KEEP an eye on the sky this autumn.
Arlin Krueger at the National Aeronautics and Space
Administration (NASA) Goddard Space Flight Center in Greenbelt, Md.,
says that "people can expect ... rosy sunsets" over much of the
Northern Hemisphere as the volcanic plume spreads from Mt.
Pinatubo's eruption in the Philippines.
But atmospheric scientists such as Dr. Krueger are interested in
more than sunsets.
Pinatubo's eruption has given them an unplanned opportunity to
study the effect that such a massive stratospheric injection of
volcanic material has on weather and atmospheric chemistry.
Krueger has explained it is "possible that the cloud will reflect
back into space some of the sunlight that would have reached the
ground, resulting in a small change in the heat balance of the
Earth." Many scientists will be watching for a possible cooling over
the Northern Hemisphere, or even globally, of a few tenths of a
Michael Coffey is a member of a team at the National Center for
Atmospheric Research (NCAR) in Boulder, Colo., that is analyzing the
cloud's composition. He uses spectroscopic data gathered from an
aircraft to identify chemical compounds. By establishing early
conditions in the cloud and watching how these change, he and his
colleagues hope to learn more about the complex chemistry that
converts volcanic gases into climate-driving aerosol particles.
Dr. Coffey also notes that the unexpected eruption "is going to
test" the prediction of his NCAR colleague Guy P. Brasseur that
these particles will provide reaction sites for chemistry that will
enhance destruction of stratospheric ozone.
Until now, the biggest eruption of this century, in terms of
material shot to stratospheric heights, was that of El Chichon in
Mexico in 1982.
Plume-tracker Lamont Poole, assistant head of the
aerosol-research branch at the NASA Langley Research Center in
Hampton, Va., estimates that Pinatubo's cloud "certainly is at least
in the same class as El Chichs, if not larger."
Not all eruptions have major atmospheric implications. Speaking
recently at his agency's regional office in Menlo Park, Calif.,
United States Geological Survey volcanologist Robert I. Tilling
explained the difference, saying:
"Volcanoes are similar in that they all offer channels for molten
rock (magma) from deep below to travel to the surface. Volcanoes
differ, however, in that the type of rock that forms the magma and
the amount of gas in the rock determines whether the material will
flow as a heavy molten liquid (as in Hawaii) when it reaches the
surface, or whether it will explode from the volcano as ash and
other bits of volcanic material."
He added, "Some of this explosive material can travel at high
speeds down the sides of a volcano in destructive pyroclastic flows,
and other material rises high into the atmosphere and later falls to
earth as volcanic ash."
This spring's eruption of Pinatubo and of Mount Unzen in Japan,
which involved pyroclastic flows, reflect the relative movement of
great pieces (plates) of Earth's crust along the western Pacific
rim. Here, an oceanic plate slides beneath the plate carrying the
Philippines and Japan. Their interaction generates molten rock that
rises and eventually erupts explosively. …