The capacity of climate to influence tectonics has been of growing interest for over a century. Likewise, the dramatic effect of rainfall on the evolution of mountainous landscapes is widely debated among geologists.
In a new study, scientists from the University of Bristol calculated the impact of rainfall, giving detailed insights on how peaks and valleys have developed over millions of years. The study, which focused on the mightiest mountain ranges- Himalaya– could forecast the possible impact of climate change on landscapes and, in turn, human life.
Their study also proves rain really can move mountains.
Lead author Dr. Byron Adams, Royal Society Dorothy Hodgkin Fellow at the university’s Cabot Institute for the Environment, said: “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. 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 reliable earth processes are intimately connected.”
The study was based in the central and eastern Himalaya of Bhutan and Nepal. Using cosmic clocks within sand grains, scientists measured the speed at which rivers erode the rocks beneath them.
Dr. Adams said, “When a cosmic particle from outer space reaches Earth, it is likely to hit sand grains on hillslopes as they are transported toward rivers. 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 challenging to produce, and the statistical interpretation of all the data together is complicated.”
Scientists overcame this challenge by combining regression techniques with numerical models of how rivers erode.
They tested several numerical models to reproduce the observed erosion rate pattern across Bhutan and Nepal. Fascinatingly, one of the models accurately predicts the measured erosion rates. The model also allowed them to quantify how rainfall affects erosion rates in rugged terrain.
Research collaborator Professor Kelin Whipple, Professor of Geology at ASU, said: “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.”
The study findings also carry significant implications for land use management, infrastructure maintenance, and hazards in the Himalayas.
In the Himalayas, there is the ever-present risk that high erosion rates can drastically increase sedimentation behind dams, jeopardizing critical hydropower projects. The findings suggest more significant rainfall can undermine hillslopes, increasing the risk of debris flows or landslides, some of which may be large enough to dam the river creating a new hazard—lake outburst floods.
Dr. Adams added: “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.”
The research was funded by the Royal Society, the UK Natural Environmental Research Council (NERC), and the National Science Foundation (NSF) of the US.
- B. A. Adams et al. Climate controls on erosion in tectonically active landscapes, Science Advances (2020). DOI: 10.1126/sciadv.aaz3166