19 December 2014
Rocks: the larger they are, the faster they crumble
Posted by kwheeling
By Rex Sanders
Sooner or later, mountains crumble into boulders, boulders crumble into rocks and pebbles, and so on, until wind and rivers carry sand and dust into the ocean, completing the geologic rock cycle.
“But how [rocks] go from the mountain into that ocean bottom, that’s what is not understood very well,” said Jaakko Putkonen, a geologist with the University of North Dakota.
Scientists from UND and other institutions discovered that chunks of rock break off of boulders in Antarctica once every 1,900 years on average, and those smaller chunks break apart once every 510,000 years. Putkonen thinks that heat flow through rocks might be responsible for the dramatic difference in how fast bigger rocks crumble compared to smaller rocks. Knowing how fast boulders break down into smaller and smaller chunks will help geologists understand how the mountains crumble to the sea everywhere.
Putkonen presented the team’s findings in a poster session Wednesday afternoon at the American Geophysical Union Fall Meeting in San Francisco.
“The take home message is that boulders break up, but the rocks and pebbles persist,” said Putkonen.
Boulders in the dry valleys of Antarctica have been sitting there for millions of years, while in most parts of the world boulders disintegrate within a few hundred thousand years, according to Putkonen.
Antarctic dry valleys receive no rain and less than four inches (100 millimeters) of snow every year. And the average air temperature is around minus 6 degrees Fahrenheit (minus 21 Celsius). In such a cold, dry environment Putkonen said the usual forces that break up rocks don’t work or work very, very slowly – forces like water freezing and expanding in cracks or lichens dissolving the rocks for nutrients. And since no visible plants or animals live there, the rocks are easy to see and don’t get pushed around.
The scientists studied 58 boulders that had trails of rocks and pebbles on the hillside below. Putkonen and his colleagues carefully measured the sizes and positions of 419 rocks or pebbles below those boulders. Then they computed how often the boulders generated the smaller chunks and how often those broke down into even smaller chunks.
“Big boulders break down. Once it reaches a critical size, the rate slows down,” said Putkonen.
Based on earlier research using boulders in New Mexico, Putkonen thinks that heat flow through rocks determines the critical rock size where the breakdown rate slows down. In the dry valleys of Antarctica, that critical size of rock is about as big as a fist. Chunks of rock larger than a fist break down faster because one side, warmed by the sun, can get much hotter than the other, creating stress that fractures the rock. Smaller chunks of rock maintain a more uniform temperature and disintegrate about 250 times slower.
Rex Sanders is a science communication graduate student at UC Santa Cruz