Sections of the Ring of Fire,
subduction zones that surround much of the Pacific Ocean, are underwater.
Submarine volcanoes at these convergent plate boundaries are much like their
counterparts on land. Many of these volcanoes form new islands that last only
a short time.
Perhaps the most famous submarine volcano is Krakatau,
a submerged caldera located between Java and Sumatra. The 1883 eruption
killed at least 36,400 people. Most of these people were killed by tsunami.
Metis Shoal, a submarine volcano near the Tonga Islands, has erupted nine times since 1851.
The summit of the volcano is only a few meters below sea level. The 1979
eruption lasted more than two months, producing a small island that lasted
several months before being washed away, and sending rafts of pumice as large
as 15x30 miles (25x45 km) floating to the northwest. The most recent eruption
began in early June of 1995. As an island grew above sea level a lava dome,
about 90 feet feet (30 m) high and 450 feet (150m) is diameter, formed in
only a few days. Explosions threw ash to heights of 1500 feet (500 m). A
week later the dome was three times larger. The dome stopped growing in late
June and may resist erosion for some time. Photo of the dome courtesy Brad
Scott, June 28 1995, while on board the Tongan tug Hifofua.
Kavachi, a submarine volcano in the Solomon Islands, has built itself above
sea level at least nine times since 1950. The volcano has had 25 known
submarine eruptions since 1938. Most of these eruptions include small to
moderate explosions. This photograph was taken by John Grover in the early
1960s. Slide courtesy U.S. Geological Survey.
Another very active submarine volcano is Monowai seamount, about midway
between the Tonga and Kermadec island groups. The volcano is about 3,000
feet (1,000 m) high and less than 600 feet (200 m) below sea-level. Monowai
has erupted at least eight times since 1977. An eruption was suspected in
1944. Most eruptions were detected using acoustics but during the 1977-1979
eruption upwelling and discolored seawater was observed. Monowai
erupted most recently in September of 1996 and in April of 1997. Top:
Bathymetric map of Monowai seamount. The contour interval is 100 m. Bottom:
Profile across Monowai seamount. Vertical exaggeration = 3.7. From Davey
(1980).
The New World Seamount (top corner of this image), north of Lihir island in
Papua New Guinea is 2.5 miles (4 km) wide at its base and has a conical peak.
The volcano rises about 1,800 feet (600 m) off the seafloor and has a deep
central crater. Sheet and pillow lava flows, volcanic breccias and lag
deposits have been photographed at the summit. This seamount is extinct and
never rose above sealevel. The subaerial SAR data of Lihir Island was supplied
with permission by Lihir Mining Co. Ltd., while the bathymetry data was
collected during the R/V SONNE SO-94 Research Cruise (EDISON PROJECT)
organized by Freiberg University of Mining & Technology and funded by the
German Federal Ministry for Research and Technology (BMFT Grant 03G0094A to
P. Herzig). Additional information about the seamount and other seamounts
and volcanoes in the area is available on the CSIRO Exploration & Mining
(Magmatic-Hydrothermal Cu-Au Group) homepage.
The New World Seamount and the volcanoes that make Lihir Island are
associated with the subduction of the Pacific Plate under the North Bismarck
microplate. The tectonics in this part of the world is very complicated. A
spreading center creates the North and South Bismarck microplates. The
Pacific plate is subducted under the North Bismarck Plate. The Solomon Sea
microplate is subducted under the South Bismarck microplate. Map from Herzig
and others (1994).
Some submarine volcanoes host economic deposits of metals. For example,
Kuroko deposits, named for an occurrence in Japan, are hosted in dacite and
rhyolite domes and their associated breccias that formed on the basaltic ocean
floor. Deposits of zinc, copper, lead, and gold formed when the volcano was
active. The volcanoes are later uplifted above sea level or added to the
margin of the continent. Modern volcanoes that are forming this type of
deposit are found in back-arc basins, areas of rifting behind active volcanic
arcs. Understanding modern volcanic environments helps geologist search for
more metal deposits. This cross-section of a typical Kuroko deposit is from
Sato (1974) and Franklin and others (1981).
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