Risks to Local Population

The populations around Irazu volcano have to worry about lahars and pyroclastic flows if they are to the north or northwest of the volcano, and ash deposits if they are to the west or southwest of the volcano.  The following excerpt is from Galt, 2004:

"On March 13, 1963, coinciding with the visit by U.S. President John F. Kennedy, Volcán Irazú began a marked period of activity, erupting ash and volcanic "bombs." Roofs in nearby towns collapsed under the weight of the ash. Agricultural activity, especially potato cultivation and milk production, was greatly reduced. According to the Ministerio de Agricultura y Ganadería, the eruptions affected over 45,000 hectares of agricultural land. Ash as deep as 1.5 meters accumulated quickly in the cities and towns of the Valle Central. The rainy season brought mudflows as the ash was washed through the deep gullies on the flanks of the volcano. A mudflow destroyed 400 houses in Taras de Cartago where the ash-water mixture was released from the gullies onto the valley floor. This strong eruptive activity continued into 1964. In that year a Costa Rican physician and Hungarian visitor were killed and a group of 50 people injured when they approached the crater while it continued to erupt. Since then, the Costa Rican government has prohibited tourist access to the craters. By 1965, activity had decreased to periodic releases of gases, and by March the activity ceased. In July 1965 a lake began forming in the principle crater. The lake level has fluctuated since then, disappearing some years in the dry season. The lake has been a permanent feature since 1991, though its color has changed from mustard to turquoise to green. To the west the highest point of Volcán Irazú is visible. To the east is Cráter Diego de la Haya, the site of the eruption in 1723, since filled with pyroclastic debris. To the south is Playa Hermosa, a flat area created by previous eruptions."

The main volcanic hazard is the ash, but other hazards include ejecta (bombs), pyroclastic flows, and lahars.

Bombs ejected from the summit crater by strombolian explosions can cause a danger to visitors from the nearby cities.  In 1964 seven people were killed by explosive ejecta because they went up close to the volcano to watch the eruptions (Global Volcanism Program).
Photo by M. Esquivel (published in Barquero, 1998).

Ashfall from the 1963-1965 eruption restricted visibility to 30m in San José, paralyzing traffic and causing damage to farmlands.  About 2000 dairy cows died of disease and malnutrition.  Remobilization of the ash during the rainy season caused mudflows that destroyed houses and roads and caused fatalities (Global Volcanism Program).
Photo by W. Schaer (published in Barquero, 1998).

The eruption plume in the background of this picture of the streets of San Jos
é is from the eruption on September 27, 1917.  In January of 1918, ashfall reached the city and explosive activity continued intermittently until 1921 (Global Volcanism Program).
Photo from Alvarado, 1989.

Over the 2 year period from 1963-1965, 46 mudflows occurred.  One killed at least 20 people and destroyed 400 houses and some factories (Global Volcanism Program).  In 1994, a phreatic explosion swept down this valley, destroying trees high up the banks of the river and triggering lahars for many kilometers.  The explosion in 1994 also formed a new crater in the fumarole field north of Irazú's main crater.
Photo by Jorge Barquero, 1998 (OVISCORI-UNA).

The mounds in this picture are scoria cones on the southern flank of the volcano.  Cones such as these were the source of the Cervantes lava flows (Barquero).
Photo by Ichio Moriya (Kanazawa University).

Hazards Maps

Prevailing winds carry ash to the west and southwest, while topography constrains the lahar flows and pyroclastic flows primarily to the north and northwest.  Most rockfalls occur on the north and northwestern flanks, where the walls are steep and the newest crater is located.

Analysis of Hazards

Volcanic eruptions in 1963-1965 produced ash that dammed a small river, resulting in the flooding of Cartago and causing serious damage to coffee crops.  The ash also hit San José, 54 km away, and caused some roofs to collapse (Encyclopedia Britannica).  Many of the eruptions from Irazú have resulted in lahars that wiped out hundreds of homes.  Even though the effects of the volcano on the populations nearby has been limited, the volcano still warrants monitoring because of its high score on the Threat Assessment for the National Volcano Early Warning System.  In this system, different hazard factors, historical unrest factors, and exposure factors are added up to determine the volcano's relative threat ranking.  Irazú has a VEI of 3, Holocene pyroclastic flows/lahars/lava flows, phreatic activity, sector collapse, high relief, crater lake, seismic unrest, fumarolic degassing, and has a permanent and large population nearby.  The relative threat ranking is a 235.5, indicating that the volcano needs a monitoring level of 4.  Irazú is currently getting a monitoring level of 1, and the volcano is classified as in a state of unrest (a level 2 of 5).  While lahars are a major problem for those near the volcano, the biggest issue is the ash because of the secondary effects of these deposits.

Minimizing Risk

The best way to minimize the risk to nearby residents is to monitor the volcano and have an evacuation plan in place for those areas in danger of lahar flows.  In the past, the only deaths were caused by lahars.  Ash has been a problem with crops and roof collapse, but by educating citizens of Costa Rica about what they should do if ash rained on them, that problem would be solved.  The towns downstream of a lahar flow should be made aware of their proximity to the volcano and what they should do in the event of an eruption.

Live Volcano Webcams

Interactive maps for heat sensors, recent earthquakes, and ash advisories can be found on Volcano Discovery's website here.
Unfortunately, the maps are not supported for Google sites and cannot be shown here.  Please check these out because they are awesome.

Seismic Monitoring

Monitoring of the volcano is done by OVISCORI-UNA, which is a seismic monitoring program run by the Volcanological and Seismological Observatory of Costa Rica and the National University. 

"The Volcanological and Seismological Observatory of Costa Rica, is an interdisciplinary research institute at the National University. Its mission is volcanic and seismic monitoring to document, analyze and interpret these processes and disseminate this knowledge to contribute to society to risk prevention and mitigation of disasters they create."


The OVISCORI-UNA site can be found here.