Geologic Background:
Rincon de la Vieja, the largest volcano in NW
Costa Rica, is a remote volcanic complex in the
Guanacaste Range consisting of an elongated, arcuate
NNW-SSE-trending ridge. At least nine eruptive vents are
located within an older 15-km-wide caldera remnant, with
activity migrating to the SE, where the youngest-looking
craters are located. The twin cone of 1,916-m-high Santa
Maria volcano, the highest peak of the Rincon complex, is
located at the eastern end of a smaller, 5-km-wide
caldera and has a 500-m-wide crater. A Plinian eruption
producing the 0.25 km3 Rio Blanca tephra about 3,500
years ago was the last major magmatic eruption from the
volcano. All subsequent eruptions, including numerous
historical eruptions possibly dating back to the 16th
century, have been from the Active Crater, which contains
a 500-m-wide crater lake.
Historic Activity:
- At least 16 phreatic eruptions have been recorded at
Rincon de la Vieja since its first historic activity in
1851.
- The eruptions have generally been characterized by
gas and ash emissions; lahars, formed by displacement of
the active crater lake, have frequently moved down the
north flank.
Recent Activity:
- A phreatomagmatic eruption in February 1983,
deposited tephra upto 1.5 km from the summit and
generated a small mudflow.
- An unobserved eruption occurred between September an
November 1985 when tephra was deposited in a fan-shaped
area extending about 500 m SE from the crater.
- A small eruption on December 31, 1986 deposited
tephra 500 m from the crater rim.
- On April 1, 1987 a phreatic eruption occurred in the
crater and several mudflows formed.
- On May 8, 1991 a phreatic eruption ejected small
quantities of ash and produced small lahars. Explosions
and ejection of ash was continuied till August.
- At least 16 phreatic eruptions were reported in 1991.
Eruptions ejected lake sediments and ash, and produced
both an ash-fall deposit and small mudflows. The fall
deposit covered and area of roughly 200 sq km.
- An eruption on Now. 6, 1995 followed increases in
fumarolic activity and a several-month long increase in
local earthquakes and tremor. The eruption climaxed on
the morning of the 8th. During some phases of the
eruption, lahars flowed down the Azul and Penjamo rivers
and an interfluvial ravine.
- Fumorolic activity continuing in 1997 through
1999.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 7, No. 11 to V. 14,
No. 4)
- Global Volcanism Network (V. 15, No. 9 to V. 25, No.
3).
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