Geoscience Reference
In-Depth Information
alter the l ux of carbon dioxide in the atmosphere, modifying climate. For
instance, the Mesozoic Era, when dinosaurs roamed the earth, had a much
balmier climate primarily because of increased undersea volcanism associated
with faster seal oor spreading and oceanic plateau formation. h is volcanism
increased the amount of carbon dioxide from the interior of the earth to the
atmosphere, resulting in atmospheric levels higher than 2,000 parts per mil-
lion and high global temperatures.
Over shorter timescales of months to years, volcanic eruptions have had
the opposite ef ect on temperatures. Following an eruption, particles of dust
and ash are ejected into the atmosphere, where they are suspended, blocking
sunlight and reducing solar radiation for a year or more. Volcanoes also eject
sulfur gases, which combine with water vapor in the stratosphere to form tiny
droplets of sulfuric acid. h ese droplets absorb solar radiation and scatter it
back into space, causing global temperatures to fall.
Volcanologists have compared the climate ef ects of recent eruptions
and observed that the greatest cooling is caused by sulfur-rich eruptions.
h e 1982 eruption of El Chichon in Mexico, for example, emitted a smaller
volume of ash than the 1980 eruption of Mount St. Helens in Washington
but forty times the volume of sulfur gases, and El Chichon resulted in
i ve times the amount of atmospheric cooling. Although these ef ects are
relatively short-term, they can be signii cant enough to have bigger climatic
impacts. Two major volcanic eruptions are thought to have contributed to
global cooling during the Little Ice Age. In Iceland, the Laki eruption in 1783
was the largest basaltic eruption in recorded history, with 3.4 cubic miles
of lava erupting over a period of eight months. h e eruption caused extremely
severe winters in Europe and North America. At the time, Benjamin
Franklin observed a constant fog over much of North America and Europe,
and he wondered whether the unusually low winter temperatures experi-
enced that year in the eastern United States, where temperatures were
8°F lower than they had been the previous 225 years, were related to the
Laki eruption.
h ree decades later, an even larger eruption (the largest of the past 10,000
years) occurred when Mt. Tambora erupted in Indonesia in 1815. Some
36.5 cubic miles of ash were sent up into the atmosphere, resulting in what
became known as the “year without a summer,” featuring unusually low tem-
peratures, frost, and snow even during summer months in Europe and New
England. h e amount of ash spewed into the air by Mt. Tambora was about
a hundred times that of Mount St. Helens in 1980.
