and was buried under thick layers of sediment that today fill
the Mississippi Valley. From an economic standpoint, earth-
quakes are a subject of some concern to the United States.
It may be only a matter of time before one or more major
U.S. cities will be destroyed, either in part or in whole, by an
earthquake. Property damage alone in such an event would
reach tremendous values, and the economic impact of a
major earthquake would be amplified by America’s growing
dependence on services and products furnished by such cit-
ies, from financial transfers to electronic components. Total
damage in the hundreds of billions of dollars, or even into
the trillions, is conceivable as an outcome of a “superquake”
in the densely settled areas of the United States, particularly
the California coast.
In modern times, the danger from volcanism is largely
confined to the Pacific Northwest. Here an encounter
between the North American crustal plate and the Juan
de Fuca crustal plate beneath the Pacific Ocean has
generated an active volcanic mountain range, the Cascade
Mountains, reaching from northern California through
Oregon and Washington state into the Canadian province of
British Columbia. Among the famous and spectacular vol-
canoes of the Cascades are Mount Baker; Mount Hood;
Mount Rainier; and Mount Saint Helens, which under-
went an explosive and highly destructive eruption in 1980. In
recent geologic time, the eruption and subsequent collapse of
Mazama, a huge volcano in what is now Oregon, generated
a caldera that filled with water and became Crater Lake, a
popular tourist attraction. Volcanic activity has continued at
Crater Lake since the destruction of Mazama and has created
a small new volcano, Wizard Island, within the lake. The Cas-
cade volcanoes are associated with an offshore trench belong-
ing to the Cascadia Subduction Zone, where the Juan de
Fuca plate is believed to have descended into Earth’s mantle,
melting along the way and producing magma that rises back
to the surface through eruptions of the volcanoes. In Califor-
nia, Mount Shasta and Lassen Peak represent very recent
volcanism on the geologic time scale (Lassen Peak erupted
early in the 20th century), and renewed volcanic activity is a
possibility for the near future in the vicinity of Mono Lake,
in east-central California near the Nevada border.
Although modern volcanism is largely restricted to the
far west United States, evidence of ancient volcanic activity is
abundant in many parts of the nation. In New Hampshire,
for example, ancient volcanism produced peculiar circu-
lar ridges visible today in the White Mountains. Volcanic
activity has produced spectacular calderas and other forma-
tions at Long Valley, California, site of one of the most
powerful eruptions in the history of North America; Yellow-
stone National Park in Wyoming, where hydrothermal
activity continues to this day, producing (among other phe-
nomena) the famous geyser “Old Faithful”; and the Valles
caldera in New Mexico. In the Northwest, vast flows of lava
cover the Columbia Plateau and Snake River Plain. Vol-
canism in Alaska is frequent, widespread, and spectacular.
Alaskan eruptions tend to be highly explosive, in contrast to
the relatively tranquil eruptions of Hawaii’s volcanoes. In this
century, the eruption of Mount Katmai in Alaska resulted in
the collapse of the volcano’s peak, the formation of a huge
caldera and the generation of a plain of fumaroles, the Val-
ley of Ten Thousand Smokes.
Volcanism and its associated phenomena have had con-
siderable economic and historical importance for the United
States. The deposits of gold that set off the gold rush of the
mid-19th century, thus accelerating the settlement of the west-
ern United States, are thought to have their origins in chemical
processes associated with hydrothermal activity. Also tied in
one way or another to volcanism in the western United States
are numerous deposits of rich metal ores that have yielded
great quantities of silver, tin, copper, and other economically
important metals. In some cases, the mines dug to exploit
these minerals have encompassed a cubic mile (4 km
3
) of rock
with their tunnels.
The west coast of the United States is vulnerable to tsu-
namis, or seismic sea waves, generated by earthquakes along
U.S. shores or elsewhere around the Pacific Ocean basin. Per-
haps the most famous tsunami in U.S. history accompanied
the Alaska Good Friday Earthquake of 1964. This tsunami
devastated shorelines in south Alaska, wiped out much of the
state’s commercial fishing fleet and retained enough power
to cause widespread damage at Crescent City, California.
Numerous other (though less destructive) tsunamis have
occurred along the California coast, many of them apparently
in connection with earthquakes there.
See also impact structure; plate tectonics.
Unzen, Mount volcano, Japan An andesitic strato-
volcano that had relatively minor activity for nearly 200
years until 1990. In 1792, the collapse of the Mayuyama
lava dome created an avalanche and tsunami that killed
an estimated 14,524 people. In the renewed activity, the
lava dome began a sudden growth spurt and received world
attention. As the dome quickly grew 300 feet (91 m) above
the base of the crater, thousands of residents of the slopes
and surrounding areas were evacuated. Many reporters and
volcanologists, on the other hand, flocked to the volcano
to witness and record the collapse of the unstable dome and
resulting pyroclastic flows. On June 3, 1991, a huge mass
unexpectedly broke away and rolled down the slope at 60
miles (97 km) per hour. It killed 42 of the observers. Since
then Mount Unzen has become the worldwide leader in pyro-
clastic flows, producing more than 7,000 since 1991.
upper mantle The top 621 miles (1,000 km) of the man-
tle beneath the crust. It contains the rigid mantle that is
adhered to the crust to comprise the lithospheric plates. It
also includes the asthenosphere upon which the plates float
and are driven.
Ushiorashi-dake See Daisetsu-Tokachi.
Usu, Mount See Toya.
Utah United States Although Utah is not commonly asso-
ciated with strong earthquakes, like nearby California,
Utah is extremely susceptible to strong earthquakes. Earth-
quakes are common in and near the Wasatch Mountains, and
the potential earthquake hazard there puts much of the state
268 Unzen, Mount