Hazardous Volcanic Events

There are several kinds of events caused from volcanic action that can be harmful to life and property. These include lava flows, lahars, ash falls, debris avalanches, and pyroclastic density currents.


List of Volcanic Hazards

Mitigation of hazards is an important goal of the volcanological community, including the U.S. Geological Survey . Volcanologists themselves require safety procedures for conducting hazardous scientific studies on volcanoes.

Many volcanoes around the world have been targeted for hazards research and several of the most notorious volcanoes have been designated as Decade Volcanoes for concentrated hazards research. Within striking range of 30,000,000 people around it, including Mexico City, Popocatepetl should be on the Decade Volcano list.

Access Montserrat for an on-going hazards mitigation drama. _______________________________________________________________________

Pyroclastic Density Currents

Pyroclastic density currents are are gravity-driven, rapidly moving, ground-hugging mixtures of rock fragments and hot gases. This mixture forms a dense fluid that moves along the ground with an upper part that is less dense as particles fall toward the ground. The behavior of the fluid depends upon the solids concentration relative to the amount of hot gases (i.e., solids-gas ratio). High concentration density flows are called "pyroclastic flows" and are essentially nonturbulent and confined to valleys. Low concentration density flows are called "pyroclastic surges" which can expand over hill and valley like hurricanes. Temperatures may be as hot as 900 degrees Celsius, or as cold as steam ( see "base surges" in section on Hydroclastic Processes).

Pyroclastic flows and surges are potentially highly destructive owing to their mass, high temperature, high velocity and great mobility. Deadly effects include asphyxiation, burial, incineration and crushing from impacts. Many people and the cities of Pompeii and Herculaneum were destroyed in 79 AD from an erupion of Mount Vesuvius; 29,000 people were destroyed by pyroclastic surges at St. Pierre, Martinique in 1902; >2000 died at Chichónal Volcano in southern Mexico in 1982 from pyroclastic surges. The only effective method of risk mitigation is evacuation prior to such eruptions from areas likely to be affected by pyroclastic density currents.


Lahars are part of the family of debris flows that are fluids composed of mixtures of water and particles of all sizes from clay-size to gigantic boulders. The abundance of solid matter carries the water, unlike watery floods where water carries the fragments. Debris flows have the viscous consistency of wet concrete, and there is a complete transition to watery floods. Lahars are composed of volcanic particles and originate directly or indirectly from volcanic action. Lahars can form by hot pyroclastic surges or flows entering watershed systems or flowing over snow and ice, by eruptions through crater lakes, by heavy rains on loose volcanic debris -- that is, any process by which volcanic particles can become saturated by water and move downslopes. They can move with velocities as low as 1.3 m/s to as great as 40 m/s on steep slopes (1 m/s = 2.55 miles per hour). They are known to have travelled as far as 300 km (1 km = 0.63 miles). Lahars have destroyed many villages and lives living on Indonesian volcanoes because most people live in valleys where lahars flow. The 21,000 lives lost at Armero, Colombia, was from a lahar that formed during the eruption of Nevado del Ruiz in 1985. It was generated by meltwater from the interaction of pyroclastic surges with snow and ice, from a very small eruption. Lahars can transform into regular floods as they become increasingly diluted with water downstream. This phenomenon was first discovered at Mount St. Helens where hot pyroclastic surges transformed to lahars, which further transformed to hyperconcentrated streamflow and then to normal stream-flow turbulence (floods).

Debris-flow Avalanches

The eruption of Mount St. Helens on May 18, 1980 started with a relatively small volcanic earthquake that caused collapse of the north side of the volcano because it was oversteepened and therefore unstable. When the landslide occurred, it decreased the pressure on the pressurized interior of the volcano which expanded explosively to form a lateral blast that devastated the countryside north of the volcano. Most of the debris flow avalanche was diverted down the North Fork Toutle River, but some moved directly northward over a 300 meter ridge and down into the next valley. Since the 1980 Mount St. Helens eruption, dozens of volcanoes that have given rise to avalanches have been discovered. For example, 40 avalanches exceeding 1 Km3 in volume, and 22 with a volume of less than 1 km3, are now known from the Quaternary alone, and 17 historic volcanic avalanches have been identified. The hilly topography north of Mount Shasta in northern California is now known to be the result of a have debris-flow avalanche. Some are known to extend up to 85 km from their sources and to cover tens to more than 1000 km2 in area.

Lava flows

Lava flows rarely threaten human life because lava usually moves slowly -- a few centimeters per hour for silicic flows to several km/hour for basaltic flows. An exceptionally fast flow (extremely rare) at Mt. Nyiragongo, Zaire (30-100 km/hour), overwhelmed about 300 people. Major hazards of lava flows -- burying, crushing, covering, burning everything in their path. Sometimes lava melts ice and snow to cause floods and lahars. Lava flows can dam rivers to form lakes that might overflow and break their dams causing floods. Methods for controlling paths of lava flows: (1) construct barriers and diversion channels, (2) cool advancing front with water, (3) disruption of source or advancing front of lava flow by explosives.

Tephra falls and Ballistic Projectiles formed on Land

Tephra consists of pyroclastic fragments of any size and origin. It is a synonym for "pyroclastic material." Tephra ranges in size from ash (<2 mm) to lapilli (2-64 mm) to blocks and bombs (>64 mm). Densities vary greatly, from that of pumice (<0.5)) to solid pieces of lava with density about 3.0. Blocks from basement material may exceed 3.0. Material may be juvenile (formed of magma involved in the eruption ) or accidental (derived from pre-existing rock).

Tephra fall and ballistic projectiles endanger life and property by (1) the force of impact of falling fragments, but this occurs only close to an eruption, (2) loss of agricultural lands if burial is greater than 10 cm depth, (3) producing suspensions of fine-grained particles in air and water which clogs filters and vents of motors, human lungs, industrial machines, and nuclear power plants, and (4) carrying of noxious gases, acids, salts, and, close to the vent, heat. Burial by tephra can collapse roofs of buildings, break power and communication lines and damage or kill vegetation. Even thin (<2 cm) falls of ash can damage such critical facilities as hospitals, electic-generating plants, pumping stations, storm sewers and surface-drainage systems and sewage treatment plants, and short circuit electric-transmission facilities, telephone lines, radio and television transmitters. When dispersed widely over a drainage basin, tephra can change rainfall/runoff relationships. Low permeability of fine ash deposits leads to increased runoff, accelerated erosion, stream-channel changes and hazardous floods. In contrast, thick, coarse-grained deposits closed to the source can increase infiltration capacity and essentially eliminate surface runoff.

Many of the hazards of tephra falls can be mitigated with proper planning and preparation. This includes clearing tephra from roofs as it accumulates, designing roofs with steep slopes, strengthening roofs and walls, designing filters for machinery, wearing respirators or wet clothes over the mouth and nose because tephra can contain harmful gases adsorbed on the particles as acid aerosols and salt particles.

Volcanic Gas

Magma is molten rock containing dissolved gases that are released to the atmosphere during an eruption and while the magma lies close to the surface from hydrothermal systems. The most abundant volcanic gas is water vapor; other important gases are carbon dioxide, carbon monoxide, sulfur oxides, hydrogen sulfide, chlorine, and fluorine. The gases are transported away from vents as acid aerosols, as compounds adsorbed on tephra and as microscopic salt particles. Sulfur compounds, chlorine and fluorine react with water to form poisonous acids damaging to the eyes, skin and repiratory systems of animals even in very small concentrations. The acids can destroy vegetation, fabrics and metals. Atmospheric veils of dust or acid aerosols caused by large-volume explosive eruptions can effect regional or global climate.

Most volcanic gases are noxious and smell bad, but they can cause mass fatalities. An rare case of mass deaths by volcanic gases in 1986 at Lake Nyos, in Cameroon, West Africa. Tons of carbon dioxide spilled out of Lake Nyos, and flowed silently down a canyon and through 3 village occupied by 1700 people. They and 3000 cattle died instantly from lack of oxygen.

Carbon dioxide emissions are now being monitored at Mammoth Mountain, California.


A tsunami is a long-period sea wave or wave train generated by a sudden displacement of water. Tsunamis travel at very high speeds through deep water as low broad waves and build to great heights as they approach the shallow bottom of shores. Most are caused by fault displacements on the sea floor, but many have been caused by volcanic action. The eruption of Krakatau in 1883 produced tsunamis that killed 36,000 people. The pyroclastic flow generated by this eruption displaced the water that initiated the tsunamis.

Copyright (C) 1997, by Richard V. Fisher. All rights reserved.