NSF PR 98-78 - November 20, 1998
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Ancient Antarctic Environment Rocked By Volcanic Eruptions
The first evidence of large volcanic eruptions that
shook Antarctica around 25 million years ago has unexpectedly
been discovered in rock cores retrieved from the seabed
as part of an international ocean-floor drilling project
in which the National Science Foundation (NSF) is
a partner.
Scott Borg, who heads the geology and geophysics program
for NSF's Antarctic Science Section, said the rock
cores, drilled from the sea floor off the Victoria
Land coast near Cape Roberts, show surprising evidence
of enormous volcanic eruptions. These eruptions are
believed to have significantly altered global temperatures
at the time.
"The discovery of the volcanic material is really quite
exciting,'' Borg said. "It is clearly evidence of
a major eruption, several times larger than Mount
St. Helens (in Washington State) and possibly comparable
with the eruption that destroyed Pompeii."
The evidence of volcanic activity was gathered under
the auspices of the Cape Roberts Project. For the
past two Antarctic field seasons, NSF-sponsored scientific
teams, in collaboration with scientists from Australia,
Britain, Germany, Italy and New Zealand, have been
drilling the seabed to obtain samples that would illustrate
the climatic and geologic history of Antarctica during
the last 100 million years. Drilling this year had
reached a depth of approximately 110 meters below
the seafloor when this unexpected evidence of volcanic
activity was encountered.
The evidence consists of layers of volcanic debris
that were erupted explosively into the atmosphere
and then settled through the air and into the ocean
onto the seafloor. The thickness and coarseness of
the main debris layer indicates a large-volume eruption
that generated an ash cloud that reached into the
stratosphere. The discovery of these volcanic layers
underlying the seabed demonstrates a far more spectacular
history of volcanic activity than was previously suspected
for the Ross Sea region of Antarctica, but it is also
useful because it provides material for accurately
dating the strata.
The layers of volcanic debris are encased within muddy
sands, indicating a relatively quiet seafloor with
occasional weak currents before and after the eruptions.
This relatively quiet environment, however, was disrupted
at least twice and possibly as many as four times,
by large and rapid inputs of volcanic debris (mostly
pumice). The debris was supplied by voluminous eruptions
from a nearby source, but the exact location and characteristics
of that source are still unknown.
The thickest distinct layer of volcanic debris is 1.2
meters thick, which suggests an eruption as dramatic
as that of Krakatau in 1883. These layers contain
volcanic pumice up to 1 cm in size, which suggests
that the volcano was located within 50 to 100 kilometers
of the drilling site and erupted in a style reminiscent
of Vesuvius.
The eruptions recorded in this core probably had a
significant impact, not only on the Antarctic environment,
but also on the global environment of the time. Modern
examples of similar eruptions, such as Mount Pinatubo,
a much smaller event, cooled world climate by 0.5
degrees C for a year after its 1991 eruption.
Scientists will use the volcanic layers to determine
the age of their strata more accurately, because volcanic
debris can be dated precisely using isotopic techniques.
Borg said that samples of the pumice fragments are
being sent to be dated at New Mexico Tech, in Socorro,
N.M., one of eight U.S. universities taking part in
the Cape Roberts project.
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