Geography Reference
In-Depth Information
width of Texas, and the mountain is about 27 km (∼17 mi) high, more than three times
higher than Mount Everest. Mars has no plate tectonics, so Olympus Mons was fixed in
place and grew larger by long-continued eruptions.
Volcanoes are linked to three basic regions: rift-valley spreading centers, such as
Iceland on the Mid-Atlantic rift, or Mount Kilimanjaro and many other volcanoes on
the East African rift system; on the plate above subduction zones; and above intraplate
“hot spots” or mantle-plume thermal centers, where plumes of solid yet mobile mantle
rock rise toward the surface like the blobs in a lava lamp (Strahler 1998; Macdonald
1972; Oilier 1988; Scarth 1994; Decker and Decker 2006). In the latter case, the mantle
plume has a bulbous head that draws out into a narrow stalk as it rises. As the head
nears the top of the mantle, decompression melting generates basaltic magmas, some
of which can pour out on the surface in vast floods; or, if the plate carries the prior so-
lidified outpourings along from the source hotspot, a chain of volcanoes can result. The
Hawai'ian Islands are a northwest-southeast string, with the oldest to the northwest and
the youngest to the southeast (Macdonald and Abbott 1970). This strung-out pattern is
the result of the Pacific Plate moving to the northwest and the islands continually being
moved away from the hot spot (Condie 2005; Decker and Decker 2006). The youngest
island, Hawai'i, with the Mauna Loa volcano on it, now overlies the hot spot. Hawai'i
continues to drift to the northwest, with the result that, although the Kiluaea volcano
there is still active, already a new eruptive center is developing underwater offshore to
the southeast. Eventually the island of Hawai'i will become entirely extinct as it drifts
away from the hot spot and the new underwater volcano rises above the waves. The vol-
canoes of the Snake River Plain and Yellowstone National Park represent a similar situ-
ation and result from lava outpourings from the Yellowstone Plume and hot spot (Condie
2005; Greeley and King 1977; Decker and Decker 2006).
VOLCANIC MOUNTAIN HAZARDS
People have lived around volcanic mountains for thousands of years, in part because
of the fertile soils there and limited other available land, but also because, most of the
time, volcanoes are not hazardous. But as anyone knows who has watched the com-
mon presentations of volcanic disasters on television in recent years, a wide variety of
deadly and destructive events are possible—even probable in some areas. Volcanoes kill
and destroy in several different ways: lava flows, ash falls, pyroclastic flows, lahars, and
phreatic explosions. Pyroclastic flows (also known as fiery clouds, glowing avalanches,
or nuees ardentes) are mixtures of hot gases infused with incandescent ash and lar-
ger fragments. Because the pyroclastics are buoyed up by the hot gases, the whole is
heavier than air, and they race down the steep slopes of volcanoes commonly at speeds
greater than 200 km/hr (125 mi/hr). In 1902 a small volcano, Mt. Pelee, on the island of
Martinique in the Lesser Antilles, produced a fiery cloud that destroyed the city of St. Pi-
erre, along with 28,000 of its inhabitants (Anonymous 1982; Decker and Decker 2006).
Lahars, or volcanic mud and debris flows, are common on volcanoes that have lakes or
glaciers, which can melt to provide plentiful water to mobilize loose pyroclastics into a
devastating slurry of rapidly flowing, wet debris. A lahar on the volcano Nevado del Ruiz
in Colombia killed 23,000 people in 1985. The volcanic heat melted glacier ice, which
mobilized mud and pyroclastics, which then raced down the steep slopes of the volcano
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