Cochise College Student Papers in Geology
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Roger Weller, geology instructor
wellerr@cochise.edu
Metamorphic Rocks
by Sean Fossgreen
Physical Geology
Fall 2009
A Gneiss Story
The process by which the mineral clay becomes the metamorphic rock gneiss (pronounced NICE) is one of geology’s most complex. The process involves many different stages and changes.
Clay
Clay is
a naturally occurring material composed of fine-grained minerals.
Clay shows varying plasticity when
wet, and which can be hardened when dried or fired into pottery.
Clay minerals are typically formed over long periods of time by the gradual
chemical weathering of
rocks. Clay—depending on the region it comes from—is usually a shade of red,
brown, or purple. It has been used for things such as pottery and roof tiles
since ancient times.
Photo from:
http://www.dep.state.fl.us/geology/geologictopics/rocks/common_clay.htm
Shale
The next step in the process is shale. Shale is made of fine grained particles
that have been lithified through compression and cementation—classifying it as a
sedimentary rock. Shale has a tendency to easily split in flaky, fragile
layers. Generally, the cleavage surface is more bumpy and irregular than
slate—with which it is commonly confused. When tapped with a blunt object,
shale produces a dull thud sound.
Photo courtesy of Roger Weller
Slate
After shale, the process then turns to metamorphism. Slate is a flat, usually
dark-colored rock that breaks in very thin, flat layers. Slate is formed
through regional metamorphism, resulting in a higher compression of the
particles which comprise it, rather than re-crystallization of the particles.
The higher compression of the particles in this rock makes it flatter than
shale. When tapped, slate makes a ringing noise like ceramic, as opposed to
shale’s dull sound. Slate’s flat and smooth nature made it perfect for things
such as chalkboards in early schoolhouses. They were also perfect for use as
roof tiles, and many older houses still utilize slate as roof tiles.
Photo courtesy of Roger Weller
Slate Roof: Wales
Photo from:
http://en.wikipedia.org/wiki/File:St_Fagans_Tannery_7.jpg
Phyllite
From Slate, the process moves to phyllite. Phyllite is a foliated metamorphic
rock. Whereas slate has a fairly dull surface because its metamorphic minerals
are extremely fine grained, phyllite has a sheen from tiny grains of mica. This
is so because of the re-crystallization that has begun to take place at this
point in the journey, signifying that not only regional, but contact
metamorphism has begun to play its part. Also, while slate usually breaks in
very flat sheets, phyllite tends to have a corrugated cleavage. This gives
phyllite a slight wavy appearance.
Photo courtesy of Roger Weller
Schist
Phyllite metamorphoses even further to become schist. Schist is one of the most
common metamorphic rocks. Schist always contains more than 50% platy or
elongated mineral formations. The mica crystals (having formed even larger in
schist than in phyllite) create a coarse texture. In addition to a change in
texture, the mica crystals in schist give it a more sparkly appearance than any
of its protoliths, beginning with clay.
Photo courtesy of Roger Weller
Gneiss
The final step of this process is gneiss. After being even further
metamorphosed—through both contact and regional metamorphism—schist is
transformed into gneiss. Gneiss is usually medium- to coarse-grained and mostly
re-crystallized. Many times, gneiss looks very similar to granite. This is
because many of the same minerals are found in both rocks. When they are almost
indistinguishable, the gneiss is referred to as “granitoid” or “granite-like.”
Photo courtesy of Roger Weller
Cartoon courtesy Roger Weller
Sources:
http://skywalker.cochise.edu/wellerr/aawellerweb.htm
http://geology.about.com/
http://hyperphysics.phy-astr.gsu.edu/Hbase/Geophys/gneiss2.html
http://hyperphysics.phy-astr.gsu.edu/hbase/geophys/schist.html
http://www.britannica.com/EBchecked/topic/458510/phyllite
