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Glossary
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Below you will find key words and concepts that you should remember from this chapter.
andesite  Fine-grained igneous rock of intermediate composition. Up to half of the rock is plagioclase feldspar with the rest being ferromagnesian minerals.
(See page(s) 253)
basalt  A fine-grained, mafic, igneous rock composed predominantly of ferromagnesian minerals and with lesser amounts of calcium-rich plagioclase feldspar.
(See page(s) 253)
block  Large angular pyroclast.
(See page(s) 255)
bomb  Large spindle- or lens-shaped pyroclast.
(See page(s) 255)
caldera  A volcanic depression much larger than the original crater.
(See page(s) 255)
cinder cone  A volcano constructed of loose rock fragments ejected from a central vent.
(See page(s) 258)
circum-Pacific belt  Major belt around the edge of the Pacific Ocean on which most composite volcanoes are located and where many earthquakes occur.
(See page(s) 261)
columnar structure (columnar jointing)  Volcanic rock in parallel, usually vertical columns, mostly six-sided; also called columnar jointing.
(See page(s) 266)
composite volcano (stratovolcano)  A volcano constructed of alternating layers of pyroclastics and rock solidified from lava flows.
(See page(s) 260)
crater  A basinlike depression over a vent at the summit of a volcanic cone.
(See page(s) 255)
extrusive rock  Any igneous rock that forms at Earth’s surface, whether it solidifies directly from a lava flow or is pyroclastic.
(See page(s) 244)
fine-grained rock  A rock in which most of the mineral grains are less than one millimeter across.
(See page(s) 253)
flank eruption  An eruption in which lava erupts out of a vent on the side of a volcano.
(See page(s) 255)
intermediate rock  Rock with a chemical content between felsic and mafic compositions.
(See page(s) 250)
lava  Magma on Earth’s surface.
(See page(s) 244)
mafic rock  Silica-deficient igneous rock with a relatively high content of magnesium, iron, and calcium.
(See page(s) 250)
magma  Molten rock, usually mostly silica. The liquid may contain dissolved gases as well as some solid minerals.
(See page(s) 244)
Mediterranean belt  A major concentration of earthquakes and composite volcanoes that runs through the Mediterranean Sea, crosses the Mideast and the Himalaya, and passes through the East Indies.
(See page(s) 261)
obsidian  Volcanic glass.
(See page(s) 253)
phenocryst  Any of the large crystals in porphyritic igneous rock.
(See page(s) 253)
pillow structure (pillow basalts)  Rocks, generally basalt, formed in pillow shaped masses fitting closely together; caused by underwater lava flows.
(See page(s) 266)
plateau basalts  Layers of basalt flows that have built up to great thicknesses.
(See page(s) 265)
porphyritic rock  An igneous rock in which large crystals are enclosed in a matrix (or ground mass) of much finer-grained minerals or obsidian.
(See page(s) 253)
pumice  A frothy volcanic glass.
(See page(s) 254)
pyroclastic flow  Turbulent mixture of pyroclastics and gases flowing down the flank of a volcano.
(See page(s) 244)
pyroclast  Fragment of rock formed by volcanic explosion.
(See page(s) 251)
rhyolite  A fine-grained, felsic, igneous rock made up mostly of feldspar and quartz.
(See page(s) 253)
shield volcano  Broad, gently sloping cone constructed of solidified lava flows.
(See page(s) 256)
silicic (felsic) rock  Silica-rich igneous rock or magma with a relatively high content of potassium and sodium.
(See page(s) 250)
tuff  A rock formed from fine-grained pyroclastic particles (ash and dust).
(See page(s) 255)
vent  The opening in Earth’s surface through which a volcanic eruption takes place.
(See page(s) 255)
vesicle  A cavity in volcanic rock caused by gas in a lava.
(See page(s) 254)
viscosity  Resistance to flow.
(See page(s) 250)
volcanic breccia  Rock formed from large pieces of volcanic rock (cinders, blocks, bombs).
(See page(s) 255)
volcanic dome  A steep-sided, dome- or spine-shaped mass of volcanic rock formed from viscous lava that solidifies in or immediately above a volcanic vent.
(See page(s) 263)
volcanism  Volcanic activity, including the eruption of lava and rock fragments and gas explosions.
(See page(s) 244)
volcano  A hill or mountain constructed by the extrusion of lava or rock fragments from a vent.
(See page(s) 244)
lava flows  river or sheet-like extrusions of lava on Earth's surface.
(See page(s) 244)
texture  the size, shape, and arrangement of mineral or other constituent grains in a rock.
(See page(s) 253)







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