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 dcollins@warren-wilson.edu.
All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion. These photos and discussions may be used for private individual use or educational use. Any commercial use without written permission of the photoprovider is forbidden.
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