Geologic Background:
The highest of the Marianas arc volcanoes,
Agrigan contains a 500-m-deep, flat-floored caldera. The
elliptical island is 8 km long; its 965-m-high summit is
the top of a massive 4,000-m-high submarine volcano, the
second largest in the Marianas Islands. Deep radial
valley dissect the flanks of the thickly vegetated
stratovolcano. The elongated caldera is 1 x 2 km wide and
is breached to the NW, from where a prominent lava flow
extends to the coast and forms a lava delta. The caldera
floor is surfaced by fresh-looking lava flows and also
contains two cones that may have formed during the
volcano's only historical eruption in 1917. The island's
youngest lava flows traveled NNW through a breach in the
caldera wall, forming a delta near the ocean. A
300-m-diameter cone on the floor of the 4 square
kilometer central caldera may have formed during the 1917
eruption. A rift zone trending N10°E passes through
the caldera and includes young cones on the N and S
coasts. Extensive pyroclastic deposits cover older lava
flow sequences dominated by basalts, but also including
basaltic andesites and andesites.
Historic Activity:
- During the last eruption, in April 1917, blocks up to
1 cubic meter fell on the south coast, 5 km away, and as
much as 3 m of ash and lapilli were deposited on a coast
village during 2 days of activity, prompting its
evacuation. A 300-m-diameter cone on the floor of the 4
square km central caldera may have formed during the 1917
eruption.
- Fumarolic activity was occurring from one of the
cones on the caldera floor 1976.
Recent Activity:
- August 1, 1990 overflight showed increased fumarolic
activity. Ground survey in Sept.-Oct. showed no sign of
increased activity. Remeasurement of ground control
stations showed no significant changes in May 1992.
Data Sources:
- Global Volcanism Network (V. 15, No. 7 to V. 17, No.
6)
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