harzards

Volcanic materials ultimately break down to form some of the most fertile soils on Earth, cultivation of which fostered and sustained civilizations. People use volcanic products as construction materials, as abrasive and cleaning agents, and as raw materials for many chemical and industrial uses. The internal heat associated with some young volcanic systems has been harnessed to produce geothermal energy. For example, the electrical energy generated from The Geysers geothermal field in northern California can meet the present power consumption of the city of San Francisco. Volcanoes have played a key role in forming and modifying the planet upon which we live. More than 80 percent of the Earth's surface (above and below sea level) is of volcanic origin. Gaseous emissions from volcanic vents over hundreds of millions of years helped cool off the earth removing heat from its interior and formed the Earth's earliest oceans and atmosphere, which supplied the ingredients vital to evolve and sustain life. Over geologic eons, countless volcanic eruptions have produced mountains, plateaus, and plains, which subsequent erosion and weathering have sculpted into majestic landscapes and formed fertile soils. Volcanic materials provide nutrients to the surrounding soil. Volcanic ash often contains minerals that are beneficial to plants, and if it is very fine ash it is able to break down quickly and get mixed into the soil. I suppose another benefit might be the fact that volcanic slopes are often rather inaccessible, especially if they are steep. Thus they can provide refuges for rare plants and animals from the ravages of humans and livestock. volcanic gases are the source of all the water (and most of the atmosphere) that we have today. The process of adding to the water and atmosphere is pretty slow, but if it hadn't been going on for the past 4.5 billion years or so we'd be pretty miserable. Volcanic deposits are also used as building materials. In the 1960's Robert Bates published Geology of the Industrial Rocks and Minerals. He noted that basalt and diabase are quarried in the northeastern and northwest states. Most of the basalt and diabase is used for crushed stone: concrete aggregate, road metal, railroad ballast, roofing granules, and riprap. High-denisity basalt and diabase aggregate is used in the concrete shields of nuclear reactors. Some diabase is used for dimension stone ("black granite"). Finally, Pumice, volcanic ash, and perlite are mined in the west. Pumice and volcanic ash are used as abrasives, mostly in hand soaps and household cleaners. The finest grades are used to finish silverware, polish metal parts before electroplating, and for woodworking. Bates reports that in ancient Rome lime and volcanic ash were mixed to make cement. In modern times pumice and volcanic ash have been used to make cement for major construction projects (dams) in California and Oklahoma. Pumice and volcanic ash continue to be used as lightweight aggregate in concrete, especially precast concrete blocks. Crushed and ground pumice are also used for loose-fill insulation, filter aids, poultry litter, soil conditioner, sweeping compound, insecticide carrier, and blacktop highway dressing. Perlite is volcanic glass (made of rhyolite) that has incorporated 2-5% water. Perlite expands rapidly when heated. Perlite is used mostly as aggregate in plaster. Some perlite is used as aggregate in concrete, especially in precast walls.

Many kinds of volcanic activity can endanger the lives of people and property both close to and far away from a volcano. Most of the activity involves the explosive ejection or flowage of rock fragments and molten rock in various combinations of hot or cold, wet or dry, and fast or slow. Some hazards are more severe than others depending on the size and extent of the event taking place and whether people or property are in the way. And although most volcano hazards are triggered directly by an eruption, some occur when a volcano is quiet.

Volcanic eruptions are one of Earth's most dramatic and violent agents of change. Not only can powerful explosive eruptions drastically alter land and water for tens of kilometers around a volcano, but tiny liquid droplets of sulfuric acid erupted into the stratosphere can change our planet's climate temporarily. Eruptions often force people living near volcanoes to abandon their land and homes, sometimes forever. Those living farther away are likely to avoid complete destruction, but their cities and towns, crops, industrial plants, transportation systems, and electrical grids can still be damaged by tephra, lahars, and flooding.

Volcanic activity since 1700 A.D. has killed more than 260,000 people, destroyed entire cities and forests, and severely disrupted local economies for months to years. Even with our improved ability to identify hazardous areas and warn of impending eruptions, increasing numbers of people face certain danger. Clearly, scientists face a formidable challenge in providing reliable and timely warnings of eruptions to so many people at risk.

Volcano Landslides

Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. These mixtures of debris move in a wet or dry state, or both. Landslides commonly originate as massive rockslides or avalanches which disintegrate during movement into fragments ranging in size from small particles to enormous blocks hundreds of meters across. If the moving rock debris is large enough and contains a large content of water and fine material (typically, >3-5 percent of clay-sized particles), the landslide may transform into a lahar and flow downwards along the valley for more than 100 km from a volcano!

Volcano landslides range in size from less than 1 km³ to more than 100 km³. The high velocity (>100 km/hr) and great momentum of landslides allows them to run up slopes and to cross valley divides up to several hundred meters high. For example, the landslide at Mount St. Helens on May 18, 1980, had a volume of 2.5 km³, reached speeds of 50-80 m/s (180-288 km/hr), and surged up and over a 400 m tall ridge located about 5 km from the volcano!

Landslides are common on volcanoes because their massive cones typically rise hundreds to thousands of meters above the surrounding terrain, and are often weakened by the very process that created them--the rise and eruption of molten rock. Each time magma moves toward the surface, overlying rocks are shouldered aside as the molten rock makes room for itself, often creating internal shear zones or over-steepening one or more sides of the cone. Magma that remains within the cone releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering rock minerals to clay. Furthermore, the tremendous mass of thousands of layers lava and loose fragmented rock debris can lead to internal faults and fault zones that move frequently as the cone "settles" under the downward pull of gravity.

These conditions permit a number of factors to trigger a landslide or to allow part of a volcano's cone to simply collapse under the influence of gravity:

Volcanoes are susceptible to landslides for several reasons. Their cones are steep and often rise several thousand meters above the surrounding land. They are built layer upon layer of weakly consolidated and brittle rock debris. Because of the frequent intrusion of magma into their cones and the shear mass of lava that form their cones, internal faults or shear zones commonly develop. Finally, hot, acid-rich water commonly circulates throughout the cone to form an extensive hydrothermal system. Over time, this hot water changes hard volcanic rocks to weak, muddy and clay-rich material. A volcano with a hydrothermal system is like a house infested with termites--the house gradually weakens to the point that it may collapse. The collapse of a volcano, and the resulting lahar, are natural, expectable events during its life history and long after it stops erupting.

The collapse or "flank failure" of a volcano will generate a fast-moving landslide that usually transforms into a lahar after traveling a few kilometers. Depending on the size of the landslide, its water content, and extent to which the volcano's rocks have been weakened and turned into clay by a hydrothermal system, the resulting lahar may travel more than 100 km downstream. Such large muddy lahars are extremely dangerous.

A landslide typically destroys everything in its path and may generate a variety of related activity. Historically, landslides have caused explosive eruptions, buried river valleys with tens of meters of rock debris, generated lahars, triggered waves and tsunami, and created deep horseshoe-shaped craters.

By removing a large part of a volcano's cone, a landslide may abruptly decrease pressure on the shallow magmatic and hydrothermal systems, which can generate explosions ranging from a small steam explosion to large steam- and magma-driven directed blasts. A large landslide often buries valleys with tens to hundreds of meters of rock debris, forming a chaotic landscape marked by dozens of small hills and closed depressions. If the deposit is thick enough, it may dam tributary streams to form lakes in the subsequent days to months; the lakes may eventually drain catastrophically and generate lahars and floods downstream.

Landslides also generate some of the largest and most deadly lahars, either by transforming directly into lahars or, after it stops moving, from dewatering of the deposit. Historically, however, the most deadly volcano landslide occurred in 1792 when sliding debris from Mt. Mayuyama near Unzen Volcano in Japan slammed into the Ariaka Sea and generated a wave on the opposite side that killed nearly 15,000 people.

On a volcano, landslides typically carve deep gashes into its cone or create large horseshoe-shaped craters hundreds of meters deep and more than a kilometer in width.

Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter. Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun.

As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice (if present) and the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size. But as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size.

Large muddy lahars commonly begin as volcanic landslides. Part of a volcano may suddenly break loose under the constant pull of gravity, especially (1) during a large earthquake that strongly shakes the ground; (2) following a period of heavy rains that saturate the ground; (3) when magma rises into the core of a volcano and wedges the cone apart; or (4) during a sideways-directed explosion that rips through one or more sides of a volcano.

Numerous terms are used by scientists to describe the properties of lahars (for example, mudflows, debris flows, hyperconcentrated flows, and cohesive and non-cohesive flows).

Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Some of the largest lahars begin as landslides of saturated and hydrothermally altered rock on the flank of a volcano or adjacent hillslopes. Landslides are triggered by eruptions, earthquakes, precipitation, or the unceasing pull of gravity on the volcano.

Lahars almost always occur on or near stratovolcanoes because these volcanoes tend to erupt explosively and their tall, steep cones are either snow covered, topped with a crater lake, constructed of weakly consolidated rock debris that is easily eroded, or internally weakened by hot hyrothermal fluids. Lahars are also common from the snow- and ice-covered shield volcanoes in Iceland where eruptions of fluid basalt lava frequently occur beneath huge glaciers

Lahars racing down river valleys and spreading across flood plains tens of kilometers downstream from a volcano often cause serious economic and environmental damage. The direct impact of a lahar's turbulent flow front or from the boulders and logs carried by the lahar can easily crush, abrade, or shear off at ground level just about anything in the path of a lahar. Even if not crushed or carried away by the force of a lahar, buildings and valuable land may become partially or completely buried by one or more cement-like layers of rock debris. By destroying bridges and key roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity, especially if the lahars leave deposits that are too deep, too soft, or too hot to cross.

After a volcanic eruption, the erosion of new loose volcanic deposits in the headwaters of rivers can lead to severe flooding and extremely high rates of sedimentation in areas far downstream from a volcano. Over a period of weeks to years, post-eruption lahars and high-sediment discharges triggered by intense rainfall frequently deposit rock debris that can bury entire towns and valuable agricultural land. Such lahar deposits may also block tributary stream valleys. As the area behind the blockage fills with water, areas upstream become inundated. If the lake is large enough and it eventually overtops or breaks through the lahar blockage, a sudden flood or a lahar may bury even more communities and valuable property downstream from the tributary.

Community After Lake Breakout-- Lahars from September 1994 though and after the date of this photograph buried Bacolor to depths of 5 meters in the town and more than 10 meters in some outlying villages.

Lahars Triggered by Sudden Landslides at Volcanoes-- During the day of peak rainfall from Hurricane Mitch on October 30, 1998, part of Casita Volcano collapsed to form a landslide (note scar at top of volcano). The sliding debris quickly transformed into a lahar that eroded rocks and vegetation from along the stream channel, destroyed two towns, and killed more than 2,000 people. In this photograph, the erosive power of landslides and lahars is clearly visible (note areas devoid of vegetation)

Scientists often use more specific terms than lahar when referring to moving masses of water and rock debris. The various terms are defined by the relative amounts of rock debris (sediment) and water in the flow, often as a ratio of sediment to water. And because a flow of water and sediment can erode or deposit sediment and incorporate water along its journey downstream, this ratio changes with increasing distance from a volcano. So, for even a single flow, scientists will commonly use different terms when describing the flow.

Debris Flow-- Dense flows that consist of a relatively high percentage of coarse rock particles are debris flows. The size of sediment transported by debris flows ranges in size from clay and silt (less than 0.06 mm) to boulders as large as 10 m in diameter. A typical debris flow consists of about 2 parts sediment for every one part water. Thus, debris flows may consist of more than 80 percent sediment by weight!

Mudflow-- dominantly sand and silt-sized particles (less than 2 mm in diameter), is often called a mudflow. Even though mudflows can transport large boulders and can have sediment concentrations as great as debris flows, their sediment composition typically consists of at least 50 percent sand, silt, and clay-size particles ("mud" refers to silt- and clay-size particles). Mudflows commonly occur on volcanoes with large deposits of ice and snow on their summits. Mudflows typically begin with the rapid melting of a large amount of ice. The melting may be caused by an eruption or simply by friction in an ice avalanche that has broken free because of an earthquake or collapse of an overloaded ice mass. As the melt water flows down the volcano's flank, it mixes with the usually abundant loose soil and ash to form a muddy liquid about the consistency of wet cement. The mud follows stream and river valleys down and away from the volcano to become a fast-moving (40-50 mph) wall of mud that will carry away anything in its path. Mudflows at Mount St. Helens carried huge trucks and machinery many miles and tore bridges and houses from their foundations. A mudflow roared down a river valley on the side of Nevado del Ruiz in Columbia in 1985 and swept through a town of 25,000 people, killing nearly everyone and leaving nothing standing. Mudflows are a particular problem on high volcanoes that have glaciers on them, such as those in the Andes Mountains in South America and the Cascades in the western United States. As you might expect, the larger the amount of ice available to melt, the larger the potential mudflows. About 3.5 billion cubic feet of ice melted during the 1980 Mount St. Helens eruption. Mudflows from that eruption flowed many tens of miles down local river drainages. Mudflows around Mount Rainier, with its much larger mass of glaciers, are an ongoing problem. Some Rainier mudflows have traveled 50 to 100 miles away from the mountain. Mudflows are a significant danger during eruptions, because the heat from the inside of the mountain or from falling hot ash is capable of melting large amounts of ice. However, mudflows can also occur when there is no eruption. An earthquake may shake off a mass of ice that melts because of friction as it rolls down the mountain, mixing with loose material to make a mudflow. A heavy snow fall or rapid spring melt may trigger a burst of ice or water from a glacier that may also form a mudflow. A debris flow composed of relatively small rock particles,

Hyperconcentrated Streamflow-- A flow containing between 40 and 80 percent sediment by weight is often referred to as hyperconcentrated streamflow. Debris flows and mudflows represent the most dense and concentrated mixtures of flowing sediment and water; they commonly are composed of more than 80 percent sediment by weight. Normal streamflow, which may contain as much as 40 percent sediment by weight, is the least dense and concentrated mixture of flowing sediment and water. Hyperconcentrated flows are finer grained than debris flows and mudflows, usually consisting of predominantly of sand-size particles. As a debris flow or mudflow moves down a river valley, they will eventually become more dilute by mixing with water in the river and by losing some of the sediment. When the percentage of sediment by weight drops below 80 percent, the flow transforms into hyperconcentrate streamflow.

Cohesive Lahars-- Debris flows or mudflows that contain more than 3 to 5 percent of clay-size sediment are sometimes referred to as cohesive lahars. Scientists may sometimes conclude that a relatively high concentration of clay in these flows indicates it began as a large landslide from the flank of a volcano. The interior parts of many volcanoes have been hydrothermally altered and consist of many clay particles.

Non-cohesive Lahars-- Debris flows or mudflows that contain less than 3 to 5 percent of clay-size sediment are sometimes referred to as non-cohesive lahars. Such a relatively low proportion of clay in this volcanic debris is considered by some scientists to be evidence that the lahar did not originate as a volcanic landslide, but rather in another way. For example, by the mixing of water melted from snow and ice with volcanic debris.

Magma contains dissolved gases that are released into the atmosphere during eruptions. Gases are also released from magma that either remains below ground or is rising toward the surface. In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems.

At high pressures deep beneath the earth's surface, volcanic gases are dissolved in molten rock. But as magma rises toward the surface where the pressure is lower, gases held in the melt begin to form tiny bubbles. The increasing volume taken up by gas bubbles makes the magma less dense than the surrounding rock, which may allow the magma to continue its upward journey. Closer to the surface, the bubbles increase in number and size so that the gas volume may exceed the melt volume in the magma, creating a magma foam. The rapidly expanding gas bubbles of the foam can lead to explosive eruptions in which the melt is fragmented into pieces of volcanic rock, known as tephra. If the molten rock is not fragmented by explosive activity, a lava flow will be generated.

Together with the tephra and entrained air, volcanic gases can rise tens of kilometers into Earth's atmosphere during large explosive eruptions. Once airborne, the prevailing winds may blow the eruption cloud hundreds to thousands of kilometers from a volcano. The gases spread from an erupting vent primarily as acid aerosols (tiny acid droplets), compounds attached to tephra particles, and microscopic salt particles.

Volcanic gases undergo a tremendous increase in volume when magma rises to the Earth's surface and erupts. For example, consider what happens if one cubic meter of 900°C rhyolite magma containing five percent by weight of dissolved water were suddenly brought from depth to the surface. The one cubic meter of magma now would occupy a volume of 670 m³ as a mixture of water vapor and magma at atmospheric pressure! The one meter cube at depth would increase to 8.75 m on each side at the surface. Such enormous expansion of volcanic gases, primarily water, is the main driving force of explosive eruptions.

Volcano Tectonic Style Temperature	Kilauea Summit Hot Spot 1170°C	Erta` Ale Divergent Plate 1130°C	Momotombo Convergent Plate 820°C
H₂0	37.1	77.2	97.1
C0₂	48.9	11.3	1.44
S0₂	11.8	8.34	0.50
H₂	0.49	1.39	0.70
CO	1.51	0.44	0.01
H₂S	0.04	0.68	0.23
HCl	0.08	0.42	2.89
HF	---	---	0.26

Types of volcanic gases

The most abundant gas typically released into the atmosphere from volcanic systems is water vapor (H₂0), followed by carbon dioxide (C0₂) and sulfur dioxide (S0₂). Volcanoes also release smaller amounts of others gases, including hydrogen sulfide (H₂S), hydrogen (H₂), carbon monoxide (CO), hydrogen chloride (HCL), hydrogen fluoride (HF), and helium (He). There is a table of volcanic gas compositions, in volume percent concentrations on the left.

Potential effects of volcanic gases

The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide, and hydrogen fluoride. Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from a volcano. Globally, large explosive eruptions that inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth's ozone layer. Because carbon dioxide gas is heavier than air, the gas may flow into in low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals, and vegetation. A few historic eruptions have released sufficient fluorine-compounds to deform or kill animals that grazed on vegetation coated with volcanic ash; fluorine compounds tend to become concentrated on fine-grained ash particles, which can be ingested by animals.

Sulfur dioxide (SO₂)-- The effects of SO₂ on people and the environment vary widely depending on the amount of gas a volcano emits into the atmosphere, whether the gas is injected into the troposphere or stratosphere and the regional or global wind and weather pattern that disperses the gas. Sulfur dioxide (SO₂) is a colorless gas with a pungent odor that irritates skin and the tissues and mucous membranes of the eyes, nose, and throat. Sulfur dioxide chiefly affects upper respiratory tract and bronchi. The World Health Organization recommends a concentration of no greater than 0.5 ppm over 24 hours for maximum exposure. A concentration of 6-12 ppm can cause immediate irritation of the nose and throat; 20 ppm can cause eye irritation; 10,000 ppm will irritate moist skin within minutes.

Emission rates of SO₂ from an active volcano range from <20 tonnes/day to >10 million tonnes/day according to the style of volcanic activity and type and volume of magma involved. For example, the large explosive eruption of Mount Pinatubo on 15 June 1991 expelled 3-5 km ³ of dacite magma and injected about 17

million tonnes of SO₂ into the stratosphere. The sulfur aerosols resulted in a 0.5-0.6°C cooling of the Earth's surface in the Northern Hemisphere. The sulfate aerosols also accelerated chemical reactions that, together with the increased stratospheric chlorine levels from human-made chlorofluorocarbon (CFC) pollution, destroyed ozone and led to some of the lowest ozone levels ever observed in the atmosphere. Downwind from the vent, acid rain and air pollution is a persistent health problem when the volcano is erupting.

Eruptions of Kilauea Volcano release large quantities of sulfur dioxide gas into the atmosphere that can lead to volcanic air pollution on the Island of Hawaii. Sulfur dioxide gas reacts chemically with sunlight, oxygen, dust particles, and water to form volcanic smog known as vog.

Measurements from recent eruptions such as Mount St. Helens, Washington (1980), El Chichon, Mexico (1982), and Mount Pinatubo, Philippines (1991), clearly show the importance of sulfur aerosols in modifying climate, warming the stratosphere, and cooling the troposphere. Research has also shown that the liquid drops of sulfuric acid promote the destruction of the Earth's ozone layer.

Hydrogen sulfide (H₂S)-- Hydrogen sulfide (H₂S) is a colorless, flammable gas with a strong offensive odor. It is sometimes referred to as sewer gas. At low concentrations it can irritate the eyes and acts as a depressant; at high concentrations it can cause irritation of the upper respiratory tract and, during long exposure, pulmonary edema. A 30-minute exposure to 500 ppm results in headache, dizziness, excitement, staggering gait, and diarrhea, followed sometimes by bronchitis or bronchopneumonia.

Carbon dioxide (CO₂)-- Volcanoes release more than 130 million tonnes of CO₂ into the atmosphere every year. This colorless, odorless gas usually does not pose a direct hazard to life because it typically becomes diluted to low concentrations very quickly whether it is released continuously from the ground or during episodic eruptions. But in certain circumstances, CO₂ may become concentrated at levels lethal to people and animals. Carbon dioxide gas is heavier than air and the gas can flow into in low-lying areas; breathing air with more than 30% CO₂ can quickly induce unconsciousness and cause death. In volcanic or other areas where CO₂ emissions occur, it is important to avoid small depressions and low areas that might be CO₂ traps. The boundary between air and lethal gas can be extremely sharp; even a single step upslope may be adequate to escape death. When a burning piece of wood is lowered into a hole that has a high concentration of CO₂, the fire goes out. Such a condition can be lethal to people and animals. Air with 5% CO₂ causes perceptible increased respiration; 6-10% results in shortness of breath, headaches, dizziness, sweating, and general restlessness; 10-15% causes impaired coordination and abrupt muscle contractions; 20-30% causes loss of consciousness and convulsions; over 30% can cause death!

Scientists have calculated that volcanoes emit between about 130-230 million tonnes (145-255 million tons) of CO₂ into the atmosphere every year. This estimate includes both subaerial and submarine volcanoes, about in equal amounts. Emissions of CO₂ by human activities, including fossil fuel burning, cement production, and gas flaring, amount to about 22 billion tonnes per year (24 billion tons). Human activities release more than 150 times the amount of CO₂ emitted by volcanoes--the equivalent of nearly 17,000 additional volcanoes like Kilauea (Kilauea emits about 13.2 million tonnes/year)!

Hydrogen Chloride (HCl)-- Chlorine gas is emitted from volcanoes in the form of hydrochloric acid (HCl). Exposure to the gas irritates mucous membranes of the eyes and respiratory tract. Concentrations over 35 ppm cause irritation of the throat after short exposure; >100 ppm results in pulmonary edema, and often laryngeal spasm. It also causes acid rain downwind from volcanoes because HCl is extremely soluble in condensing water droplets and it is a very "strong acid" (it dissociates extensively to give H⁺ ions in the droplets).--

Hydrogen Fluoride (HF)-- Fluorine is a pale yellow gas that attaches to fine ash particles, coats grass, and pollutes streams and lakes. Exposure to this powerful caustic irritant can cause conjunctivitis, skin irritation, bone degeneration and mottling of teeth. Excess fluorine results in a significant cause of death and injury in livestock during ash eruptions. Even in areas that receive just a millimeter of ash, poisoning can occur where the fluorine content of dried grass exceeds 250 ppm. Animals that eat grass coated with fluorine-tainted ash are poisoned. Small amounts of fluorine can be beneficial, but excess fluorine causes fluorisis, an affliction that eventually kills animals by destroying their bones. It also promotes acid rain effects downwind of volcanoes, like HCl.

Secondary Gas Emissions (Lava haze or laze)-- Another type of gas release occurs when lava flows reach the ocean. Extreme heat from molten lava boils and vaporizes seawater, leading to a series of chemical reactions. The boiling and reactions produce a large white plume, locally known as lava haze or laze, containing a mixture of hydrochloric acid and concentrated seawater.
Hydrochloric acid forms when lava enters the ocean boiling and vaporizing the sea water. Chloride in the sea salt combines with hydrogen in the water to form hydrochloric acid in the plume. This is a short-lived local phenomenon that only affects people or vegetation directly under the plume.

Lava flows are streams of molten rock that pour or ooze from an erupting vent and are probably the best known volcanic hazard. Lava is erupted during either non-explosive activity or explosive lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including the type of lava erupted and its viscosity, the steepness of the ground over which it travels, whether the lava flows as a broad sheet, through a confined channel, or down a lava tube, and the rate of lava production at the vent. Fluid basalt flows can extend tens of kilometers from an erupting vent. The leading edges of

basalt flows can travel as fast as 10 km/hour on steep slopes but they typically advance less than 1 km/hour on gentle slopes. But when basalt lava flows are confined within a channel or lava tube on a steep slope, the main body of the flow can reach velocities >30 km/hour. Viscous andesite flows move only a few kilometers per hour and rarely extend more than 8 km from their vents. Viscous dacite and rhyolite flows often form steep-sided mounds called lava domes over an erupting vent. Lava domes often grow by the extrusion of many individual flows >30 m thick over a period of several months or years. Such flows will overlap one another and typically move less than a few meters per hour.

Everything in the path of an advancing lava flow will be knocked over, surrounded, or buried by lava, or ignited by the extremely hot temperature of lava. When lava erupts beneath a glacier or flows over snow and ice, meltwater from the ice and snow can result in far-reaching lahars. If lava enters a body of water or water enters a lava tube, the water may boil violently and cause an explosive shower of molten spatter over a wide area. Methane gas is produced as lava buries vegetation, can migrate in subsurface voids and explode when heated. Thick viscous lava flows, especially those that build a dome, can collapse to form fast-moving pyroclastic flows.

Deaths caused directly by lava flows are uncommon because most move slowly enough that people can move out the way easily and flows usually don't travel far from the vent. Death and injury can result when onlookers approach an advancing lava flow too closely or their retreat is cut off by other flows. Deaths attributed to lava flows are often due to related causes, such as explosions when lava interacts with water, the collapse of an active lava delta, asphyxiation due to accompanying toxic gases, pyroclastic flows from a collapsing dome, and lahars from meltwater.

Other natural phenomena such as hurricanes, tornadoes, tsunami, fires, and earthquakes often destroy buildings, agricultural crops, and homes, but the owner(s) can usually rebuild or repair structures and their businesses in the same location. Lava flows, however, can bury homes and agricultural land under tens of meters of hardened black rock; landmarks and property lines become obscured by a vast, new hummocky landscape. People are rarely able to use land buried by lava flows or sell it for more than a small fraction of its previous worth.

Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive

eruption of molten or solid rock fragments, or both. They may also result from the nonexplosive eruption of lava when parts of dome or a thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow. Here is a QuickTime video of pyroclastic flow (1.79 M).

Explosive Eruption of Magma-- Explosive eruption of magma and solid-rock fragments or the collapse of a vertical eruption column of ash and larger rock fragments may generate pyroclastic flows.

Collapse of Lava Flows-- The fall of fresh lava and hot rock debris from a lava dome or thick lava flow can generate scores of pyroclastic flows. The repeated collapse of a growing lava dome atop Unzen Volcano caused thousands of small but dangerous pyroclastic flows between 1991 and 1995.

Secondary Pyroclastic Flows-- Avalanching of hot, thick pyroclastic flow deposits in valleys downslope from Mount Pinatubo, Philippines, triggered many small pyroclastic flows for more than two years after its climactic eruption on June 15, 1991. Some of these flows formed deposits as long as 10 km, 1 km wide, and 10 m thick. Such secondary pyroclastic flows present post-eruption hazards that were not recognized until they were observed at Pinatubo

A pyroclastic flow will destroy nearly everything in its path. With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour, pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses.

Pyroclastic flows vary considerably in size and speed, but even relatively small flows that move <5 km from a volcano can destroy buildings, forests, and farmland. And on the margins of pyroclastic flows, death and serious injury to people and animals may result from burns and inhalation of hot ash and gases.

Pyroclastic flows generally follow valleys or other low-lying areas and, depending on the volume of rock debris carried by the flow, they can deposit layers of loose rock fragments to depths ranging from less than one meter to more than 200 m. Such loose layers of ash and volcanic rock debris in valleys and on hillslopes can lead to lahars indirectly by damming or blocking tributary streams, which may cause water to form a lake behind the blockage, overtop and erode the blockage, and mix with the rock fragments as it rushes downstream. Or, increasing the rate of stream runoff and erosion during subsequent rainstorms. Hot pyroclastic flows and surges can also directly generate lahars by eroding and mixing with snow and ice on a volcano's flanks, thereby sending a sudden torrent of water surging down adjacent valleys

Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Such fragments range in size from less than 2 mm (ash) to more than 1 m in diameter. Large-sized tephra typically falls back to the ground on or close to the volcano and progressively smaller fragments are carried away from the vent by wind. Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano.

Tephra consists of a wide range of rock particles (size, shape, density, and chemical composition), including combinations of pumice, glass shards, crystals from different types of minerals, and shattered rocks of all types (igneous, sedimentary, and metamorphic). A great variety of terms are used to describe the range of rock fragments thrown into the air by volcanoes. The terms classify the fragments according to size, shape, or the way in which they form and travel.

Ash Flows are dense masses of gas and tiny fragments of lava that flow down the sides of volcanoes at great speeds. Much less well known (and understood) than lava flows or explosive blasts, ash flows are a combination a blast cloud and a lava flow. They form when gas-saturated lavas comes near the surface of the Earth and the pressure in the lava becomes low enough to allow the dissolved gas to form bubbles. If the bubbles form fast enough, the lava breaks into tiny fragments of liquid rock (called ash) that are carried out of the surface vent with the gas. If the ratio of gas to fragments is large (lots of gas), the ash is carried by the gas into blast clouds that can reach the upper atmosphere. If the ratio of gas to ash is small (lots of fragments), the ash can drag the gas downward into red-hot flows. These red-hot flows are partially controlled by gravity, and since the gas makes the friction between the ash particles very small, they can flow very far (up to hundreds of miles), very fast (over a hundred mph). The relatively small red-hot ash flows that have been seen by geologists are called glowing avalanches because of their similarities to snow avalanches. They usually covers a large area and disrupts the lives of far more people than the other more lethal types of volcano hazards. Unfortunately, the size of ash particles that fall to the ground and the thickness of ashfall downwind from an erupting volcano are difficult to predict in advance. Not only is there a wide range in the size of an eruption that might occur and the amount of tephra injected into the atmosphere, but the direction and strength of the prevailing wind can vary widely. The photo above is by Courtesy of NGDC/NOAA.

Ash flows ultimately stop because the gas finally escapes from between the particles. Glowing avalanches usually leave thin deposits of loose fragments like sand mixed with gravel. The ash fragments are hot, however, and deposits of ash have been found which are so thick that the hot particles actually fused together to form solid masses of rock after they stopped flowing. Geologists have never actually watched the formation of such a deposit, but the amount of ash in these deposits is so large that it requires ash flows hundreds of times larger than the glowing avalanches that have been seen. The destructive power in ash flows is a result of both the high flow speed and the high temperatures of the gas and fragmented rock. Even relatively small ash flows can be incredibly destructive: a glowing avalanche destroyed the town of St. Pierre in less than 30 seconds. The force of the flow literally ripped buildings apart all over town; only the sturdiest of stone walls were left standing. All but two of the 30,000 inhabitants of St. Pierre were asphyxiated or burned to death by the dense, superhot cloud. Ash flows occur in explosive eruptions and have been a significant problem with eruptions in Japan and the American Cascades.

Ash Falls are less devastating than ash flows but can be very disruptive to modern life. Ash falls are the blanketing deposits dropped downwind by the clouds of ash thrown into the atmosphere by explosive volcanoes. These deposits may be a thin dusting or a thick layer of grit. They appear grayish to whitish and look like fallen snow, but, unlike snow, ash deposits do not melt and must be physically removed. The ash is heavy and can collapse roofs, break branches, and coat the leaves of plants. Unless the plants are cleaned (manually or by rain), they can die. This is a significant problem for crops. The ash is particularly hard on machines. Some ash particles are so small that they can pass through the engine's air filters and ruin the engine.

House destroyed by ashfall, Rabaul Caldera, Papua New Guinea

Pumice and ash cover cars, Mount Pinatubo, Philippines

Volcanic ash is highly disruptive to economic activity because it covers just about everything, infiltrates most openings, and is highly abrasive. Airborne ash can obscure sunlight to cause temporary darkness and reduce visibility to zero. Ash is slippery, especially when wet; roads, highways, and airport runways may become impassable. Automobile and jet engines may stall from ash-clogged air filters and moving parts can be damaged from abrasion, including bearings, brakes, and transmissions. Roofs may collapse from added weight, farmland will be covered, power systems may shut down, waste water systems may clog, gutters may fill and collapse... The image USGS EROS Data Center by on the right shows the ash layer on cars near Mount Pinatubo.

The Explosive Blast is the "Feature Presentation" of a explosively erupting volcano. It is an outburst of fragments of rock and lava driven by expanding gases which were dissolved in the erupting lava at great depths. These blasts may throw great blocks of rock many miles. However, the superheated blast cloud itself, which expands out from the volcano at hundreds of miles per hour, enveloping and searing anything in its path is more destructive. The destructive power of the blasts lies in the high velocity winds (exceeding wind speeds in hurricanes) within the cloud and the very high temperatures of the gas. The blasts are capable of destroying all life within many miles of the volcano in a matter of minutes. The main blast at Mount St. Helens destroyed more than 230 square miles of forest in a few seconds.

Glacial Outbursts as the name suggests, are masses of water or ice suddenly released from a glacier. Outbursts may be caused by rapid melting, an earthquake, or heat from lava moving inside a volcano. Glacial outbursts are primarily water, but they can turn into mudflows if they flow over ground with abundant soil or gravel. Glacial outbursts and mudflows can occur on any mountain with glaciers or heavy snow pack, but since many volcanoes grow to altitudes at which glaciers form, outbursts and mudflows are frequently significant volcano-related hazards as well.

Some, but not all, earthquakes are related to volcanoes. For example, most earthquakes are along the edges of tectonic plates. This is where most volcanoes are too. However, most earthquakes are caused by the interaction of the plates not the movement of magma. Earthquakes around volcanoes are caused by movement of lava inside the volcano. The lava literally pushes rock aside as it moves underground. The rock is brittle and breaks as it is moved, releasing seismic energy that we record as earthquakes. Most of the earthquakes are small to medium in size. When they occur in large groups during a short period of time, they are called swarms. These occur often and can keep the ground continuously shaking for long periods of time. Some earthquakes can be large, such as the 5.1-magnitude earthquake that started the May 18 eruption of Mount St. Helens which was the result of the north half of the mountain breaking free. The photo on the left is taken by Courtesy of NGDC/NOAA.

Tidal Waves are giant water waves that form when volcanoes erupt in or near large bodies of water. Waves can be caused by large earthquakes associated with the volcano, by movement of seafloor rocks during caldera eruptions, by giant rock avalanches, or by boiling or expuling water out of a hot, collapsed crater.

Yes, during a volcanic eruption lightning is often observed in the eruption cloud. We don't completely understand regular lightning and volcanic lightning is similarly not understood. It must have something to do with all the particles of ash in the cloud moving around and bumping into each other, producing strong static charges (similar to how rubing a comb across the carpet produces static charges).

How do volcanoes affect the atmosphere and climate?

There are two things to think about. The first is how the weather near an erupting volcano is being affected. The second is how large eruptions will affect the weather/climate around the world. I think more people are worried about the second issue than the first.

The main effect on weather right near a volcano is that there is often a lot of rain, lightning, and thunder during an eruption. This is because all the ash particles that are thrown up into the atmosphere are good at attracting/collecting water droplets. We don't quite know how the lightning is caused but it probably involves the particles moving through the air and separating positively and negatively charged particles. Another problem in Hawaii involves the formation of vog, or volcanic fog. The ongoing eruption is very quiet, with lava flowing through lava tubes and then into the ocean. Up at the vent is an almost constant plume of volcanic fume that contains a lot of sulfur dioxide. This SO₂ combines with water in the atmosphere to form sulfuric acid droplets that get carried in the trade winds around to the leeward side of the Big Island. The air quality there has been really poor since the eruption started in 1983 and they are getting pretty tired of it.

As for the world-wide affects of volcanic eruptions this only happens when there are large explosive eruptions that throw material into the stratosphere. If it only gets into the troposphere it gets flushed out by rain. The effects on the climate haven't been completely figured out. It seems to depend on the size of the particles (again mostly droplets of sulfuric acid). If they are big then they let sunlight in but don't let heat radiated from the Earth's surface out, and the net result is a warmer Earth (the famous Greenhouse effect). If the particles are smaller than about 2 microns then they block some of the incoming energy from the Sun and the Earth cools off a little. That seems to have been the effect of the Pinatubo eruption where about a 1/2 degree of cooling was noticed around the world. Of course that doesn't just mean that things are cooler, but there are all kinds of effects on the wind circulation and where storms occur. Some people think that large eruptions can cause the weather phenomena called "El Nino" to start. This is a huge disruption of the Earth's atmospheric circulation. The connection hasn't been accepted by everybody though. An even more controversial connection involves whether or not volcanic activity on the East Pacific Rise (a mid-ocean spreading center) can cause warmer water at the surface of the East Pacific, and in that way generate an El Nino. Dr. Dan Walker at the University of Hawaii has noticed a strong correlation between seismic activity on the East Pacific Rise (which he presumes indicates an eruption) and El Nino cycles over the past ~25 years.

As a long-term average, volcanism produces about 5X10^11 kg of CO₂ per year; that production, along with oceanic and terrestrial biomass cycling maintained a carbon dioxide reservoir in the atmosphere of about 2.2X10^15 kg. Current fossil fuel and land use practices now introduce about a (net) 17.6X10^12 kg of CO₂ into the atmosphere and has resulted in a progressively increasing atmospheric reservoir of 2.69X10^15 kg of CO₂. Hence, volcanism produces about 3% of the total CO₂ with the other 97% coming from anthropogenic sources.

How do volcanoes affect plants and animals?

Livestock and other mammals have been killed by lava flows, pyroclastic flows, tephra falls, atmospheric effects, gases, and tsunami. They can also die from famine, forest fires, and earthquakes caused by or related to eruptions. Mount St. Helens provides an example. The Washington Department of Game estimated that 11,000 hares, 6,000 deer, 5,200 elk, 1,400 coyotes, 300 bobcats, 200 black bears, and 15 mountain lions died from the pyroclastic flows of the 1980 eruption. Aquatic life can be affected by an increase in acidity, increased turbidity, change in temperature, and/or change in food supply. These factors can damage or kill fish. Eruptions can influence bird migration, roosting, flying ability, and feeding activity. The impact of eruptions on insects depends on the size of the eruption and the stage of growth of the insect. For example, ash can be very abrasive to wings.

Plants are destroyed over a wide area, during an eruption. The good thing is that volcanic soil is very rich, so once everything cools off, plants can make a big comeback! How quickly do plants begin to grow back? The answer is that it depends on how much rain falls in the particular area. For example, on the rainy side of the island of Hawaii, flows that are only 2 years old already have ferns and small trees growing on them. Probably in 10 years they'll be covered by a low forest. On the dry side of Hawaii there are flows a couple hundred years old with hardly a tuft of grass in sight. This means that when you are looking at old lava flows and trying to determine how old they are based on the amount of vegetation, you have to take the climate into effect as well.

Lava flows covering the Kamoamoa area of Hawaii Volcanoes National Park. Photograph by Steve Mattox, November 14, 1992.

The long-term effects of an eruption on wildlife are usually quite small. Certainly at Mt. St. Helens scientists saw that both plants and animals returned to the utterly devastated areas within only a year or so of the eruption. It is usually the short-term effects that are really bad. For example, there was a very big eruption of Santa Maria volcano (Guatemala) in 1902. The eruption itself killed a few hundred to perhaps 1500 people as well as thousands of birds. Pretty soon there were so many insects including disease-carrying mosquitoes that eventually 3000-6000 people died from malaria.

As for the dinosaurs, there are various variations on the main theory. In general it is proposed that volcanic activity put so much ash and/or gas into the atmosphere that the earth's temperature either got too hot for the dinosaurs or got too cold for the dinosaurs. It sounds kind of funny that either can happen but it is true. If the ash particles are really small (<2 microns) then they block out incoming sunlight and the earth gets cool. If they are bigger than 2 microns (but still pretty small) then they let sunlight in but don't let heat radiation from the surface out, and the earth gets warm. Anyway, if you have enough large explosive eruptions, then the theory says that there will be enough ash in the stratosphere to have one of these effects. You need an eruption (or series of eruptions) that is much bigger than anything we have ever witnessed. The reason that you need to put the ash into the stratosphere is that if it is only in the troposphere (where weather clouds are), then it will get rained out very quickly and it won't be around long enough to have a climatic effect. Of course the more famous idea is that a huge meteorite came in and hit the earth, throwing up enough gas and dust into the stratosphere to have the same heating or cooling effect. One line of support for this is that at the geologic time boundary where the dinosaurs died out (the Cretaceous-Tertiary boundary) there is a layer of clay that is rich in an element called iridium. Iridium is not very common on Earth, but it is proposed to be more abundant in asteroids and meteorites. One way to produce such a layer at the same instant that the dinosaurs died out is therefore to have a meteorite bring it in. One major problem with the volcanic hypothesis is that volcanoes, especially the explosive ones, don't produce much iridium. Basaltic volcanoes, such as those in Hawaii produce more iridium but they are not very explosive.

A more recent idea that tries to get around these problems is that instead of a huge explosive eruption, you have a long-term basaltic eruption that mainly puts SO₂ gas into the troposphere. The gas will be converted into small droplets of sulfuric acid which will block incoming sunlight. Because it is only in the troposphere much of the acid may get rained out, but if you have an eruption that continues long enough it can keep up with the rain to produce an Earth-covering haze. What kind of eruption might this be? There are places on Earth where huge volumes of basaltic lavas are found. They are called flood basalts, and the most famous are the Columbia River Basalts in Washington/Oregon, and the Deccan Traps in India. The name "flood basalts" gives an indication of how most people consider them to be erupted, namely as huge fast-moving floods of basalt. However, recent work by a number of scientists at the University of Hawai'i have shown that these flood basalts look more like the slow-moving type of basalt lava (pahoehoe) than the fast-moving type (aa). This leads next to the conclusion that perhaps these flood basalts were not emplaced as huge floods in short periods of time but rather as slower-moving flows over a long period of time (such as 1-2 hundred years). The eruptions would still have been much bigger than those we see here in Hawaii, however. This new idea thus provides a long-term source for lots of SO2, and a possible explanation for the iridium (which, again, is found in basalt lava). It wasn't the Columbia River Basalts that were the cause for the dinosaur extinction because the dinosaurs disappeared about 65 million years ago, and the Columbia River Basalts are only about 12 million years old. Perhaps it was the eruption of the Deccan traps, since their age is about right.