SeaWiFS Views Volcanoes
SeaWiFS was in just the right place, at just the right time, to
capture this view of the ash plume from an eruption of Mount Etna in Sicily.
Mount Etna is a huge (3310 m high) volcano, the largest volcano in Europe.
While it has produced some voluminous flank eruptions that have threatened
nearby towns, Etna commonly has activity only in its summit craters. Since
1997, there has been episodic activity at Etna's summit. In the past few
months, in intervals as short as 12 hours or as long as 10 days, there have
been remarkable eruptions when large lava fountains have jetted from Etna's
Southeast Crater. While these eruptions are short-lived, they are also
spectacular. At night, the lava fountains can be seen from vantage points
all around the island, often up to 100 km away from the summit. If an eruption
takes place during the day, the ash cloud can also be seen from a great
distance.
On April 6, 2000, a summit eruption took place just before noon.
Boris Behncke has provided a description of this
eruption, with pictures taken from the summit. SeaWiFS passed over the
Mediterranean just as the eruption was ending, as the large cloud of ash was
drifting eastward. This image appears "pixelated", i.e., the clouds appear
somewhat square, because Sicily was near the edge of the SeaWiFS observation
swath. Due to the curvature of the Earth and the observation angle, the
resolution of this area of the image is degraded compared to the center of the
swath, where the resolution is 1 km.
Though the primary mission of SeaWiFS
is to obtain views of the ocean, SeaWiFS has also provided observations of
atmospheric aerosols. The first Science
Focus! page showed observations of a dust storm in China. SeaWiFS data is
also useful for detecting smoke from ground fires, and as shown above and
below, it can detect plumes of ash and steam from volcanoes.
Volcanoes do
not usually have a direct influence on the biological activity of surrounding
oceans. But they can have an indirect influence, if an eruption is large
enough. Fine dust and sulfur aerosols from the massive eruption of Mt.
Pinatubo in 1991, distributed around the world in the stratosphere, reduced the
amount of sunlight reaching the ocean surface. This reduction in
insolation reduced both the sea surface temperature and the primary
productivity in the tropical Pacific Ocean for several months.
Iron from ash
deposited on the ocean floor around volcanic islands can also nourish
productivity in iron-deficient waters. The plume
of productivity that is found near the Galapagos Islands is the best
example of this phenomenon. During the eruption of the new volcanic island
Surtsey off of Iceland, dissolved silicate concentrations were elevated in
surrounding waters, and this led to augmented phytoplankton productivity near
the eruption. Silica is a vital nutrient for diatoms, a form of phytoplankton,
which form delicate microscopic shells of silica.
Here are some other views of volcanic plumes from SeaWiFS:
Sakura-Jima Volcano, Japan
Note the light-colored plume drifting
southward from the volcano into the clear area, and to the west, a pretty
display of von Karman vortices in the clouds. Image acquired December 6, 1999.
San Cristobal Volcano, Nicaragua
Two views of a long plume from
San Cristobal volcano in Nicaragua, drifting westward over the Pacific Ocean.
These views were obtained two days apart (February 23 and 25, 2000), and look
slightly different due to a slight difference in the observation angle.
San Cristobal volcano: February 2000 eruption report
North Island, New Zealand
This SeaWiFS image from February 1,
1999, shows the entire North Island of New Zealand.
(Larger version) In the center right of
the image, in the "Bay of Plenty", a thin plume of gas can be seen drifting
from White Island
, a small and intermittently active volcanic island. On the
main island, the patch of white south of Lake Taupo (refer to the map of the
Taupo volcanic
zone) is
Mount
Ruapehu, a frequently active stratovolcano which last erupted in
1995-1996. Hidden under the clouds to the north of Lake Taupo is the Rotorua
hydrothermal area and the Waimangu hydrothermal area. Early in this century,
the Waimangu hydrothermal area was the location of the largest geyser in the
world. When Waimangu Geyser erupted, in 1900-1904, some of its eruptions were
over 500 meters high. (The current "largest geyser in the world" is considered
to be Steamboat Geyser in Yellowstone National Park, but it doesn't erupt very
often.) Also note that Lake Taupo is a lake contained in a volcanic caldera.
The Lake Taupo volcano has erupted as recently as 181 A.D., an extremely large
and violent eruption.
Popocatepetl, Mexico
On December 18-19, 2000,
Popocatepetl volcano in Mexico, which is near Mexico City and has been in a
state of mild activity for several years, had an elevated level of activity.
SeaWiFS observed the puffing plume of ash from a position almost directly above
the volcano.
Here's
what the volcano looked like from the ground as the sun was setting, captured
by the monitoring of Centro Nacional de
Prevencion de Desastres (CENAPRED).
ACKNOWLEDGMENT
The authors wish to thank Boris Behncke, who maintains the excellent Italy's Volcanoes Web
site, and the Institute for Geological and
Nuclear Sciences in New Zealand, which provided the Taupo Volcanic Zone
map. We also wish to thank Norman Kuring of the SeaWiFS Project for his
routine skill at creating remarkable images from SeaWiFS data.
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