Bhutan and Nepal. Ultimately only one model was able to accurately predict the measured erosion rates,” said Dr. Adams. “This model allows us for the first time to quantify how rainfall affects erosion rates in rugged terrain.”
“Our findings show how critical it is to account for rainfall when assessing patterns of tectonic activity using topography, and also provide an essential step forward in addressing how much the slip rate on tectonic faults may be controlled by climate-driven erosion at the surface,” said study co-author Professor Kelin Whipple.
The findings could also be used to inform land use management and infrastructure maintenance based on projected hazards in the Himalaya. Here, there is an ongoing risk that high erosion rates will dramatically increase sedimentation behind dams and jeopardize hydropower projects.
In addition, the results of the study indicate that higher rates of rainfall will increase the threat of landslides or debris flows that are large enough to cause lake outburst floods in the Himalayas.
“Our data and analysis provides an effective tool for estimating patterns of erosion in mountainous landscapes such as the Himalaya, and thus, can provide invaluable insight into the hazards that influence the hundreds of millions of people who live within and at the foot of these mountains,” said Dr. Adams.
The research was funded by the Royal Society, the UK Natural Environmental Research Council (NERC) and the National Science Foundation (NSF). Building on this work, Dr. Adams is currently exploring how landscapes respond to large volcanic eruptions.
“This new frontier of landscape evolution modeling is also shedding new light on volcanic processes. With our cutting-edge techniques to measure erosion rates and rock properties, we will be able to better understand how rivers and volcanoes have influenced each other in the past,” said Dr. Adams.
“This will help us to more accurately anticipate what is likely to happen after future volcanic eruptions and how to manage the consequences for communities living nearby.”
The study is published in the journal Science Advances.