Tuesday, June 18, 2013

CATACLYSMIC CANYONS


Geologic processes in general take a lonnnng time. For example, the Colorado River has been eroding its way through bedrock and scouring out the Grand Canyon for the last 6 million years – and it is still eroding. The majority of canyons, including the Grand Canyon for most of its period of formation, are created when the rising of bedrock is fast enough to entrap a river in its own valley: as uplift continues, the river has little choice but to continue to erode downwards in the same old river bed, ever deepening as thousands and millions of years go by.

Some the earth’s most astounding and inspiring features are gorges that are deep, steep and narrow: located near to me in Greece, the Portitsa Gorge (photo) is ~170 m deep with vertical walls and is only ~10 - 20 m wide. Many narrow canyons and gorges like this one have formed so rapidly as to be an event of a mere blink of the geologic eye.

These spectacular landforms are created when a dam bursts – natural dams, but dams nonetheless. The dams can be created by landslides, by frozen plugs of ice or glaciers, by rapid deposition of glacial moraines or a combination of all these things. With the formation of such natural dams, a lake forms upstream, growing larger and increasing pressure on the ad hoc dam. With sudden flooding upstream or the melting of ice within the dam, a sudden cataclysmic release of the lake water is triggered, generating powerful floods (in some cases even megafloods). In fluctuating climate conditions such as mark the latest Ice Ages, the scenario of dam formation – lake waters accumulating – dam broken and flooding can repeat numerous times. Each flood and its lode of rocks, boulders, gravels and sand are capable of scouring into the nooks and crannies of the dam, rapidly forming a steep, spectacular gorge. Perhaps in decades, perhaps several hundred years or a thousand or two – again, in geologic terms, in the blink of an eye.

Criteria for recognizing such an origin for canyons include:
-- the lack of river terraces upstream of the canyon suggesting the past presence of a lake.
-- the presence of flood phenomena in the landscape downstream of the gorge.
-- evidence of intensive water erosion within the gorge and along the walls of the gorge: these might include large rather spherical scours into bedrock known as “kolks.”

Not surprisingly, the presence of moraines, landslides, and ice dams seem to recall one to the Ice Ages, and most of the steep, narrow gorges of this nature date from the latest Ice Age.

Ralph Waldo Emerson reminded us to “…adopt the pace of nature: her secret is patience.” But in the case of these spectacular steep canyons and gorges, nature herself seemed impatient and in a rush to create these topographic wonders.


Photo of Portitsa Gorge, Greece with permission from Kostas Petrakis: http://kostaspetrakis.com/

Further reading:
http://www.livescience.com/8312-canyons-form-quickly-gusher-suggests.html
http://gsabulletin.gsapubs.org/content/100/7/1054
http://www.nps.gov/yell/naturescience/geocanyon.htm
http://hugefloods.com/
http://www.nps.gov/grca/naturescience/index.htm



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