Student Papers in Geology-Cochise College
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Roger Weller, geology instructor

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 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.
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          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


          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

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          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


          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


          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