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Chapter 4 - Volcanism and Extrusive Rocks
midterm number 1
29
Geology
Undergraduate 1
09/20/2009

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Term
pyroclasts
Definition
rock fragments
Term
lava
Definition
magma on earth's surface
Term
extrusive rock
Definition
surface rock resulting form volcanic activity
Term
silicic or felsic rocks
Definition

silica rich 

65% or more

Term
mafic rocks
Definition

silica deficient rocks

50%

Term
intermediate rocks
Definition
chemical content between that of silicic and mafic.
Term
pyroclastic flow
Definition
a mixture of gas and pyroclastic debris that is so dense that it hugs the ground as it flows rapidly into low areas.
Term
rhyolite
Definition

a silicic rock

usually cream colored, tan or pink

made up of mostly feldspar but always includes some quartz

Term
andesite
Definition

crystallizes from an intermediate lava

moderately gray or green color

Term
basalt
Definition
has a relatively low amount of silica (50%)
Term
texture
Definition
refers to a rock's appearance with respect to the size, shape, and arrangement of its grains or other constituents. 
Term
fine grained rock
Definition
one in which most of the mineral grains are smaller than 1 millimeter.
Term
obsidian
Definition

volcanic glass

one of the few rocks that is not composed of minerals.

fine grained or glassy texture distinguishes extrusive rocks from most intrusive rocks.

Term
porphyritic
Definition
one in which larger crystals are enclosed in a groundmass of much finer grained minerals or obsidian. the larger crystals are termed phenocrysts.
Term
vesicles
Definition
cavities in extrusive rock resulting form gas bubbles that were in lava, and the texture is called vesicular.
Term
pumice
Definition
a frothy glass with so much void space that it floates in water.
Term
blocks
Definition

solid rock has been blasted apart by volcanic explosion

pyroclastic fragments are angular, not rounded

Term
bomb
Definition

spindle or lens shaped pyroclast

occurs when lava is ejected into the air.

Term
tuff
Definition
rock composed of fine grained pyroclastic particles
Term
volcanic breccia
Definition
a rock that includes larger pieces of volcanic rock
Term
vent
Definition
opening through which eruption takes place
Term
flank eruption
Definition
lava pours from a vent on the side of a volcano
Term
caldera
Definition
volcanic depression much larger than the original crater
Term
shield volcanoes
Definition
broad, gently sloping volcanoes constructed of solidified lava flows.
Term
cinder cone
Definition
a volcano constructed of pyroclastic fragments ejected from a central vent
Term
composite volcano
Definition
one constructed of alternating layers of pyroclastic fragments and solidified lava flows.
Term
volcanic domes
Definition
steep sided or spine shaped masses of volcanic rock formed from  viscous lava that solidifies in or immediately  above a volcanic vent
Term
columnar structure or jointing
Definition
individual layers may appear to be formed of parallel, vertical columns, mostly six-sided.
Term
pillow structure
Definition
rocks, gernerally basalt, occuring as pillow shapped, rounded masses closely fitted together.
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