U.S. Department of the Interior U.S. Geological Survey Lahar Hazards at Concepción Volcano, Nicaragua Open-File Report 01–457 Lahar Hazards at Concepción Volcano, Nicaragua 1 Cover photograph Lithograph showing Lake Nicaragua and Concepcion Volcano adapted from Squier (1856). 2 Lahar Hazards at Concepción Volcano, Nicaragua Lahar Hazards at Concepción Volcano, Nicaragua By J.W. Vallance, S.P. Schilling, G. Devoli, and M.M Howell U.S. GEOLOGICAL SURVEY Open-File Report 01-457 Vancouver, Washington U.S.A. 2001 Lahar Hazards at Concepción Volcano, Nicaragua 3 U.S. DEPARTMENT OF THE INTERIOR Gale Norton, Secretary U.S. GEOLOGICAL SURVEY Charles G. Groat, Director This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. For additional information write to: Copies of this report can be purchased from: Scientist-in-Charge U.S. Geological Survey U.S. Geological Survey Information Services 1300 SE Cardinal Court, Bldg. 10 P.O. Box 25286 Vancouver, WA 98683 Denver, CO 80225 (360) 993-8900 (303) 202-4210 FAX: (360) 993-8980 4 Lahar Hazards at Concepción Volcano, Nicaragua CONTENTS Introduction ................................................................................................................................................ 7 Debris Avalanches, Landslides, and Lahars .............................................................................................. 9 Future Landslides and Lahars at Concepción Volcano .............................................................................10 Lahar-Hazard-Zonation Map ..................................................................................................................... 10 Lahar-hazard zones .............................................................................................................................. 11 Hazard Forecasts and Warnings ................................................................................................................ 11 Protecting Communities and Citizens from Lahar Hazards ....................................................................... 11 References................................................................................................................................................. 12 Additional Suggested Reading ................................................................................................................... 12 End Notes .................................................................................................................................................. 13 PLATE [In pocket] 1. Lahar hazards of Concepción volcano, Nicaragua. Proximal and distal hazard zones from lahars at Concepción volcano. FIGURES 1. Locations of major cities and significant Quaternary volcanoes in Nicaragua including Concepción volcano and schematic map showing the areal extent of and two cross- sections for the Casita, Nicaragua debris flow of 1998 .................................................................... 8 Lahar Hazards at Concepción Volcano, Nicaragua 5 6 Lahar Hazards at Concepción Volcano, Nicaragua Lahar Hazards at Concepción Volcano, Nicaragua By J.W. Vallance, S.P. Schilling, G. Devoli1 and M.M. Howell INTRODUCTION These pyroclastic rocks are rather weak, and tend Concepción is one of Nicaragua’s highest and to break apart easily. The loose volcanic rock most active volcanoes [1] (numerals in brackets remobilizes during heavy rain to form lahars. refer to end notes in the report). The symmetrical Volcanic explosions have produced blankets of cone occupies the northeastern half of a dumbbell- tephra that are distributed downwind, which on Isla shaped island called Isla Ometepa. The dormant Ometepe is mostly to the west. Older deposits at volcano, Maderas, occupies the southwest half of the west end of the island that are up to 1 m thick the island (figure 1). A narrow isthmus connects indicate larger explosive events have happened at Concepción and Maderas volcanoes. Concepción Concepción volcano in prehistoric time. Like volcano towers more than 1600 m above Lake pyroclastic-flow deposits, loose tephra on the steep Nicaragua (figure 1) and is within 5 to 10 km of slopes of the volcano provides source material that several small towns situated on its aprons at or heavy rainstorms and earthquakes can mobilize to near the shoreline. These towns have a combined trigger debris flow. population of nearly 5,000. The volcano has North-south-trending fractures cutting across frequently produced debris flows (watery flows of the volcano are associated with small volcanic mud, rock, and debris—also known as lahars when flank vents called spatter cones, cinder cones, and they occur on a volcano) that could inundate these maars located on the middle north flank and on the nearby populated areas. lower south flank of Concepción volcano down to Concepción volcano has erupted more than 25 Lake Nicaragua [1]. Immature soils on deposits of times in the last 120 years [1]. Its first recorded fine ash to the west of these vents show that several activity was in AD 1883. Eruptions in the past of them have been active within recent prehistoric century, most of which have originated from a time. small summit crater, comprise moderate In late October and early November 1998, explosions, ash that falls out of eruption plumes torrential rains of Hurricane Mitch caused (called tephra), and occasional lava flows. Near numerous slope failures in Central America, the the summit area, there are accumulations of rock most catastrophic of which occurred at Casita that were emplaced hot (pyroclastic deposits), volcano, Nicaragua on October 30, 1998. At most of which were hot enough to stick together Casita, five days of heavy rain triggered a during deposition (a process called welding). 1.6-million-cubic-meter rock and debris avalanche 1Instituto Nicaragüense de Estudios Territoriales (INETER), Managua, Nicaragua. Lahar Hazards at Concepción Volcano, Nicaragua 7 830 A 820 FLOW I FLOW I deposits 810 A' FLOW I FLOW II Head scarp La Ollada 1998 collapse 800 Crater 1405 Volcán Casita 790 Pre-flow land surface Sites of previous 00 0 780 13 1 20 FLOW II deposits flank collapses 0 770 10 1 00 0 50 100 150 200 250 300 10 Overflow to 0 Horizontal Distance, in meters 9 0 80 Argelia 320 Deposit thickness diagrammatic 310 15 cm levee 300 B' B A' Argelia Altitude, in meters above sea level Channel fill FLOW I deposits Versalles A Bella Vista 290 tree FLOW I 280 700 270 La Pelona FLOW II 600 Caldera 260 Site of flow 0 200 400 600 800 1,000 1,200 1,400 1,600 cross-section Horizontal Distance, in meters 5 at right 00 4 0 0 FLOW I Santa Narcisa 300 EL PORVENIR B' ROLANDO NICARAGUA B V. Cosiguiña RODRIGUEZ Site of flow cross-section V. Casita at right V. Momotombo Lake Minagua Boulders >3m V. Masaya Managua intermediate V. Mombacho diameter End of debris 0 V. Concepcíon 0 2 l flow e V. Maderas n n a h C 0 50 100 Km o t r e u M Runout of no hyperconcen- o M trated hlow and l sediment-laden E streamflow 012Km Figure 1. Locations of major cities and significant Quaternary volcanoes in Nicaragua including Concepción volcano and schematic map showing the areal extent of and two cross-sections for the Casita, Nicaragua debris flow of 1998 adapted from Scott et al. (in press). 8 Lahar Hazards at Concepción Volcano, Nicaragua that generated a 2- to 4-million-cubic-meter debris two years. Rapid erosion of these deposits flow that swept down the steep slopes of the probably constitutes much of the source for volcano, spread out across the volcano’s apron, the laharic material to the N and W of the destroyed two towns, and killed more than 2500 cone. The towns of San Jose del Sur (6.2 km people [2]. The avalanche did not dam the upper SSW), San Marcos (5.6 km NNW), and La drainages or impound water. Rather, it appears Flor (5.3 km NW) are in a particularly that the Casita debris flow evolved directly as the dangerous situation because they are in the avalanche moved down slope [2]. paths of lahars descending these gullies. [1]. On October 30, 1998 between 10:30 and 11:00 AM, residents south of Casita heard a Several very active, young symmetrical roaring noise like helicopters or thunder. Some volcanoes in Nicaragua seem not to be as thought an earthquake was occurring. Three to susceptible to slope failures as large as the one that five minutes thereafter, a wave of muddy debris occurred at Casita during Hurricane Mitch. San 1.0-1.3 km wide and an average of 3.5 m deep Cristóbal, Telica, Momotombo and Concepción destroyed all traces of two towns (figure 1). volcanoes are all young, symmetrical cones that are Observations by survivors record an enormous formed of fresh rock. During Hurricane Mitch, flood on the slopes of the volcano and a wall of small lahars and floods occurred at these volcanoes mud on the volcano apron. A person on the but none approaching the size of the big Casita volcano slopes saw a “black curtain of water with flow. In contrast, Casita volcano is an older edifice trees.” On the apron of the volcano where the flow where alteration processes have locally affected the spread out, survivors describe the flow as, “an rocks and where erosion has deeply incised its infernal wave of mud, rocks, and trees,” or, “an slopes. Alteration and erosion both weaken the enormous mass of mud.” [2]. The debris flow edifice, making it more susceptible