Geo Jocks: Shrinking Mountains

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Of all Earth’s natural wonders, mountains stand out as one of the most majestic, unignorable phenomena. They are prone to frigid, unpredictable and often uninhabitable climates, with air too thin to breathe at the highest elevations. And on a clear day, a view of icy pinnacles can arguably be one of the most beautiful vistas in the world.

Mountains appear to be static forces unmovable throughout geologic time. In reality, the alpine ranges on today’s earth have been growing for millions of years to reach their current magnitude. Although the Earth’s mountains continue to grow, other forces wear them down. New research suggests mountains may have been taller in the past — specifically, data from Alaska’s St. Elias range show that over the last million years the mountains have been shrinking.

Mountains are born through the activity of tectonic plates, which collide and scrunch up like an accordion over millions of years to create mountain ranges. Erosion keeps mountain growth in check, caused predominantly by rainfall and the movement of glaciers. The most recent research has shown that glaciation during ice ages can wear mountains down faster than they are growing.

The international study, conducted by the Integrated Ocean Drilling Program, investigated the relative impacts of climate and tectonics on orogenic growth in the St. Elias range. Researchers have studied the rocks that make up the St. Elias peaks for years, but the recent study found that the key to the range’s erosion rates lies in sediments buried on the ocean floor, in the Gulf of Alaska. The sediments — in the form of a submarine fan — are carried by rivers down the mountain slopes and deposited into the ocean.

To determine sediment accumulation rates, scientists calculated the age and thickness of the sediment samples at different depths. By comparing the accumulation rate to the rate of uplift from tectonic action — which has remained steady over the last 6 million years — they were able to deduce which force has a greater effect on the range.

The sediments were much younger than the researchers hypothesized, meaning erosion rates have increased in the last several glacial cycles. The most curious find was the time at which erosion rates made a sudden increase — when the glacial-interglacial cycle suddenly changed tempo. Around 1.2 million years ago, the glacial cycles switched from being a short 40 thousand years long, to longer and more intense 100 thousand year cycles. At that point, erosion rates overtook tectonic uplift by 50 to 80 percent, meaning that the mountains started shrinking due to erosion by glaciers.

The St. Elias range study is but one example of glacier-induced high erosion rates. The delicate balance — or battle — of tectonic uplift and erosion rates in other mountain ranges remains unknown. There is much to investigate still, and one can’t help but ask: With the increasing rate of glacial melt in recent years, will tectonic uplift overtake the race once again?