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Rhyolite
A fine-grained to aphanitic, in many cases light-coloured, volcanic
rock composed of quartz and potassium- and sodium-rich feldspar, +/-
rare sodic plagioclase. It is an extrusive equivalent of a granite.
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Shown is a fine-grained to aphanitic, felsic (i.e., light-coloured)
volcanic rock exhibiting a banding produced by flow when the rock was
molten. This specimen is from Bloody Reach, Bonavista Bay.
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Volcanic Breccia
A breccia is a rock composed of angular fragments in a matrix of finer
particles. A volcanic breccia is produced by a volcanic explosion, i.e.,
a pyroclastic rock.
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The photo shows a coarse-grained volcanic rock consisting of welded,
white, angular fragments of rhyolite in a matrix of volcanic ash and
fine-grained volcanic rock fragments. This specimen is also from the
Bloody Reach area of Bonavista Bay.
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Columnar Joints
A system of polygonal fractures that cracks a rock unit into columns as
a result of cooling of molten magma. It is characteristic of lava flows
and shallow intrusive rocks.
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The hexagonal pillars or columns, such as those shown, have developed
in white rhyolite from the Springdale area, of central Newfoundland.
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Pillow Lava
Rounded to oval, bulbous structures resembling pillows that formed when
molten lava was extruded into water. The moulding of pillows around each
other can be used to determine which are the youngest and oldest rocks
in the sequence(i.e., the younging direction or tops). The younger or
overlying pillows sag down into the V-shaped spaces between the
underlying pillows. Also sediments may fill in the V-shaped spaces. The
top of the sequence is in the direction of the wide part of the V.
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The photo shows basaltic lava forming ovoid, pillow-shaped masses
draped over each other. The pillows are close-fitting, the concavities
of one matching the convexities of another. The spaces between these
pillows are few and small and are filled with either volcanic or
sedimentary material. This sample of pillow lava is from the Indian Bay
Big Pond area, Bonavista Bay.
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