This granite in the
Sierra Nevada Mts. of California has been weathered chemically.
The mineral feldspar reacts with water, especially when warmed by the
sun to form clay minerals. As you know clay is not particularly
strong and so the rock begins to fail and the remaining crsytals of
quartz, feldspar and biotite fall from the rock and collect as a kind
of sand below.
Many rocks of Yellowstone National Park have been weathered by the
infiltration of water heated by magma beneath the park. The hot water
reacts with feldspars in the rocks to form clay. In the upper photo
cliffs and pinnacles of weathered but more competent rock rise from
rock that has been weathered to a fine grit. The semi-circle in the
center of the lower photo was form by the slippage of grit and weakened
rock down the steep slope of the canyon wall. This is the
beginning of the transport or erosion of material that will carry the
weathered particles to a sedimentary basin, most commonly the ocean
floor.
