Geology Reference
In-Depth Information
Volcanoes are found in various shapes and sizes, but all
form where lava and pyroclastic materials are erupted
from a vent.
The summits of volcanoes have either a crater or a much
larger caldera. Most calderas form following voluminous
eruptions, and the volcanic peak collapses into a partially
drained magma chamber.
Shield volcanoes have low, rounded profi les and are com-
posed mostly of mafi c fl ows that cool and form basalt.
Small, steep-sided cinder cones form around a vent where
pyroclastic materials erupt and accumulate. Composite
volcanoes are made up of lava fl ows and pyroclastic mate-
rials of intermediate composition and volcanic mudfl ows.
Viscous bulbous masses of lava, generally of felsic com-
position, form lava domes, which are dangerous because
they erupt explosively.
Fluid mafi c lava from fi ssure eruptions spreads over large
areas to form a basalt plateau.
Pyroclastic sheet deposits result from huge eruptions of
ash and other pyroclastic materials, particularly when
calderas form.
Geologists have devised a volcanic explosivity index
(VEI) to give a semiquantitative measure of the size
of an eruption. Volume of material erupted and
height of the eruption plume are criteria used to
determine the VEI; fatalities and property damage
are not considered.
Approximately 80% of all volcanic eruptions take place in
the circum-Pacifi c and the Mediterranean belts, mostly at
convergent plate boundaries. Most of the rest of the erup-
tions occur along mid-oceanic ridges or their extensions
onto land.
The two active volcanoes on the island of Hawaii and
one just to the south lie above a hot spot over which the
Pacifi c plate moves.
To effectively monitor volcanoes, geologists evaluate
several physical and chemical aspects of volcanic regions.
Of particular importance in monitoring volcanoes and
forecasting eruptions is detecting volcanic tremor and
determining the eruptive history of a volcano.
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Important Terms
aa (p. 112)
basalt plateau (p. 119)
caldera (p. 115)
Cascade Range (p. 123)
cinder cone (p. 116)
circum-Pacifi c belt (p. 122)
columnar jointing (p. 112)
composite volcano (stratovolcano)
(p. 117)
crater (p. 114)
fi ssure eruption (p. 119)
lahar (p. 117)
lava dome (p. 117)
lava tube (p. 111)
Mediterranean belt (p. 123)
nuée ardente (p. 117)
pahoehoe (p. 112)
pillow lava (p. 112)
pyroclastic sheet deposit
(p. 122)
shield volcano (p. 115)
volcanic ash (p. 112)
volcanic explosivity index (VEI)
(p. 128)
volcanic tremor (p. 128)
volcanism (p. 108)
volcano (p. 114)
Review Questions
1. Water-saturated fl ows of volcanic debris called _____
are common on _____ volcanoes.
a. _____ nuée ardentes/shield;
b. _____ lava fl ows/lava dome;
c. _____ lahars/composite;
d. _____ pillow lava/submarine;
e. _____ fi ssure eruptions/cinder cones.
2. The shaking that takes place when magma moves
beneath the surface is called
a. _____ volcanic tremor;
b. _____ columnar joints;
c. _____ pyroclastic intrusion;
d. _____ basalt plateau;
e. _____ volcanic explosivity index.
3. A lava fl ow made up of angular blocks and fragments is
a/an _____ fl ow.
a. _____ lava tube;
b. _____ aa;
c. _____ pahoehoe;
d. _____ lahar;
e. _____ caldera.
4. The chain of volcanoes stretching from northern Cali-
fornia into British Columbia, Canada, is known as the
a. _____ circum-Pacifi c Mountains;
b. _____ mid-oceanic ridge volcanic chain;
c. _____ Rocky Mountains;
d. _____ Cascade Range;
e. _____ Columbia River basalt.
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