Volcanic hazards

Hazards associated with volcanic activity include explosions that eject liquid magma and rock fragments ranging from dust-sized particles to blocks of solid rock, lava flows, glowing avalanches, ash flows, volcanic mudflows, and gases. Active volcanoes typically lie within relatively narrow zones in close proximity to tectonic plate margins, coinciding with principal earthquake zones. The type of volcanism, with associated hazards, differs from one kind of plate boundary to another.

Magmas and Gases

Volcanoes are essentially vents in the Earth's surface through which magma—a combination of molten rock, dissolved gases, and suspended crystals—and associated gases and ash erupt or are ejected from the interior onto the Earth's surface. The nature of the eruption is largely dependent on the proportion of gas to molten rock and how readily the gas escapes. Eruptions can occur once or frequently, with the proportion of gas to molten rock varying widely. Hazards associated with volcanic activity commonly occur as lava flows, glowing avalanches, ash flows, volcanic mudflows, tephra falls, glacier bursts, and volcanic gases.

Gases

Volcanic gases originate in the magmas that rise out of the Earth to activate the volcano. Gases in magma are soluble at the high temperatures found deep in the Earth, but as magma rises, the pressure exerted at depth decreases, and some of the gas is no longer in solution. The gas, which first appears as small and widely spaced bubbles, increases in proportion and begins to coalesce as the magma continues to rise. As the gas pressures then increase, an explosion is prevented by the confining pressure of the magma and, partially, by the viscosity of the magma.

However, when the gas pressures are high enough, the enclosing liquid bursts, gas escapes, and an explosion occurs. If the confining pressures are low and the enclosing magma is relatively fluid, gas bubbles escape readily, with a mild magma eruption at the surface. If the confining pressures are high, however, and continue to increase, with gas bubbles escaping with much difficulty, eruptions are very violent and of an explosive nature. If the magma comes in contact with water in the form of seawater, surface water, or groundwater, violent steam explosions are possible, as documented at the Kilauea volcano in Hawaii. Magma evolution, however, eventually causes an increase in viscosity and gas content, leading to progressively shorter and thicker lava flows, forming domes composed of many individual flows formed by the extrusion of highly fluid lava and increasingly explosive eruptions.

Volcanoes can be broadly divided into two groups based on magma compositionbasaltic volcanism and acidic volcanism. Basaltic volcanism is associated with the oceanic crust, commonly producing large quantities of lava with minimal explosive activity. Those active volcanoes situated on the Hawaiian Islands and Iceland's Kirkefell are examples of this type of volcanism. Acidic volcanism is associated with destructive tectonic plate margins and tends to be violent, with an unpredictable pattern of behavior and long periods between eruptions. The relatively high viscosity of the magma can induce a buildup of gas pressures, resulting in massive explosions such as those of Krakatau and, more recently, Mount St. Helens. This type of volcanism can produce flowing gas clouds or surges of pyroclastic material without any warning during periods of prolonged and less turbulent activity. Hot gases charged with varying proportions of red-hot ash can descend a mountainside at speeds greater than 100 kilometers (62 miles) per hour, bringing instant destruction as in the case of Pompeii by the eruption of Vesuvius in 79 CE.

Location of Volcanoes

The most explosive volcanoes exist in zones where subduction of oceanic plates occurs. The volcano develops on the overlying plate within a few tens of kilometers from its edge. The eruptions of this type of volcano are characterized by tephra, constituting from 45 to 99 percent of the volcanic product. Mount St. Helens and the other volcanoes that form the Cascade chain are examples of this type of volcanism. Associated hazards include heavy ashfalls, glowing avalanches, ash flows, mudflows, and lava flows.

Ejecta and Lava Flows

Explosive-type volcanoes result in the ejection of liquid magma or fragments, or a combination of both, into the air. These fragments, called pyroclastic ejecta, or simply tephra, fall back to the ground or are carried away by the wind hundreds to thousands of kilometers distant. Sand-sized and smaller fragments are referred to as ash or dust; fragments measuring two to sixty millimeters in diameter are called lapilli; and fragments having diameters greater than sixty millimeters are called bombs or blocks, depending on whether the material was ejected in a liquid or angular chunks of solid rock, respectively.

Glowing avalanches, nuées ardentes, are rapidly expanding clouds of dust that are typically black in daylight but a glowing dull red at night. The principal part of this phenomenon, however, is not the dust cloud itself but rather the avalanche of incandescent lava blocks, sand, and dust beneath it. Nuées ardentes can travel at great speeds, with documented cases exceeding 150 kilometers (93 miles) per hour. As a result of such great speed, when opposing hill slopes or the bend of a valley wall are encountered, the avalanche is capable of climbing vertically as much as several hundred meters. Ash flows closely resemble glowing avalanches in mobility and cause; however, the flow is composed primarily of an emulsion of bits of very hot glass, referred to as ash, and occasional lumps of pumice. Debris avalanches and landslides result from the sudden downslope movement of loose materials on the flank of a volcano.

Mudflows

Mudflows, also called lahars or debris flows, are slurries of solid fragments in cool to hot water, associated with explosive-type volcanoes where loose fragmented material is abundant. Flowing downhill under the influence of gravity, the flows are largely controlled by topography and can attain speeds of 90 kilometers (55 miles) per hour, with some documented at 200 kilometers (125 miles) per hour. Mudflows have destroyed more property than any other single volcanic process. Also attributed to them is the loss of thousands of lives. Following the eruption of Indonesia's Mount Merapi, the threat of lahars during the 2011 rainy season forced the evacuation of thousands of people. Seasonal mudslides or debris flows are common on glacier-clad volcanoes and often occur without a volcanic eruption.

Glacier Bursts

Glacier bursts occur by volcanic eruptions generated beneath glaciers, as in Iceland. For a few hours, these bursts may achieve volumes greater than that of the world's largest river, with documented cases exceeding velocities of 92,000 cubic meters per second and volumes of liquid exceeding six cubic kilometers. Globally, around fifteen million individuals are at risk of being impacted by glacier bursts, particularly those living near the Himalayan mountain range.

Tsunamis

As an effect of volcanic activity, huge waves known as tsunamis may be generated by abrupt displacement of the seabed. Great volumes of water are involved, and the travel speeds of the waves can reach 800 kilometers (500 miles) per hour. A tsunami may arrive at a shore, after being slowed by the shallower water and increasing in amplitude, as a breaker up to twenty meters high. The tsunami resulting from the explosion of Krakatau in 1883 caused nearly all the deaths from that disaster. In 1896, a tsunami was responsible for the loss of 27,000 lives in Japan. More than a century later, the March 11, 2011, earthquake and tsunami in Japan caused nearly 16,000 deaths, most of which were attributed to drowning. The volcanic eruption on Hunga Tonga–Hunga Haʻapai in 2023 also resulted in a tsunami. Fortunately, tsunamis are relatively infrequent, occurring less than once every ten years.

Although most tsunamis originate in the Pacific Ocean, not all of them do. The 2004 Indian Ocean Tsunami was triggered by a 9.1-magnitude earthquake near Banda Aceh, Indonesia. The resulting tsunami traveled about 4,800 kilometers (3,000 miles), killed more than 230,000 people, and left millions more homeless in eleven countries.

Evaluating Volcanic Risks

Evaluating volcanic risks and hazards on or near a volcano relies on understanding its historical behavior. The assumption is that a volcano is most likely to behave like it has in the past. This evaluation is based on the recorded history of eruptions and geological studies of the cone's composition and structure.

Attention to volcanic eruptions is increasingly directed toward the assessment of risks to life and property. A problem arises, however, in defining when a volcano is actually dead or extinct and, thus, presents no further risk. For example, volcanoes can remain quiet or dormant for thousands of years and then abruptly erupt. In some of the most destructive volcanic events, no documentation of previous eruptions exists and the volcanoes were thus considered to be inactive; the eruption of Vesuvius in 79 CE is an example.

Beginning in the twentieth century, maps have been prepared to identify areas around known dangerous or active volcanoes based on historical evidence. Noted have been the eruptive characteristics of a particular volcano, especially the effect of topography on pathways of flows. Certain areas are typically coded to determine the expected risks given differing activity types. For example, the primary risks associated with the Hawaiian volcanoes (Kilauea and Mauna Loa) are almost wholly from lava flows, whereas with the Cascade and Alaskan volcanoes, the primary risks are ashfalls and mudflows. Thus, topography is considered principally with the former, whereas meteorological conditions are monitored with the latter.

Earth scientists contribute to reducing volcanic hazards by understanding the nature and geographic distribution of these hazards. Attention presently is directed toward the assessment of risks to life and property. The solution is to minimize these risks for those who live close to volcanoes. Thus, the practical goals of volcanology are to be in a position to provide ample warning of the timing, type, and location of future eruptions, to minimize the effects of eruptions and associated phenomena, and to rehabilitate devastated areas efficiently and effectively. Great loss of life was averted during the 1991 eruption of Mount Pinatubo in the Philippines by the action of volcanologists, whose warnings led to the evacuation of sixty thousand people before the main eruption.

With sufficient warning, people and property can be evacuated. Prediction of the timing of an eruption uses a combination of indicators. No volcanic district as a whole is monitored sufficiently to avoid the loss of life and property completely. Monitoring the temperature of fumaroles, hot springs, and crater lakes may serve to warn of an eruption because, in some cases, a temperature rise has precluded an eruption. Changes in gas discharge and composition also may forewarn an imminent eruption. Additionally, an alteration of the strength or orientation of the Earth's magnetic field may precede an eruption. This indicator is based on the hypothesis that the increased temperature of a volcano should reflect a decrease in the overall strength of its local magnetic attraction.

Innovative methods of attempting to predict volcanic eruptions, which appear to show much promise, have included monitoring of deformation of the ground surface and earthquake behavior. Eruptions are commonly preceded by local earthquakes, which result from the opening of fissures at depth through which the magma rises toward the surface. Earthquakes are also produced by the movement of blocks in and around the volcano as it swells or, in the case of Kilauea, as an indication that the underlying magma reservoir is being filled and an eruption is possible. Volcanic tremors (rhythmic vibrations of the ground surface, reflecting the movement of magma) often occur just before or during eruptions, although tremors may also occur without an eruption. Changes in the Earth's electrical currents within the volcano—where rapid changes in Earth currents have occasionally been observed before eruptions—are monitored and inferred to precede a potential eruption. Finally, the disturbed and excited behavior of animals on or near the volcano previous to its eruption has been observed, suggesting that certain animals may sense earthquakes that are too small in magnitude to be felt by humans.

Volcanic Costs and Benefits

Volcanoes are among the most impressive and devastating of phenomena. However, volcanoes have done more good than harm by creating thousands of square kilometers of fertile land. Additionally, the use of volcanic heat in the form of natural steam and hot water continues to be developed to produce electricity and heat at a low cost and with minimum environmental pollution. Most episodes of volcanic unrest typically end without an eruption. Eruptions are, thus, the exception rather than the rule.

Nevertheless, millions of people continue to live near active volcanoes, risking the potential destruction often unpredictably associated with them. As developable land becomes scarcer, urban development progressively encroaches upon these areas. During the last five hundred years, more than 200,000 people have lost their lives as a result of, at minimum, five hundred active volcanoes—and that figure does not include the casualties resulting from tsunamis. Lives were lost not only directly by the volcanic activity itself but also by the destruction of food crops and livestock, which led to deaths from starvation.

Principal Terms

ash flow: a density current composed of a highly heated mixture of volcanic gases and ash, which travels down the flanks of a volcano or along the ground surface

composite volcano: a volcano built of alternating layers of lava and pyroclastic deposits, along with abundant dikes and sills

lahar: a mudflow composed chiefly of volcanic debris on the flanks of a volcano

magma: a body of molten rock, including any dissolved gases and suspended crystals

mudflow: a general term for a flowing mass of predominantly fine-grained Earth material that possesses a high degree of fluidity during movement

pyroclastic: composed of clastic (rock-derived) material formed by volcanic explosion or aerial expulsion from a volcanic vent

shield volcano: a volcano in the shape of a flattened, broad, and low dome built by flows of very fluid lava or ash flows

tephra: a general term for all pyroclastic material and shreds of liquid magma formed by volcanic explosion or aerial expulsion from a volcanic vent

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