Physics Photo of the Week

FEBRUARY 10, 2012

Acidic Erosion of Granite
This photo shows erosion channels that developed in the rock underneath a blanket of moss on Wheeler Mountain in northern Vermont.

Wheeler Mountain and neighboring mountains in northern Vermont are granite batholiths - solid granite that forms dome-like mountains. Granite consists of quartz (SiO2), feldspars (KAlSi3O8), and mica.  Mica with the feldspars is the predominant mineral.  This area of the mountain had been covered with a thick layer of moss that had "recently" disappeared - perhaps in the past half century.  Druing the many years that moss covered the rock, the dead organic matter in the moss layer tended to make the percolating rainwater slightly acidic, thus dissolving some of the rock, namely the feldspars, that are susceptible to acids.

The large mottled appearance of the rock surface is due to the flacking of 2-3 cm diameter chips of the granite due to rain, and repeated freezing and thawing during the long, severe winters.  The actual individual crystallites of the feldspars are actually much smaller - 1-2 mm diameter as revealed by the texture in the erosion channel.

The photo at right shows a larger field of view of the area.  The edge of existing moss can be seen in the top of the photo.

My thanks to WWC geologist Robert Hastings for information on the properties and composition of feldspars. 

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to

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