Of Rivers and Ripples

“Always choose the window seat.” That was the advice given by Taylor Perron, Cecil and Ida Green Assistant Professor of Geology, and the featured speaker at the October 15, 2012, School of Science breakfast at MIT’s Faculty Club. A geomorphologist, Perron studies the processes that create landscapes on Earth, other planets, and moons. More than most, he is keenly interested in the amazing diversity of landscapes that can be seen from an airplane window.

That morning, Perron’s stated goal was to help his audience see familiar landscapes in a different way. “We are all geomorphologists,” he said. In his talk, Perron explored the origin of two common landscapes, river networks and sand ripples, that often contain organized, repeating patterns. Despite their abundance and uniformity, there is no underlying theory to explain how either of these landscapes form.

Using a combination of mathematical models, highresolution topography, and field measurements, Perron and his research team have discovered that branching river networks arise from an instability in the erosional mechanisms that shape Earth’s surface and that the size of the smallest branches depends on rainfall, the strength of the underlying rock, and the gradual downhill movement of the soil.

Sand ripples generated by water waves are a familiar feature of beaches; however, we know remarkably little about how their patterns adjust as waves and tides shift. Perron and his research team have conducted laboratory wave tank experiments and numerical simulations to discover the meaning of “irregularities” or “defects” in wave ripples that have been observed in modern environments and in ancient rocks. They have found that some defects are unique signatures of changes in wave height or water depth, whereas others are similar to patterns observed in many settings with approximately parallel features, such as animal stripes and optical wave fronts.

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