Canyon carved by three short, savage floods

http://www.bbc.co.uk/news/science-environment-31356229

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Geologists have discovered that a huge canyon in Iceland was created by just a few days of catastrophic flooding, separated by thousands of years.

The Jokulsargljufur canyon, 28km long and up to 100m deep, is home to Europe's most powerful waterfall.

By analysing the chemistry of rocks in the canyon's walls, the researchers constructed a timeline of how the canyon was shaped over time.

Most changes seem to have taken place during three brief but brutal periods.

Interestingly, these key flooding events occurred two, five and nine thousand years ago - separated by millennia of relative stability.

Writing in the journal Proceedings of the National Academy of Sciences (PNAS), the scientists say their findings demonstrate the importance of brief but cataclysmic events in shaping landscapes.

"We think of natural environments as being formed over thousands of years, but sometimes they are shaped very suddenly," said Edwin Baynes, a PhD student at the University of Edinburgh and the paper's first author.

"This insight into one of Iceland's magnificent landscapes helps us better understand these processes, and illustrates their legacy."

Cosmic timeline

To build their erosion timeline, Mr Baynes and his colleagues used the fact that atoms on the earth's surface are bombarded by cosmic rays. The concentration of resulting "cosmogenic isotopes" found in a rock surface allows geologists to calculate how long that rock has been exposed to the atmosphere.

This dating process pointed to a series of tumultuous floods in the canyon's history, triggered by the volcanoes that lurk beneath Iceland's largest ice sheet. Eruptions from these volcanoes would have released vast amounts of water from glaciers.

If the resulting flow of water in Jokulsargljufur canyon was above a certain threshold, it would start to topple and transport huge columns of basalt rock - originally formed by cooling lava.

These columns are the structure underlying several large waterfalls in the canyon, including the mighty 54m Dettifoss.

In the rare, destructive floods described by the study, those basalt walls were eaten away so rapidly that the waterfalls were forced upstream by up to 2km - retreating at a startling rate of hundreds of metres within days.

The researchers say their findings underline the importance of considering such rapid change when studying not only landscapes on earth, but also those of other planets.