Geography Reference
In-Depth Information
south of Seattle. Mount St Helens experienced a Plinian
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eruption that liberated
1 km³ of ignimbrites and devastated a very large, although practically uninhabited
area. The United States Geological Survey (USGS) lists 16 potentially active
volcanoes and shows that certain Quaternary eruptions were more violent than that
of Mount St Helens. While they may be less active than Indonesian volcanoes, the
mountains of the Pacific Northwest are unstable. Certain geological studies indicate
that the region of Yellowstone National Park is a potential “super-volcano” whose
mega eruption once covered half of the United States in ash about 700 million years
ago. This risk remains nonetheless hypothetical. Isolation and the harshness of
winter in the highlands of Yellowstone Park, situated between 1,600 and 3,200 m of
altitude, is a much more significant obstacle to settlement in the region. In the West,
as in the rest of the country, climatic factors have more influence than topography
and geology in explaining the settlement of the United States.
1.3. Unfavorable climatic factors
Despite the latitude of the country, the prevailing climates in the United States
can hardly be characterized as “temperate” in the sense of being moderate or
restrained. They are temperate since, for most of the territory, each year is marked
by alternating cold and hot seasons. Most of the US experiences a continental
climate, typified by extreme seasonal contrasts. The mountain barriers that follow
the Pacific coast reduce the influence of the ocean to a narrow coastal area.
Meanwhile, the corridor of the Great Plains, stretching from the Gulf of Mexico
to the Arctic Ocean, reinforces the continental climate in central and eastern regions,
which is dominated by the clash of polar and tropical air masses, which results in
frequent tornadoes.
The western part of the continent has an arid climate, while the Atlantic coast,
and the shores of the Gulf of Mexico are very humid, and subject to an average of
two hurricanes per year. Compared with Europe, winters are harsh despite the
relatively low latitude. The North gets a great deal of snow and freezing
temperatures, while the areas sheltered from frost are limited to a southern strip.
Summers are very hot except in the Pacific Northwest and in some Northeastern
counties located near Canada.
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. Plinian eruptions are characterized by their explosive nature resulting in the formation of
pyroclastic flows which sweep down the flanks of the volcano and cause the most loss of life.
Another very dangerous phenomenon is “lahars”, violent mudflows caused by the sudden
melting of snow which spread over valleys with great energy and speed.
