Landslides and lahar at Nevado del Huila Volcano, Colombia

Earthquake on June 6, 1994, Triggers Landslides and Catastrophic Lahar Near Nevado del Huila Volcano, Colombia

Landslide scars on hillslopes above Río Paez, near Huila Volcano, Colombia

Photograph by R.L. Schuster on July 3, 1994

Brown-colored scars on steep hillsides sweep into the Río Paez at the base of Nevado del Huila (elevation 5,262 m), a large volcanic complex in Colombia. The scars are pathways of dozens of landslides that removed wet soil, volcanic ash, and vegetation from the hillsides and produced a destructive lahar in the Río Paez valley (flowing toward bottom of photograph). The southwest flank of the volcano is visible in upper left.

Within minutes of a magnitude 6.4 earthquake beneath the SSW flank of Nevado del Huila volcano, dozens of landslides swept from the steep valleys above Río Paez into the river. The landslides quickly turned into debris flows, which joined to produce a single enormous flow of water, rocks, soil, and trees in Río Paez. The lahar destroyed most or parts of several towns along the river, including Dublin, Irlanda, Toez, and Belalcazar, killed several hundred people, and displaced about 20,000 people from their homes. Six bridges and >100 km of roads were destroyed.

The huge flow wave originated from both hydrothermally altered rocks of Huila Volcano and the ash-mantled terrain surrounding the volcano. Whether triggered by a large earthquake, intense rainfall (for example, see Casita Volcano), or both, this tragic event illustrates that both volcanic cones and steep hillslopes around them can collapse to form landslides and debris flows.

People from around the world have repeatedly sensed an approaching lahar as a rumbling noise or ground vibration. In some cases, had they known the cause of the noise and tremor, people killed by lahars may have had time to get to high ground before it reached them. In the case of lahars triggered by a large earthquake, people often mistake the shaking of the ground from an approaching lahar as a second earthquake, and so they stayed where they were. People living in valleys downstream from mountainous volcanic terrains can reduce their risk by moving immmediately to high ground for a few hours after a strong earthquake--just in case a destructive flow is headed their way. This strategy is very important for communities located at the mouth of a tributary canyon, an area that can be swept by lahars from both the canyon and the main river valley.