It's a simple phenomenon that scientists have long believed applies to large-scale landforms as well — that is, rivers cut faster into mountains that receive heavier precipitation. It's thought that if rainfall patterns influence how rivers cut into rock, over time, the cumulative erosion and its effects on rock deformation can ultimately control how entire mountain ranges take shape.
However, this seemingly intuitive theory — that precipitation influences how quickly landscapes erode — has been difficult to verify, because many other factors, such as rock strength and tectonic-plate motions, can also influence erosion rates.
Now researchers in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS) have tested this theory by studying the relationship between precipitation and erosion on the Hawaiian island of Kauai, which has one of the world's steepest gradients in annual rainfall. The center of the island receives more than 9 meters (about 350 inches) of rain per year, while its shores remain relatively dry, with as little as half a meter (about 20 inches) of rainfall annually.
The researchers charted the island's precipitation and estimated how much land has eroded over Kauai's 4-million-year history. They found a clear pattern: The more rain a region receives, the more efficiently its rivers cut into rock, forming deep canyons in the wettest areas. The group used these measurements to test a widely used but rarely tested mathematical formula for erosion, and found that when they factored precipitation rates into the equation, they could accurately predict how rivers carved out the island over time.
"We now have empirical support for an idea that has been around for a while," says Ken Ferrier, who led the study while a postdoc at MIT and is now a postdoc at Harvard University. "That idea is that precipitation really should affect how quickly rivers cut through rock, which has many implications for how landscapes evolve."
Ferrier published the results of the study this week in the journal Nature. The study's co-authors are MIT graduate student Kimberly Huppert and Taylor Perron, the Cecil and Ida Green Assistant Professor of Geology in EAPS.
Rain versus the volcano
According to the researchers, Kauai's steep rainfall gradient and uniform volcanic rock make it an "exceptional natural laboratory" for testing the relationship between precipitation and erosion. Wind patterns sweep rain clouds from the ocean toward the peak of the island's volcano, where they rain out most of their moisture before passing over the rest of the island. As a result, annual rainfall is highest in the island's center, with a dramatic drop-off toward the coasts, and is also higher on the side of the island that faces the wind. If rainfall indeed has an effect on erosion, the team reasoned, then the island's erosion rates should exhibit a similarly dramatic pattern.
To test their theory, the researchers first looked at Kauai's current topography, which features large canyons funneling into the middle of the island, with smaller valleys on the outskirts. They then created a map of what the island looked like when it first formed more than 4 million years ago, before erosion altered its surface. To do that, the researchers identified gently sloping, nearly planar surfaces around the island that likely are remnants of the volcano's original terrain. They then used a simple mathematical equation to, in essence, stretch the remnant surfaces together into a roughly conical shape — what Kauai's topography likely resembled when the island first formed.
Ferrier and his colleagues then measured the difference between the modern topography and this reconstructed topography to estimate the amount of rock eroded over time — and divided this difference by the age of the uppermost volcanic flows to calculate an erosion rate. The researchers performed this exercise for more than 13,000 locations along 32 rivers throughout the island, measuring the erosion rates along each river. They then plotted these erosion rates against precipitation rates across the island and found that, after correcting for each river's steepness and the size of its drainage basin, rivers that received more rainfall eroded the land faster than those with less rain.
Feeding the flow
The researchers compared their measured erosion rates to a mathematical equation widely used to predict a river's erosion rate. This equation attributes the erosion rate to the river's steepness and the rate of flow through its channel, but the flow rate is typically assumed to depend only on the size of the river's drainage basin, ignoring spatial differences in rainfall. Other factors that might influence erosion rate, but which are not explicitly included in this equation, include the type of rock being eroded and the kinds of vegetation in the area.
Ferrier used measured precipitation rates to calculate the flow rate at every point along each river, and found a strong correlation between the equation's predicted erosion rates and the measured erosion rates — a result that indicates how much precipitation really matters when it comes to predicting how a landscape will erode."This is exciting because it shows that some bold ideas that have been proposed about landscapes are probably right," Perron says. "For example, if it rains more on one side of a mountain range, it might actually make the mountain range asymmetric and change its width. Just by changing atmospheric processes, you can change how the solid Earth is deforming. Now there is some empirical support for these ideas."
Sarah McDonnell | EurekAlert!
A 3-D look at the 2015 El Niño
29.05.2017 | NASA/Goddard Space Flight Center
'Tiny clocks' crystallize understanding of meteorite crashes
29.05.2017 | University of Western Ontario
The world's highest gain high power laser amplifier - by many orders of magnitude - has been developed in research led at the University of Strathclyde.
The researchers demonstrated the feasibility of using plasma to amplify short laser pulses of picojoule-level energy up to 100 millijoules, which is a 'gain'...
Staphylococcus aureus is a feared pathogen (MRSA, multi-resistant S. aureus) due to frequent resistances against many antibiotics, especially in hospital infections. Researchers at the Paul-Ehrlich-Institut have identified immunological processes that prevent a successful immune response directed against the pathogenic agent. The delivery of bacterial proteins with RNA adjuvant or messenger RNA (mRNA) into immune cells allows the re-direction of the immune response towards an active defense against S. aureus. This could be of significant importance for the development of an effective vaccine. PLOS Pathogens has published these research results online on 25 May 2017.
Staphylococcus aureus (S. aureus) is a bacterium that colonizes by far more than half of the skin and the mucosa of adults, usually without causing infections....
Physicists from the University of Würzburg are capable of generating identical looking single light particles at the push of a button. Two new studies now demonstrate the potential this method holds.
The quantum computer has fuelled the imagination of scientists for decades: It is based on fundamentally different phenomena than a conventional computer....
An international team of physicists has monitored the scattering behaviour of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy.
We can refer to electrons in non-conducting materials as ‘sluggish’. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence...
Two-dimensional magnetic structures are regarded as a promising material for new types of data storage, since the magnetic properties of individual molecular building blocks can be investigated and modified. For the first time, researchers have now produced a wafer-thin ferrimagnet, in which molecules with different magnetic centers arrange themselves on a gold surface to form a checkerboard pattern. Scientists at the Swiss Nanoscience Institute at the University of Basel and the Paul Scherrer Institute published their findings in the journal Nature Communications.
Ferrimagnets are composed of two centers which are magnetized at different strengths and point in opposing directions. Two-dimensional, quasi-flat ferrimagnets...
24.05.2017 | Event News
23.05.2017 | Event News
22.05.2017 | Event News
29.05.2017 | Physics and Astronomy
29.05.2017 | Physics and Astronomy
29.05.2017 | Earth Sciences