Geoscience Reference
In-Depth Information
(a)
(b)
Figure 13.38 Eruption of Mount St. Helens.
(a) The near-
perfect conical shape of Mount St. Helens before the May
1980 eruption. (b) The catastrophic eruption on May 18,
1980. (c) Mount St. Helens after the eruption. The blast was
so explosive that much of the peak was blown off, reducing
the height of the mountain from 2950 m (9677 ft) to about
2550 m (8364 ft) and leaving a wide crater. Note the lava
dome (arrow) in the center of the crater.
(c)
occur on Mount Rainier, for example, it would truly be a cata-
strophic event. In addition to the destruction caused by the blast,
the eruption would cause ancient glaciers on the mountain sum-
mit suddenly to melt, resulting in a massive flood of volcanic
debris and water (called a
lahar
) that would sweep toward the
populated areas near Tacoma, Washington. This flood would
likely cause many deaths and massive property loss.
cratic Republic of the Congo in eastern Africa, they usually re-
sult in the formation of a
shield volcano
that has shallow, slop-
ing sides (Figure 13.39). This type of volcano develops because
successive eruptions of fluid magma cause the volcano to build
upward gradually over a much broader area than composite
volcanoes. A number of prominent shield volcanoes occur on
Earth in association with subduction zones and rift zones. One
of the best known is Skjaldbreiður, which straddles the Mid-
Atlantic Ridge in Iceland. The name of the volcano is derived
from the Icelandic term for
broad shield
and is the source of the
more general term that is applied to similar volcanoes around
the world. In Alaska, a prominent shield volcano is Mount San-
ford, which is part of the Wrangell Volcanic Field in the east-
ern part of the state. This volcano began to form about 900,000
years ago and was most recently active approximately 320,000
years ago. This shield volcano is unusual because one of the
slopes is very steep, rising 2400 m (8000 ft) over the course of
1.6 km (1 mi).
Fluid Volcanoes
In addition to the volcanoes that erupt explosively, a number
of prominent volcanoes have very fluid eruptions with flowing
rivers of lava. In contrast to the highly viscous magma associ-
ated with explosive volcanoes, the magma that results in fluid
eruptions contains far less silica and is thus less viscous. As a
result, the lava associated with such an eruption flows across the
surface until it cools to form basalt. Such eruptions can occur
for weeks and months at a time and generally take place more
frequently than explosive eruptions.
Although fluid eruptions are occasionally associated with
composite volcanoes, such as Mount Nyaragongo in the Demo-
Shield volcano
A very broad volcano with shallow slopes
that forms in association with nonviscous lava flows.
