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REPORT:
Lahar Hazards at Concepción Volcano, Nicaragua


-- J.W. Vallance, S.P. Schilling, G. Devoli, and M.M. Howell, 2001,
Lahar Hazards at Concepción Volcano, Nicaragua U.S. Geological Survey Open-File Report 01-457

Introduction

Concepción is one of Nicaragua's highest and most active volcanoes. The symmetrical cone occupies the northeastern half of a dumbbell-shaped island called Isla Ometepa. The dormant volcano, Maderas, occupies the southwest half of the island. A narrow isthmus connects Concepción and Maderas volcanoes. Concepción volcano towers more than 1,600 meters above Lake Nicaragua and is within 5 to 10 kilometers of several small towns situated on its aprons at or near the shoreline. These towns have a combined population of nearly 5,000. The volcano has frequently produced debris flows (watery flows of mud, rock, and debris -- also known as lahars when they occur on a volcano) that could inundate these nearby populated areas.

Concepción volcano has erupted more than 25 times in the last 120 years [1]. Its first recorded activity was in AD 1883. Eruptions in the past century, most of which have originated from a small summit crater, comprise moderate explosions, ash that falls out of eruption plumes (called tephra), and occasional lava flows. Near the summit area, there are accumulations of rock that were emplaced hot (pyroclastic deposits), most of which were hot enough to stick together during deposition (a process called welding). These pyroclastic rocks are rather weak, and tend to break apart easily. The loose volcanic rock remobilizes during heavy rain to form lahars. Volcanic explosions have produced blankets of tephra that are distributed downwind, which on Isla Ometepe is mostly to the west. Older deposits at the west end of the island that are up to 1 m thick indicate larger explosive events have happened at Concepción volcano in prehistoric time. Like pyroclastic-flow deposits, loose tephra on the steep slopes of the volcano provides source material that heavy rainstorms and earthquakes can mobilize to trigger debris flow.

North-south-trending fractures cutting across the volcano are associated with small volcanic flank vents called spatter cones, cinder cones, and maars located on the middle north flank and on the lower south flank of Concepción volcano down to Lake Nicaragua [1]. Immature soils on deposits of fine ash to the west of these vents show that several of them have been active within recent prehistoric time.

In late October and early November 1998, torrential rains of Hurricane Mitch caused numerous slope failures in Central America, the most catastrophic of which occurred at Casita volcano, Nicaragua on October 30, 1998. At Casita, five days of heavy rain triggered a 1.6-million-cubic-meter rock and debris avalanche that generated a 2-to 4-million-cubic-meter debris flow that swept down the steep slopes of the volcano,spread out across the volcano's apron, destroyed two towns, and killed more than 2500 people [2]. The avalanche did not dam the upper drainages or impound water. Rather, it appears that the Casita debris flow evolved directly as the avalanche moved down slope [2]. 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 1.0-1.3 km wide and an average of 3.5 m deep destroyed all traces of two towns (figure 1). Observations by survivors record 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,,"an enormous mass of mud." [2]. 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.

Small debris flows occurred at Concepción volcano during Hurricane Mitch but none so large as the one that occurred at Casita. The largest is more than an order of magnitude smaller than the Casita debris flow. It descended SSW about 5 km toward San José del Sur (plate 1). A note in BVGN in March 1993 indicates that the area to the North and the SSW gully are the main foci of lahars in the recent past [1]. Deposits suggest that all of these lahars are similar in size and at least an order of magnitude smaller than that at Casita in 1998.

Lahars continue to form during rainstorms, with the main loci of activity being the SW gully and the N flank. The road N of the volcano was cut by a lahar in late 1992. Erosion remains rapid on higher slopes, where some gullies had widened by 2-5 m since 1990. The 1957/74 pyroclastic deposits are especially vulnerable to erosion and gully headwalls have almost intersected the crater rim to the N, W, and SW in the last two years. Rapid erosion of these deposits probably constitutes much of the source for the laharic material to the N and W of the cone. The towns of San Jose del Sur (6.2 km SSW), San Marcos (5.6 km NNW), and La Flor (5.3 km NW) are in a particularly dangerous situation because they are in the paths of lahars descending these gullies. [1].

Several very active, young symmetrical volcanoes in Nicaragua seem not to be as susceptible to slope failures as large as the one that occurred at Casita during Hurricane Mitch. San Cristóbal, Telica, Momotombo and Concepción volcanoes are all young, symmetrical cones that are formed of fresh rock. During Hurricane Mitch, small lahars and floods occurred at these volcanoes but none approaching the size of the big Casita flow. In contrast, Casita volcano is an older edifice where alteration processes have locally affected the rocks and where erosion has deeply incised its slopes. Alteration and erosion both weaken the edifice, making it more susceptible to failure.

Volcanoes emit gases that react with water to make acid, especially sulfuric acid. These acidic waters then react with the rocks to weaken them. With time the above hydrothermal processes weaken older volcanic edifices and make them more susceptible to landslides. Rain-induced landslides in altered, weakened rock are apt to form large lahars.

This report describes the hazards of landslides and lahars in general, and discusses potential hazards from future landslides and lahars at Concepción volcano in particular. The report also shows, in the accompanying lahar-hazard-zonation map, which areas are likely to be at risk from future landslides and lahars at Concepción. For a broader volcano-hazard assessment of the volcano see references in end note [1].


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02/14/02, Lyn Topinka