Page 60 - Environment: The Science Behind the Stories

the crust. As a tectonic plate moves across a hotspot, repeated
eruptions from this source may create a linear series of volca-
noes. The Hawaiian Islands provide an example of this pro-
cess (Figure 2.22a).
At some volcanoes, lava flows slowly downhill, such as
at Mount Kilauea in Hawaii (Figure 2.22b), which has been
erupting continuously since 1983! At other times, a volcano
may let loose large amounts of ash and cinder in a sudden
explosion, such as during the 1980 eruption of Mount St.
Helens (see Figure 17.11, p. 475). Sometimes a volcano can
unleash a pyroclastic flow—a fast-moving cloud of toxic gas,
ash, and rock fragments that races down the slopes at speeds
up to 725 km/hr (450 mph), enveloping everything in its path.
Such a flow buried the inhabitants of the ancient Roman cities
of Pompeii and Herculaneum in A.D. 79, when Mount Vesu-
vius erupted.
Figure 2.21 The 2010 earthquake in Haiti devastated the Besides affecting people, volcanic eruptions exert envi-
capital city of Port-au-Prince and killed an estimated 230,000 ronmental impacts (Table 2.3). Ash blocks sunlight, while sul-
people. One reason for the extensive loss of life was that many of fur emissions lead to a sulfuric acid haze that blocks radiation
Haiti’s buildings were not constructed to withstand earthquakes. and cools the atmosphere. Large eruptions—such as that of
Mount Pinatubo in the Philippines in 1991 (p. 475)—can
depress temperatures throughout the world. When Indo-
along mid-ocean ridges or over subduction zones as one tec- nesia’s Mount Tambora erupted in 1815, it cooled average
tonic plate dives beneath another. Due to its position along global temperatures by 0.4–0.7 °C (0.7–1.3 °F), enough to
subduction zones, Japan has over 100 active volcanoes, which cause crop failures worldwide and make 1816 “the year with-
is 10% of the world total and more than any other nation. out a summer.”
Mount Fuji, one of Japan’s most prominent and recognizable One of the world’s largest volcanoes—so large it is called
natural features, is one such active volcano. a “supervolcano”—lies in the United States. The entire basin
Lava may also be emitted at hotspots, localized areas of Yellowstone National Park is an ancient supervolcano that
where plugs of molten rock from the mantle erupt through has at times erupted so massively as to cover large parts of

ALASKA CHAPTER 2 • E ART h’s Physi CAL

RUSSIA
CANADA

Pacific Ocean
This location on UNITED
the crust was Direction of plate movement STATES
over the hotspot (Islands get older)
60 million years
ago.
Present-day MEXICO
Hawaiian Islands
Older submerged s ys TE m s: mATTER , E NER gy, AN d
Islands
Midway Is. This location on the crust
Laysan Is. is over the hotspot today.

(a) Current and former Hawaiian Islands, formed as crust moves over a (b) Mt. Kilauea erupting
volcanic hotspot
Figure 2.22 The Hawaiian Islands are the product of a hotspot on Earth’s mantle. The Hawaiian Islands gE o L ogy
(a) have been formed by repeated eruptions from a hotspot of magma in the mantle as the Pacific Plate passes
over the hotspot. The Big Island of Hawaii is most recently formed, and it is still volcanically active. The other
islands are older and have already begun eroding away. To their northwest stretches a long series of former
islands, now submerged. The active volcano Kilauea (b), on the Big Island’s southeast coast, is currently
located above the edge of the hotspot. 59

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