When the Spanish first visited the Cordillera de los Maribios in AD 1524, they reported that San Cristóbal volcano was erupting and “giving off visible flames.” Apparently, the volcano remained active thoughout much of the 16th century. The volcano erupted strongly in AD 1684 and again in AD 1685. Aside from fumarolic activity and minor ash emissions, a quiescent period of almost three centuries followed. Beginning in 1972, vigorous eruptions resumed. Activity typically included sharp explosions, gas emissions, and ash fall to the south and west. The most recent eruptions of this type occurred in 1999 to 2000.
Population centers, especially to the south, occupy the aprons of the volcano complex and are at risk from phenomena common at stratovolcanoes. About 15 km southwest of San Cristóbal is the commercial and political center of Chinandega, which has a population of more than 50,000. El Viejo, also to the southwest, has a population of about 25,000. Wind typically distributes ash and coarser particles ejected into the air (tephra) to the west or southwest of San Cristóbal volcano. During rainy periods, tephra on the steep slopes of the volcano can be mobilized to form debris flows (watery flows of mud, rock, and debris—also known as lahars when they occur on a volcano).
South of Casita, two towns with populations in the range of 5,000 to 15,000 inhabitants are Chichigalpa and Posoltega. Though Casita volcano has apparently not been active in historical time, or about the last 500 years, it has the potential to produce debris flows and debris avalanches that could inundate these nearby populated areas.
In late October and early November 1998, torrential rains of Hurricane Mitch caused numerous slope failures in Central America. The most catastrophic occurred at Casita volcano, on October 30, 1998. At Casita, five days of heavy rain triggered a 1.6-million-cubic-meter rock and debris avalanche that generated an 2- to 4-million-cubic-meter debris flow that swept down the steep slopes of the volcano. The debris flow spread out across the volcano’s apron, destroyed two towns, and killed more than 2500 people. In contrast to suggestions in initial accounts, the avalanche did not dam the upper drainages or impound water that later broke out to form the debris flow. Rather, it appears that the Casita debris flow evolved as the avalanche moved down slope.
On October 30, 1998, between 10:30 and 11:00 AM, residents south of Casita heard a roaring noise like helicopters or thunder. Some thought an earthquake was occurring. Three to five minutes thereafter, a wave of muddy debris 1.0 to 1.3 km wide and an average of 4.5 m deep destroyed all traces of two towns.
Observations by survivors indicate an enormous flood on the slopes of the volcano and a wall of mud on the volcano apron. A person on the volcano slopes saw a “black curtain of water with trees.” On the apron of the volcano where the flow spread out, survivors describe the flow as--“an infernal wave of mud, rocks, and trees,” or “enormous mass of mud.”. The debris flow moved about 10 km from its source. It also generated floods that moved an additional 10 km downstream, destroying roads and bridges and inundating homes.
In prehistoric time, Casita erupted explosively to form ash-fall deposits (tephra), debris avalanches, lava flows, and hot flowing mixtures of ash and rock (called pyroclastic flows). The chronology of activity at Casita is rather poorly known. Its last documented eruption occurred 8300 years ago, and included a pyroclastic flow. Tephra deposits exposed in the east crater suggest the possibility of subsequent eruptions. Work prior to Hurricane Mitch suggested that a part of the volcano’s apron that included the area inundated during the 1998 event south of Casita was a lahar pathway. Erosion during Hurricane Mitch revealed that at least three large lahars descended this pathway to distances of up to 10 km.
This report describes the hazards of landslides and lahars in general, and discusses potential hazards from future landslides and lahars at San Cristóbal and Casita volcanoes in particular. The report also shows, in the accompanying lahar-hazard-zonation maps, which areas are likely to be at risk from future landslides and lahars at Casita and San Cristóbal.
Return to:
[Report Menu]
