ground to bits in the explosion of a volcano, it becomes fine
ash. Under certain circumstances, gases released from solu-
tion in molten rock during an eruption may build up inside
the volcano and eventually produce a colossal explosion.
This is what happened at Krakatoa, where the effect of clog-
ging up the vent of the volcano was to stop it up and allow
gases released from the magma to attain tremendous pres-
sures. Ultimately, this situation caused the island to explode
with devastating effects. At Mount Saint Helens in Wash-
ington State in 1980, gases built up under the north flank
of the volcano until that flank burst open and sent a pow-
erful blast sweeping over the landscape, leveling forests. The
potential for such events is relatively high where subduction
zones carry water-saturated sediments into Earth’s interior
near chains of volcanoes, such as the Cascade Mountains,
of which Mount Saint Helens is part.
mudflows or lahars are another destructive manifes-
tation of water in volcanic eruptions. Heat released from an
eruption on the summit of a volcano may melt ice or expel
the contents of a crater lake mixed with vast quantities of
ash, sending great flows of mud down the flanks of a moun-
tain. These flows may accumulate to great depths and bury
entire communities. Many volcanoes exhibit the phenomenon
of phreatic eruptions, in which heated water burst out
explosively in the form of steam. When phreatic eruptions
occur in combination with eruptions of magma, the result is
called a phreato-magmatic eruption.
hydrothermal activity, the result of water heated
underground by bodies of molten rock near Earth’s surface,
is a highly complex phenomenon that has had a considerable
impact on human history. At some locations, the heated under-
ground water may be tapped for generating electrical power or
for heating. Elsewhere, minerals dissolved in the water may
be deposited in the form of rich ores of gold, silver, copper,
and other metals of economic importance. Such activity is
thought to have formed many of the valuable ore deposits of
the western United States and western South America, for
example. One especially violent and photogenic kind of hydro-
thermal activity is the geyser, a fountain of water and vapor
that may reach heights of more than 100 feet (30 m). Some of
the world’s most famous geysers are located in Yellowstone
National Park in the United States and in Iceland.
Sometimes the interaction of water and volcanic heat
creates formations of rare beauty and complexity. At Mount
Rainier in the Cascade range of the United States, for exam-
ple, volcanic heat at the summit of the mountain has reached
an equilibrium with the accumulation of ice and snow to
produce an intricate and fragile network of caverns within
the ice. These caverns are large enough to accommodate a
person.
See also geothermal energy.
wavelength The distance between corresponding points on
two adjacent sinusoidal waves; for example the measured dis-
tance between peaks on two successive waves in a line. Wave-
length is measured for seismic waves.
Wegener, Alfred (1880–1930) A German meteorologist,
Wegener is widely credited with originating the concept of
continental drift, which gave rise to the theory of plate tec-
tonics, the basis for much of modern geology. Although
Wegener appears to have taken the idea of continental dis-
placements from other authors, notably the U.S. geologist
Frank Bursley Taylor, Wegener presented a comprehensive
model of continental motion and the formation of Earth’s
landmasses in his book The Origins of Continents and
Oceans. Wegener suggested that the continents are not fixed
in position but instead float atop denser rock in Earth’s man-
tle, just below the crust. Wegener believed the continents as
arranged today were once united as a single supercontinent,
which Wegener called Pangaea, meaning “all land.” This
supercontinent was surrounded by a world ocean he called
Panthalassa. The Atlantic Ocean basin, Wegener wrote,
was created during the breakup of Pangaea, as the continents
moved apart from one another. The energy for the continents’
movements, Wegener suggested, came from a tidal motion
that supposedly produced a net westward motion of land-
masses. Additional energy for continental motion was
believed to come from a Polftucht, or “flight from the poles,”
resulting from Earth’s rotation.
Wegener’s formulation had numerous flaws. He sup-
ported his account of the formation of the Atlantic Ocean
basin, for example, with inaccurate data that indicated
Greenland was moving westward several kilometers per year,
when the actual figure is nearer several centimeters per year.
For such shortcomings, Wegener’s work was criticized.
Wegener graduated from Berlin University and served
later on the faculty of Marburg University. He also served as
an officer in the German army during World War I. Wegener
himself appears to have been a colorful figure. Athletic and
adventurous, he took part in the exploration of the atmo-
sphere, setting a world record in 1902 for long-distance bal-
loon flight (56 consecutive hours aloft) in the company of his
brother Kurt. Wegener was fascinated with Greenland and
joined several expeditions to the island. On his final visit in
1930, he died, presumably of a heart attack, and was buried
on the ice cap.
welded tuff After one ashfall or ash-flow tuff is
deposited, another ashflow tuff is deposited on top. The heat
of the later flow weld underlying ash together into a very
dense, glassy, and layered rock. Successive flows continue to
weld the underlying layers together. Welded tuffs are com-
mon in rhyolite and dacite volcanoes.
Westdahl volcano, Aleutian Islands, Alaska, United States
Westdahl, in the Aleutian Islands on Unimak Island, stands
about 6,000 feet (1,830 m) high and is located approximately
700 miles (1,127 km) southwest of Anchorage. It is a large
shield volcano covered by an icecap. It has erupted eight
times in historical times, including 1795, 1796, 1820, 1827–
30, 1964–65, 1978, 1979, and 1991–92. The ash cloud from
1991 fissure eruption reached an altitude greater than four
miles (6.4 km) and lava fountains yielded flows that are
4.4 miles (7 km) long.
West Virginia United States The state of West Virginia
lies mostly within a region of minor seismic risk, although
West Virginia 283