The Grand Canyon may seem gobsmackingly enormous—because it is—but it is not without competition. The Yarlung-Tsangpo River flows eastward across the Tibetan Plateau before cutting south across the Himalayan Range and feeding into the mighty Brahmaputra River. Where it bends to the south, it runs through the Tsangpo Gorge. The river channel is only about 200 meters wide there (or roughly 656 feet), and the steep walls climb to truly awesome heights. At one point, the peaks on either side of the river rise more than 4,000 meters (nearly 2.5 miles) into the air—dwarfing the depth of the Grand Canyon.
This incredible incision is the result of the rapid growth of the Himalayas over the last 10 million years. With great uplift comes great erosion—as much as 5 to 10 millimeters (roughly .2 to .4 inches) of surface rock are removed each year across the region. That adds up in a hurry. Recent discoveries just upstream of the Tsangpo Gorge have revealed some fascinating details of its history, which featured giant floods that could do the work of thousands of years of erosion in a geological instant.
Last year, Ars reviewed a book written by University of Washington geomorphologist David R. Montgomery, who wrote about the mythology and geology of large-scale floods, in part from personal knowledge. In 2004, Montgomery and his colleagues reported their discovery of deposits left by lakes that formed in the Yarlung-Tsangpo River valley when the river was dammed by glaciers.
Ice-dammed lakes can unleash formidable torrents when the ice fails, as Washington’s Channeled Scablands attest. One of the Himalayan floods, dated to roughly 9,000 years ago, released a volume of water greater than Lake Erie. That could have moved boulders as large as 18 meters (60 feet) across and carried 1-meter (3.2 feet)-wide rocks as if they were grains of sand.
The erosive potential of an event like that is obviously significant. At times, when glaciers advanced down into the valley, floods of that magnitude could even have been fairly common, as the ice dams can re-form in just a few decades. So were these floods important contributors to the carving of the Tsangpo Gorge?
To investigate, researchers examined zircon crystals in modern river sediments, as well as in flood deposits. Zircons are remarkably tough and make handy time capsules due to the radioactive uranium they hold inside. In this case, the age of the zircon crystals can tell us where they came from. The Tsangpo Gorge begins near a transition between Tibetan and Himalayan bedrock, with the Himalayan rocks being older. By tallying up the ages of the zircons, the researchers got estimates of the percentage of sediment coming from erosion within the gorge and erosion upstream of it.
A little less than half the sediment currently flowing downstream from the Tsangpo Gorge comes from the gorge itself. Floods that occur after ice-dam failures are found as much as 150 meters (more than 490 feet) above today’s river level, making their sediments easy to recognize. In that material, the amount of material from the gorge rose to over 80 percent. Those floods did a lot of damage, and they did much of it in the gorge.
This may be because the floods caused landslides in the gorge and swiftly carried the material away. The walls of the gorge are so steep that erosion at the base could cause the entire slope above it to fail, making the floods pretty efficient at removing material from the gorge. In fact, according to the researchers’ estimates, just one of those massive floods could do the work of one- to four-thousand-years' worth of “normal” erosion. Depending on just how frequent these floods have been, it seems quite plausible that they had a significant role in the formation of the Tsangpo Gorge.
In Montgomery’s book, he discussed how difficult it was for geologists to consider catastrophic events like this after dispensing with wild legends (like Noah’s flood) in favor of slow, gradual processes. But in some cases, sudden events on a scale unfamiliar to our day-to-day experience have played an important part in shaping the world. The Tsangpo Gorge seems to be one of those cases.
Geology, 2013. DOI: 10.1130/G34693.1 (About DOIs).
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