STOP 2: Close-up views of the garnet schist, the most common rock of the Wissahickon Schist.
The garnet and mica schist of the
Wissahickon is a metamorphic rock that was once a shale, or a sedimentary
rock composed primarily of clay -- fine grained sediments that will only
fall to the bottom if the water carrying them becomes still. Heating
during burial and the additional pressure created by burial causes the
minerals in the shale to undergo chemical changes. One of the dominant
changes is the release of water from the clay and growth of minerals in
mica family. All micas are platy minerals that grow so that the plates
are oriented in the rock to minimize differences in pressure. Imagine
a pile of cards on a table. As you bring your hands around the cards
to pick them up, the cards align themselves and eventually the deck is
oriented so that the flat surface of the cards parallel your hands.
Mica grains will do the same thing as they grow. The parallel alignment
of mica grains gives the rocks a metamorphic layering (foliation) that
is visible in the photos. In the Wissahickon schist, the layers are
not flat because the rocks have been deformed during and after growth of
the muscovite or clear, silvery mica that is the dominant mineral in them.
You can see the wavy surface created by folded plates of mica.
Other minerals
also grow in response to heating and burial. In the rocks of the
Wissahickon you will find garnet -- they are dark purple or red and look
like small soccer balls, staurolite -- similar in color but rectangular
in shape, and kyanite -- a light blue to whitish mineral that forms long
thin rectangles or needles. The best place to find the kyanite is
near areas rich in quartz like those seen within the schist of the second
photo.
Geologists
use the set of minerals found in the rocks and their chemistry to estimate
temperatures and pressures that affected the rock during metamorphism.
The minerals found here imply temperatures higher than 550° C at pressures
equivalent to burial 20 to 25 km beneath the surface of the earth.
This of course raises the question of how did sedimentary rocks move from
the surface to such great depths.