Geology Reference
In-Depth Information
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the nue ardente erupted by Mount Pele in 1902, it seems
that these ancient rocks originated as pyroclastic flows—
hence the name pyroclastic sheet deposits .
They cover far greater areas than any observed during
historic time, however, and apparently erupted from long
fissures rather than from a central vent. The pyroclastic
materials of many of these fl ows were so hot that they fused
together to form welded tuff.
Geologists now think that major pyroclastic flows issue
from fi ssures formed during the origin of calderas. For instance,
pyroclastic fl ows erupted during the formation of a large cal-
dera now occupied by Crater Lake in Oregon (Figure 5.9), and
in the Yellowstone caldera in Wyoming.
Similarly, the Bishop Tuff of eastern California erupted
shortly before the formation of the Long Valley caldera. Interest-
ingly, earthquake activity in the Long Valley caldera and nearby
areas beginning in 1978 may indicate that magma is moving
upward beneath part of the caldera. Thus, the possibility of fu-
ture eruptions in that area cannot be discounted.
What Would You Do
You are an enthusiast of natural history and would like to
share your interests with your family. Accordingly, you plan a
vacation to see some of the volcanic features in U.S. national
parks and monuments. Let's assume your planned route will
take you through Wyoming, Idaho, Washington, Oregon, and
California. What specifi c areas might you visit, and what kinds
of volcanic features would you see in these areas? What
other parts of the United States might you visit in the future
to see additional evidence for volcanism?
those of shield volcanoes. In fact, there are small, low shields,
as well as fi ssure fl ows, in the Snake River Plain.
Currently, fissure eruptions occur only in Iceland. Ice-
land has a number of volcanoes, but the bulk of the island is
composed of basalt lava fl ows that issued from fi ssures. In fact,
about half of the lava erupted during historic time in Iceland
came from two fi ssure eruptions, one in A . D . 930 and the other
in 1783. The 1783 eruption from Laki fi ssure, which is more
than 30 km long, accounted for lava that covered 560 km 2 and,
in one place, fi lled a valley to a depth of about 200 m.
DISTRIBUTION OF VOLCANOES
Most of the world's active volcanoes are in well-defined
zones or belts rather than randomly distributed. The circum-
Pacifi c belt , popularly called the Ring of Fire, has more than
60% of all active volcanoes. It includes volcanoes in the An-
des of South America; the volcanoes of Central America,
Mexico, and the Cascade Range of North America; as well
as the Alaskan volcanoes and those in apan, the Philippines,
Indonesia, and New ealand (
Pyroclastic Sheet Deposits
Geologists have long been aware of vast areas covered by fel-
sic volcanic rocks a few meters to hundreds of meters thick.
It seemed improbable that these could be vast lava fl ows, but
it seemed equally unlikely that they were ash fall deposits.
Based on observations of historic pyroclastic fl ows, such as
Figure 5.15). Also in the
Eurasian
plate
Eurasian
plate
North American
plate
Juan de Fuca
plate
Caribbean
plate
Arabian
plate
Hawaiian
volcanoes
Indian
plate
Cocos
plate
Pacific
plate
South
American
plate
Nazca
plate
African
plate
Australian
plate
Antarctic plate
Divergent plate boundary
(some transform plate boundaries)
Convergent boundary
Volcano
Figure 5.15 Volcanoes of the World Most volcanoes are at or near convergent and divergent plate
boundaries. The two major volcano belts are the circum-Pacifi c belt, commonly known as the Ring of
Fire, with about 60% of all active volcanoes, and the Mediterranean belt, with 20% of active volcanoes.
Most of the rest lie near the mid-oceanic ridges.
 
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