Smithsonian Institution
Global Volcanism Network Bulletin v. 20, no. 10, October 1995
Rincon de la Vieja (Costa Rica)  New eruption; lahars damage
       a bridge and lead to evacuations

Rincon de la Vieja
Costa Rica
10.83 N, 85.33 W; summit elev. 1,916 m
All times are local (= GMT - 6 hours)

A new phreatomagmatic eruption followed three months of
declining seismicity. During 1995 the number of local
earthquakes peaked in July and then progressively decreased
(figure 1). Prior to the eruption, during October,
OVSCICORI-UNA reported that park rangers who ascended to the
main summit saw increased degassing and noted the appearance
of fumaroles along cracks at the E and NE crater margins.
Rangers described the crater lake~s color as green and the
smell as strong and sulfurous.

ICE described the eruption as phreatomagmatic, beginning at
1504 on 6 November, and climaxing on 8 November with 25
explosions. They noted the ash-bearing and steam-rich
columns rose to 1 and 4 km, respectively, above the crater.
Ash blew WSW; medium- to fine-grained ash reached up to 30
km from the volcano (Santa Rosa National Park).

According to ICE, on 9 November the eruption entered a
steam-rich phase. Columns typically rose 200 m, but
sometimes as much as 1.5 km after some steam explosions.

During the course of the eruption, ballistic ejecta were
thrown over a zone extending to ~1 km N. Ejecta formed
lahars that followed two key rivers (Penjamo and Azul
rivers) and their tributaries. Heavy rains beginning on 10
and continuing on 11 November triggered secondary lahars and
associated floods; a bridge 7 km N of the crater (Penjamo
bridge) was damaged but not destroyed, interrupting traffic
flow. During this episode, lahars along a tributary of the
Penjamo river produced a gully 8-m deep and 25-m wide,
isolating some inhabitants.

Initial inspections of ash and the lahar matrix indicated
that they mainly consisted of hydrothermally altered
fragments, lake-sediment mud, and vesiculated glassy
andesite fragments.

Some residents living near the volcano were evacuated to a
safe village 9 km NW of the crater. News reports on 8
November by both Associated Press and Deutsche Presse-
Agentur stated that about 100 families were evacuated. Two
days later Enrique Coen reported relocation of 300 families.

The volcano sits near Costa Rica~s N border, 85 km SE of
Concepcion, the volcano forming a prominent island in the W
part of Lake Nicaragua. Rincon de la Vieja is a compound
volcano that has erupted at least twelve times since 1860.

Information Contacts: E. Fernandez, E. Duarte, and V.
Barboza, Observatorio Vulcanologico y Sismologico de Costa
Rica, Universidad Nacional (OVSICORI-UNA), Apartado 86-3000,
Heredia, Costa Rica; G.J. Soto, Oficina de Sismologia y
Vulcanologia del Arenal y Miravalles: OSIVAM, Instituto
Costarricense de Electricidad (ICE), Apartado  10032-1000,
San Jose, Costa Rica; Enrique Coen, Departamento de Fisica,
University Nacional, Heredia; Costa Rica; P.O. Box 86,
Heredia 3000; Associated Press; Deutsche Presse-Agentur.