Catastrophic Landslides Take Over Landscapes


The catastrophists are coming! And they are making yet another case to support the heretical idea that some landscapes are formed by sudden, violent events rather than by the slow and continuous processes beloved by generations of geologists (with the exception, that is, of those flashy, fast-talking volcanologists and seismologists). This time it’s a study in the latest issue of the Journal of Geophysical Research-Earth Surface, which shows the erosive power of debris flows — these are flash floods of rock, lubricated by a spit of water, a.k.a., Nature’s very own runaway freight trains.

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If you’ve never seen a debris flow, here is a video to give you an idea. And for those of you with more patience, here’s another of an experimental debris flow created by Chinese scientists which really shows the erosive power of debris flows.

This new study is a rare attempt to quantify how much debris flows actually cut into bedrock and erode the landscape. The researchers found that over four years the bedrock in their study area was cut down 1.2 to 2.4 inches (30-60 millimeters) by debris flows. They also observed the details of how the bedrock was given a haircut by passing debris flows hammering on the bedrock.

Okay, so maybe less than an inch per year isn’t exactly catastrophic erosion. And, alright, I used the word heretical with my tongue in cheek. But I have my historical reasons. It was only back in the early 19th century that a British geologist named Charles Lyell looked around at the slow processes wearing down the landscape and came to the reverse, but at that time deeply heretical, conclusion that this was how Earth changes. Lyell argued there was no need for biblical catastrophes, just grain-by-grain, raindrop-by-raindrop, and freeze-and-thaw erosion of mountains to make the landscapes seen today.

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The double heresy of Lyell’s view was that for gentle weathering of the Earth to yield such big changes, the planet would have to be far older than the biblically sanctioned 6,000 or so years. Only wild men like the young Charles Darwin could stomach such an unsanctioned idea, which was dubbed uniformitarianism to make it distinct from catastrophism. 

This new work underscores how both ends of the argument are right and wrong. There are, indeed, some landscapes where short-lived, dramatic events dominate the shaping of the land. Other landscapes lack the drama and fit Lyell’s view quite well. What both landscapes share is the need for lots of time to make the land we see today.

Image: Aftermath of a debris flow in the Rocky Mountains (USDA Forest Service).

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