Nicaraguan Ecotourism/Planeta.com

Lahar on Casita Volcano: The Inevitability of a Natural Disaster
by Tom Fletcher

May/Mayo 1999

Nicaragua -- There has been a tremendous amount of commentary and even controversy surrounding the deadly mudslide in north-western Nicaragua that killed between 1800-2200 people, destroyed two villages, and became the rallying point for international environmentalists condemning Central American land use policies and deforestation. With the recent highly moving visit to the site by U.S. President Bill Clinton and subsequent international publicity, it may be time for a closer examination of what actually occurred at Casita on 30 October 1998. The following is a very preliminary assessment based on examination of satellite imagery (both visual and infrared), discussions with geologists, and first-hand examination of the slide site over the course of several visits. It is not intended as a scientific study.

Location and Description

Casita Volcano is one of a chain of volcanos known as the Cordillera Los Marrabios in Nicaragua extending from Cosiguina Volcano on the Gulf of Fonseca to Maderas Volcano in Lake Nicaragua. It is located in Chinandega Department, roughly two-thirds of the way between the cities of Chinandega and Leon. Casita is a classic cinder-cone that rises some 650 meters (2100 feet) above sea level. Although dwarfed by its near neighbors San Cristóbal (1745 m / 5600 ft), Telica (1060 m / 3400 ft), and Santa Clara (840 m / 2700 ft), Casita's form generates slopes exceeding 40% in some areas, especially near the crater rim. Vegetation on these slopes ranges from non-existent near the rim to low grasses and thorny scrub farther down to deforested, scattered trees, or light secondary forest around the base. Casita is considered dormant, with evidence of still-molten magma below the surface (active steam vents), but its last eruption was pre-historic (1).

The relatively flat land on the Pacific side of Casita, and indeed the entire Cordillera Marrabios, is heavily devoted to agriculture. The original biome is difficult to assess, but was probably a combination of dry tropical forest and savannah-like plains gently sloping down to the ocean. In the more fertile areas of this region, the land is devoted to large sugarcane plantations, cotton, and occasional ranching. In the more marginal areas closer to the mountains, agriculture is limited to subsistence or small-scale farm-to-market (mostly corn) plots. Because of slope, there were no farms actually on Casita itself. In addition, the slope also precludes extensive forest except at the very base of the volcano (where most has been cut down). The entire region is dotted with small villages.

The Casita Lahar

Shortly before 11:00 am on 30 October 1998, an approximately 100 m (330 ft) wide section consisting of some 200,000 m3 of rock and loose soil on a very steep section of the southern rim of Casita abruptly subsided. Known as a flank collapse, this subsidence started a debris avalanche. Within moments the avalanche, mixing with highly saturated soil and loose rock and scree, became a lahar (2). After traveling less than a kilometer (one half mile) laterally and a descent of some 200 m (600 ft) in elevation, the lahar was over 1000 m (3000 ft) wide and continuing to gain velocity. En route down the mountain, the flow front engulfed and destabilized much water-saturated material that was both entrained in the flow, and formed the dilute tail of the flow wave. When the lahar reached the flatter area at the base of the volcano, it was approximately 2.5 km wide (1.5 miles) and traveling in excess of 25 m/s (60 mph) (3). Although slowing slightly on the flat, the lahar was still traveling an estimated 20 m/s (35 mph) when it struck the two closely sited villages of Rolando Rodriguez (pop. approx 1,250), and El Porvenir (pop. approx 650) 3 km (1.8 mi) further down range. At this stage, based on an examination of the debris field, it is estimated that the lahar may have been as much as 10 m (30 ft) deep, with an average of 3-4 m (12-15 ft) (Scott 1999). The eastern half of the slide halted about 2 km (1 mi) further downrange, but a torrent of debris and water continued for another 6 km (3.5 mi) following a streambed and road, scouring a channel some 300 m (1000 ft) wide and up to 4-5 m (15 ft) deep until just across the main Leon-Chinandega highway on the outskirts of Posoltega. The total length of the slide is 12 km (5 mi) with a drop in elevation of some 500 m (1500 ft), most of which occurred in the first 2 km (1 mi). The final 6 km (3.5 mi) registers a drop of only 100 m (300 ft).

The duration of the event, from the first avalanche to the obliteration of El Porvenir and Rolando Rodriguez, was less than 5 minutes.

There was no eruption or earthquake recorded leading up to this devastating slide. Although the exact cause of the initiating subsidence is not yet known, a number of factors contributed to the creation of a lahar on Casita. Some of these factors are based on time, some on timing, and some on Hurricane Mitch. As was mentioned, Casita is considered dormant with, however, evidence of magma below. Over time, the normal expansion and contraction of this magma, added to hot, acid-rich water, and the earthquakes to which this region is prone, has eroded the subsurface structure of the volcano. Coupling the subsurface erosion with centuries of exposure to wind and rain erosion at the surface created a highly unstable slope that required little impetus to begin sliding. Turning a landslide into a lahar, however, requires the presence of a great deal of water. The timing of the slide, coming as it did at the end of the normal rainy season, meant that the soil around the volcano and on its slopes was highly saturated. Adding Hurricane Mitch, which dumped in excess of 480 mm (2.5 ft) of water on the volcano on 30 October (INETER 1998), meant that conditions were near perfect for the creation of a lahar.

It is unlikely that the original biome would have done much to mitigate the speed or power of the lahar (Scott 1999). Trees as large as 1 m (3 ft) in diameter were either uprooted by the flow wave or smashed to kindling by half-ton boulders. The walls of the houses in the two villages destroyed didn't appreciably slow the slide, and provided no protection. Once it began, there was nothing to stop the lahar until flat land had exhausted velocity it had gained on the slopes of the volcano. There is no evidence to indicate that human activity could either have prevented or mitigated - or added to - this lahar.

Conclusions

The Casita lahar represents the most devastating single effect of Hurricane Mitch in Nicaragua. Unfortunately, we can not blame human activity, deforestation, peasant farmers, giant landowners, poor government policies, international mining and agriculture monopolies, or others of that ilk for this disaster, as many writers apparently desire to do. The slide that initiated the lahar would have occurred eventually anyway - if not this year, then ten years from now, or whenever the unstable slope finally let go. The Casita disaster was primarily the culmination of abiotic factors that caused what would probably have been a short run-out landslide to metamorphose into a killer lahar.

To me, the idea that Casita was a natural phenomenon, as inevitable as time, is more frightening than being able to blame a human agency for the death toll. Most volcanic slopes in Central America are unstable by definition. Adding in human activity such as deforestation only increases the threat. The Casita Volcano lahar should serve as a warning - not as a political statement.

Footnotes:

1. The exact date of this eruption is unknown. A geologist colleague's cursory examination of the area indicated a date greater than 2000 years. However, I don't have hard data to back this up. In any event, the volcano has not erupted during recorded history. 2. Also known as a volcanic mudflow, a lahar is a highly fluid mixture of rock, soil and debris that "skates" on a film of water. It acts more like a viscous river than a landslide, and can reach very high velocity depending on slope. Lahars are a common threat to volcanic slopes, either during eruptions or when some other factor causes a subsidence in the presence of water. As an example, Mt. St. Helens in the U.S. experienced a lahar when the heat of its eruption caused a sudden melting of the snow cover. 3. The exact velocity of the slide at this point is not known precisely. This would be determined by the exact composition of the debris, amount of water, slope, distance traveled, etc. which has not been studied. The velocity noted above is based on similar occurrences elsewhere, but is probably close (Scott 1999).

Works Cited:

INETER 1998, "Las lluvias del siglo en Nicaragua", Instituto Nacional de Estudios Terrestrales, Managua, Nicaragua (multiple authors)

Scott, Kevin 1999 "Volcanic Landslides, Debris Avalanches, and Debris Flows in Nicaragua Resulting from Hurricane Mitch", preliminary report of a USGS mission, January 1999, USAID Managua, Nicaragua

Tom Fletcher runs Ecotourism International of Nicaragua and can be reached via email at einsa@ibw.com.ni.

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