Monadnock Erosion

February 09, 2006


Provided and copyright by: David Albeck
Summary authors & editors: David Albeck; Jim Foster

The view above of Mt Monadnock (3,165 ft or 965 m), New Hampshire, was captured on October 2, 2004. This popular New England mountain is climbed (not all reach the summit) by nearly 100,000 people per year -- more than any other mountain in the U.S. The photo shows that steep slopes aren't necessary for mountainsides to crumble. Exposure to winter temperatures suffices -- freeze-thaw cycles. You can also see that the gaps between the granite blocks are too wide to be explained by mere cracking. The detached blocks have moved downhill, even though the slope is too slight for them to slide freely. Frost wedging may have contributed to widening the gaps initially, but they're now many centimeters wide. The blocks must have been displaced by frost creep. Water seeps under the blocks during freezing episodes, and they're "lifted" as the water expands when it freezes into ice. This allows the blocks to shift slightly downhill each winter.

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