Smithsonian Institution
Bulletin of the Global Volcanism Network, v. 20, no. 11/12, November-December 1995
Rincon de la Vieja (Costa Rica)  Additional details about the 6-10
     November eruption

Rincon de la Vieja
Costa Rica
10.83 N, 85.33 W; summit elev. 1,916 m

An eruption on 6 November 1995 (Bulletin v. 20, no. 10) followed
increases in fumarolic activity and a several-month long increase
in local earthquakes and tremor (figures 15 and 16). Park rangers
who visited the summit at the start of October noted increased
fumarolic activity and witnessed landslides down the main crater's
walls. Strong sulfur smells were noted W-SW of the volcano on
multiple occasions in the days prior to 6 November (figure 17).

The seismic receiver (RIN3) sits 5 km SW of the active crater.
Although the OVSCICORI-UNA seismic system failed on 29 October (and
possibly other times during the month), it functioned reliably
again after the 31st. Low-frequency events gradually increased
during 1-6 November (figure 16), followed by a modest decline.
High-frequency events were only registered after 3 November. Tremor
was absent prior to the 6 November eruption.

OVSCICORI reported that the first phase of the eruption consisted
of vapor with subordinate ash in a discharge lasting 2 minutes.
Later, vigorous fumarolic activity led to many hours of constant
tremor. Only two more clear eruptions followed in the initial 17
hours of venting, but others followed in subsequent days. The
eruption climaxed on the morning of the 8th, when columns reached
3.5 km altitude. Fine ash blew W and NW; larger blocks and tephra
were confined to within ~1 km and the area of heavy ashfall reached
~5 km away (figure 17).

During some phases of the eruption, lahars flowed down the Azul and
Penjamo rivers and an interfluvial ravine called the Quebrada
Azufrada (figure 17). Upper reaches of these drainages sustained up
to 6 m of erosion. Lahars on the 7th were cooler and more
water-rich than those on the 8th. In addition to previously
reported damage, on 8 November lahars shut down some communications

At 0900 and 1130 on 8 November OVSICORI scientists visited the
summit area and saw impact craters as large as 2 m in diameter; the
craters were produced by 0.5-1.0 m diameter blocks, some of which
were still warm to the touch. The scientists also saw ongoing
phreatic eruptions escaping from a vent adjacent to the crater

At 0411 on the 9th a shock wave was felt 25 km SE in the city of
Liberia; the related outburst was seen from the N flank, where
residents witnessed incandescent block ejections.

Amplitudes on the seismic recorders regularly peaked at over 30 mm
on 6-9 November. The highest amplitudes, on 7-9 November, reached
nearly 60 mm. Amplitudes decreased the morning of 9 November;
following the eruption (10-14 November) amplitudes generally
remained under 10 mm with infrequent spikes to ~20 mm and a few
rare spikes to 30 mm. Tremor decreased by an order of magnitude on
10 November and it dropped to <1 hour/day on 13 November.

During fieldwork in early December, G. Soto (ICE) and G. Bouton
(IPG) inspected the near-source region. For a radial distance of ~1
km from to the crater they saw a deposit consisting of muddy ash,
lapilli, and blocks. These reached 40 cm thick on the crater's
southern outer rim at a point 150 m from the inner rim. The
deposit's thickness and grain size decreased rapidly with distance,
such that at 600 m SW of the crater the deposit was only 7 cm
thick. The deposit's basal zone was enriched in fine grained,
muddy-looking material, but throughout the deposit there occurred
lustrous black juvenile clasts. Over ~1 km^2 of the upper surface
of the deposit, there lay a blanket consisting of (a) dense,
quenched blocks, (b) breadcrust bombs with notably vesicular cores,
and (c) some highly vesiculated fragments. On 8 December at points
5 and 8 km from the summit, the Penjama and Blanco rivers,
respectively, still ran milky and were slightly acidic in taste.
That same day, the scientists saw only fumarolic activity. Although
scientists looked for a lake in the depths of the crater, they
failed to gain a clear view there.

All historical eruptions came from the Rincon crater, a depression
that heretofore contained (and may still contain) a 500-m-wide
lake. A previous phreatic eruption began on 8 May 1991 (Bulletin v.
16, nos. 4, 5, 7, and 8; v. 17, nos. 2, 3, 5, 6, 8, and 10; Bouton
and others, 1995).

Reference: Boudon, G., Rancon J.-P., Kieffer, G., Soto, G.J.,
Traineau, H., and Rossignol, J.-C., 1995, Estilio eruptivo actual
del Volcan Rincon de la Vieja: evidencias de las productos de las
erupciones de 1966-70 y 1991-92: Rothschildia, 2 (2): 10-13, Area
de conservacion de Guanacaste, Costa Rica.

Information Contacts: E. Fernandez, E. Duarte, R. Saenz, W.
Jimenez, and V. Barboza, Observatorio Vulcanologico y Sismologico
de Costa Rica, Universidad Nacional (OVSICORI-UNA), Apartado
86-3000, Heredia, Costa Rica; Georges Boudon, Institut de Physique
du Globe de Paris, 4, Place Jussieu, 75252, Paris Cedex 05, France.

Figure 15. Rincon de la Vieja's monthly totals for tremor and low-
frequency seismicity, January-September 1995. Courtesy of OVSICORI-

Figure 16. Rincon de la Vieja's seismicity, 1-13 November 1995. An
eruption began on 6 November. Courtesy of OVSICORI-UNA.

Figure 17. Map of NW Costa Rica showing key features associated
with Rincon de la Vieja's 6 November 1995 eruption. Courtesy of