Taiwan's 'vanishing canyon' erasing quake record
A gorge in Taiwan, sliced through rocks only raised by a 1999 earthquake, is disappearing at an unprecedented rate.Continue reading the main story
Geologists say the Daan River, which floods regularly and violently, will wipe the gorge off the map in 50 years.
Massive earthquakes shake this region every 300-400 years, but these results explain why so little evidence remains of previous tremors, making predictions and mapping of fault lines difficult.
The find also allows researchers to witness gorge erosion in fast-forward.
The study is published in the journal Nature Geoscience.Erasing the evidence
"The really cool thing about this place is that it's happening so fast, we can watch it," said Dr Kristen Cook, a geologist at the German Research Centre for Geosciences (GFZ) in Potsdam.
"We can see processes that you can't reconstruct."
At the time of the 1999 Jiji earthquake, a 10m high rock table was pushed upward, creating a 1km-long dam across the Daan River valley in Taiwan's western foothills.
"The amount of uplift was huge," Dr Cook told the BBC. "Imagine one side of your house going up by 10m - it's a big change."
And yet nowhere in the valley is there any evidence for previous upheaval of this kind.
"If you were trying to look in the topography for where this sort of thing might happen, you wouldn't see anything," explained Dr Cook.
Now that she and her colleagues have observed the ferocity of the river's attack on the rocks, Dr Cook is not surprised. "The river can really efficiently remove all of the evidence," she said.
Beginning in 2004, the river overcame the natural dam and material dragged along the river bed cut a brand new gorge which was 1,200m long and 20m deep by 2008.
The formation is known in Chinese as the Grand Canyon of the Daan River, and Dr Cook said it shows similarity to its mighty US namesake in Arizona.
"That's one of the exciting things - we expect the process to be the same, but sped up."Breaking new ground
Dr Cook visited the valley some 51 times while she was working at National Taiwan University from 2009 to 2013, recording detailed GPS measurements every month or two as well as time-lapse photography. Twice each year, she also measured the shape of the whole canyon at 2cm resolution using a laser scanner.
"That takes about a day - you have to set up the scanner in a bunch of different places," Dr Cook said. "You end up with something like 100 million data points."
All these measurements reveal some of the fastest erosion geologists have ever seen: the gorge is being eaten away from its upstream end at a rate of 17m per year.
The breakneck pace is a result of both the relative softness of the rocks, and the regular flooding brought by typhoons.
Furthermore, Dr Cook and her colleagues have identified an entirely new mechanism of canyon erosion, which they call "downstream sweep". It arises because the wide river floodplain suddenly funnels into the gorge, producing sharp bends in the current that grind away the rocks at the canyon's upstream end.
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