Geologic Background:
The massive Mauna Loa shield volcano rises
almost 9 km above the sea floor to form the world's
largest active volcano. Flank eruptions are predominately
from the lengthy NE and SW rift zones, and the summit is
cut by the Mokuaweoweo caldera within an older and larger
6 x 8 km caldera. Two of the youngest large debris
avalanches documented in Hawaii traveled nearly 100 km
from Mauna Loa; the second of the Alika avalanches was
emplaced about 105,000 years ago (Moore et al. 1989).
Almost 90% of the volcano's surface is covered by lavas
less than 4000 years old (Lockwood and Lipman 1987).
During a 750-year eruptive period beginning about 1500
years ago, a series of voluminous overflows from a summit
lava lake covered about one fourth of the volcano's
surface; the ensuing 750-year period, from shortly after
the formation of Mokuaweoweo caldera until the present,
covered an additional quarter of the volcano and
consisted of predominately low-level summit and rift zone
activity, often from NW rift zone vents.
Historic Activity:
- Since the first identifiable flow, that of 1843,
twenty-two flows have erupted outside the summit caldera.
About 4.2 cu km of lavas have been erupted since 1843, in
individual eruptions of less than 0.001 to more than 0.5
cu km volume.
Recent Activity:
- From 1980 to May 1983 there was an increase in number
of shallow earthquakes beneath Mauna Loa. In May 1983 a
forecast of an eruption in 1983 or 1984 was issued.
- The eruption began on March 25, 1984. At about 01:30
eruptions from fissures on floor of summit caldera began.
Fissures began to open along NE Rift Zone at about 04:00
and by 06:00 activity had ended in caldera. Flows
extended up to 27 kilometers from the vent area down the
northeast flank. The eruption ended early on April 15,
1984 after the eruption of 220,000,000 cubic meters of
lava. Flow covered 50 sq km.
- Summit area has been reinflating steadily since
eruption but seismicity has been low. By February 1989,
50% of deflation had been recovered. Continued slow
inflation of summit was continuing in 1991. Small summit
earthquake swarm on July 13, 1991.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 8, No. 5; V. 9, No. 3
and 4; V. 12, No. 10; V. 14, No. 1)
- Global Volcanism Network (V. 16, No. 7).
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