Geomorphology 33

J Bret Bennington

Geomorphology of Volcanic Hazards

Updated 3-01

Volcanic eruptions produce a variety of effects which are hazardous to lives and property. Some of these leave distinctive geomorphic fingerprints on the surrounding landscape. Geologists can examine the existing topography of a volcanic region to assess the likely hazards that might be a problem for inhabitants in the future.

Eruption Hazards

Gas

Lateral blast

Ash

Lava flows

Debris flows (landslides)

Lahars

Pyroclastic flows

The most common volcanoes in the world are stratovolcanoes and lava domes, which are also the most dangerous because their eruptions are commonly accompanied by debris flows, lahars, and pyroclastic flows that can impact populated regions far from the summit of the volcano.

Lateral blast

The explosive rupture of a stratovolcano or lava dome along it's flank can produce an expanding wave of gas and pyroclastic debris capable of destroying much of everything in it's path. The effects of a lateral blast vary greatly depending on the size of the explosive eruption.

In the intial eruption of Mt. St. Helens the lateral blast was a 7 megaton explosion that propelled volcanic debris at 300 mph, which leveled 4 billion board-feet of timber and killed 57 people, several of whom tried unsuccessfully to outrun the blast in automobiles and trucks.

Debris flows

Landslides are common on volcanoes because volcanic mountains are often steep (particularly stratovolcanoes) and loosly constructed of tephra and lava flows. Explosive volcanic eruptions cause massive debris flows along the flank of a volcano. Also, volcanic eruptions are often preceeded by swelling of the magma chamber within and by earthquakes, which can trigger failure of the flank of a volcano. Heavy rain that saturates newly erupted material can also trigger a volcanic debris flow.

Volcanic landslides have also been known to trigger lahars and even tsunami if they flow directly into the sea. Historically, the deadliest volcanic landslide occurred in 1794 when debris from Mt. Mayuyama in Japan slid into the Ariaka Sea and caused a tsunami that killed 15,000 people on the opposite shore.

Effect on the landscape - Large volcanic debris flows leave prominent scars on the sides of volcanoes. They flow downslope and bury the existing topography, leaving a chaotic terrain of small rounded hills and depressions (hummocky terrain). Debris flows can fill in existing river valleys and block flow, creating new lakes.

Lahar

Lahar is an Indonesian term that describes a rapidly flowing mixture of water and volcanic tephra, as well as ice, rock, mud, and anything else incorporated into the flow. Lahars vary greatly in size and speed of flow, but the largest have destroyed entire towns travelling at speeds in excess of 50 ft / sec. Often lahars grow in size as they flow down the flank of a volcano by sweeping up additional water and debris.

Lahars can be triggered by eruptions that melt glacial ice on the summit of stratovolcanoes or eject water from a caldera lake. Debris flows can evolve into lahars if they incorporate lake or river water while flowing down the flank of the volcano. Lahars can also be triggered by periods of heavy rainfall.

Effect on the landscape - Ancient lahars can be recognized as chaotic deposits of mud and boulders similar to glacial till that developed along the regional drainage. They infill existing river valleys and may alter existing drainage patterns.

Pyroclastic flow

These are high-density mixtures of hot, dry tephra and hot gases that flow rapidly away from volcanic vents during eruptions. Pyroclastic flows occur during explosive eruptions and also due to collapse of part of a lava dome on a steep slope. Most pyroclastic flows are gravity-driven. Often they are called by their French name: Nuee Ardente (glowing cloud).

Pyroclastic flows consist of two parts:

- basal flow of coarse fragments that moves along the ground

- turbulent cloud of ash that rises above the basal flow

Pyroclastic flows vary greatly in size but even small flows are extremely destructive due to their density, speed (up to 60 mph), and high temperature (200°-700° C). Buildings in their path are usually completely destroyed.

Famous disasters caused by pyroclastic flows include the partial destruction and total burial of Pompeii in A.D. 79 and the complete destruction of the city of St. Pierre on the island of Martinique in 1902.