Geologic Background:
The roughly 20-km-diameter Gaua Island, also
known as Santa Maria, consists of a basaltic-to-andesitic
stratovolcano with a 6 x 9 km summit caldera. Small
parasitic vents near the caldera rim fed Pleistocene lava
flows that reached the coast on several sides of the
island; several littoral cones were formed where these
lava flows reached the sea. Quiet collapse that formed
the roughly 700-m-deep caldera was followed by extensive
ash eruptions. Construction of the historically active
cone of Mount Garat (Gharat) and other small cinder cones
in the SW part of the caldera has left a crescent-shaped
caldera lake (Lake Letas). The symmetrical, flat-topped
Mount Garat cone is topped by three pit craters. The
onset of eruptive activity from a vent high on the SE
flank of Mount Garat in 1962 ended a long period of
dormancy.
Historic Activity:
- Only solfataric activity was recorded from 1868 to
1962.
- Beginning in 1962, central crater explosions with
frequent associated ash columns were reported nearly
every year until 1977.
- Information on activity from 1977 to 1990 is scarce,
but the volcano was probably quiet, with only minor steam
emissions from the SE crater.
Recent Activity:
- Increased fumarolic activity has been noted since
April 1991. The NW slopes of the cone are largely denuded
of vegetation.
- Large steam-and-gas plume with a strong SO2 odor was
observed on July 15, 1996. Strong fumarolic
degassing.
- Solfatara activity in 1999.
Data Sources:
- Global Volcanism Network (V. 16, No. 7 and V. 24, No.
9).
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