Volcano Lecture 3: Lahars Flashcards

Study Notes

Mount Rainier and Lahars

Mount Rainier serves as an example of disaster risks due to mudslides and pyroclastic flows.

Lahar Overview

Lahar refers to a class of destructive muddy flows, notably impactful in volcanic regions.
Example: The Rio Lagunillas Armero incident, which led to 23,000 fatalities, highlights the potential for tragedy that could be mitigated.
Recognized as one of the most serious ground-based hazards at many volcanoes, lahars can occur with or without eruptions.
Capable of traveling several tens of kilometers at velocities of tens of meters per hour.
Lahars can inflict damage on all structures along their paths and deposit significant sediment on valley floors.

Definition of Lahar

The term 'lahar' originates from Indonesia, describing a mixture of water and rock fragments, resembling wet concrete that contains rock debris from clay to large boulders.
Large lahars can measure hundreds of meters in width and tens of meters in depth, flowing at high speeds that are dangerous for individuals attempting to escape.

Types of Lahars

Debris flow: Contains a high ratio of coarse particles (over 80% sediment by weight).
Mudflow: Composed of smaller sand and silt-sized particles (over 80% sediment by weight).
Hyperconcentrated streamflow: Contains 40-80% sediment, primarily sand-sized.
Cohesive lahars: Contain 3-5% clay-sized sediment.
Non-cohesive lahars: Less than 3-5% clay-sized sediment.

Triggers of Lahars

During eruptions: Result from melting snow and ice due to pyroclastic flows.
After eruptions: Triggered by heavy rainfall, leading to lake breakouts or runoff.
Without eruptions: Sudden landslides or earthquakes can initiate lahars.

Example of Lahar Triggered by Melting

On May 18, 1980, lahars formed on the sides of Mount St. Helens due to melting snow and ice from hot volcanic ejecta and pyroclastic flows.

Magmatic Triggers: Armero-Nevado del Ruiz

Nevado del Ruiz, Colombia, at 5,389 meters tall, has a snow-covered area of 25 square kilometers.
The eruption melted approximately 10% of the ice cover, triggering lahars that eroded nearby soil and vegetation, flowing downstream at around 60 km per hour.

Impact of Armero-Nevado del Ruiz Eruption

In just four hours following the eruption, lahars caused 23,000 deaths, injured 5,000 individuals, and destroyed 5,000 homes—three-quarters of the town's population perished.
Despite flow depths of only 2-5 meters and the town's distance of 75 km from the volcano, many lives could have been saved with adequate warning systems.
The event revealed a significant failure in civil safety systems.

Lahars Following Eruptions

Heavy rainfall can mobilize freshly deposited materials, potentially causing further lahar flows.

Mount Pinatubo, 1991

Located in a subduction zone, Mount Pinatubo had a Volcanic Explosivity Index (VEI) of 6.
Pyroclastic flows drastically altered the landscape, filling river valleys with volcanic material up to 200 meters thick and annihilating vegetation.
The region experiences annual rainfall between 2 to 4 meters.

Lahars without Eruptions

Triggered by factors such as earthquakes, landslides, or lake outburst floods.

Lahar Impacts

Resulted in approximately 400 deaths and displaced about 200,000 individuals into temporary relocation centers and new communities.
The impact of lahars lasted a decade, leading to disruptions in transportation and agriculture.
Significant government expenditures were required for recovery and community reunification efforts.

Description

Explore the devastating impacts of lahars through this set of flashcards. Learn about mudslides, examples such as Mount Rainier, and the tragic events in Rio Lagunillas Armero. Understand how these ground-based hazards can occur with or without volcanic eruptions.