Tuesday, February 4, 2014

Two boulders, both alike in dignity...

Last month I wrote several posts about geologic concepts (cross-bedding, original horizontality, and superposition).  Every photo that I shared in those three posts was of sedimentary rocks (sandstones and shales).  This focus on sedimentary rocks was not intentional - I did not intend to ignore the fans of igneous rocks and metamorphic rocks.  In order to make up for this oversight, and to restore peace in the geologic community, I would like to share an image of a igneous rock and one of a metamorphic rock.  Both of these photos were taken over this past summer during my vacation to Maine.

The first rock is a granite boulder with a very coarse texture.  The size of the crystals in this boulder indicate that the magma that formed this rock cooled very slowly, deep below the surface of the earth, allowing large crystals to form.  Three minerals can be easily seen in this boulder:  black crystals of Biotite Mica, smoky gray Quartz, and large tan Feldspar crystals.

A coarse granite boulder

The second rock is a boulder with metamorphic origins.  Metamorphic rocks are rocks that have been changed from one rock type to another by intense heat and pressure (or through chemical interactions).  This boulder is composed of a type of rock known as Ellsworth Schist.  The dark grayish-green rock is called Chlorite.  The pale layers between the the Chlorite are bands of Quartz or Feldspar.  The layering within the rock is called foliation.  Foliations form when the the pressures that cause the rock to metamorphose are unevenly applied.  The folding within the foliations show that this rock was subjected to further pressures perpendicular to the plane of the foliations.

Details of a Ellsworth Schist boulder

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