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viernes, 14 de mayo de 2010

Volcanoes—Are You at Risk?

Volcanoes—Are You at Risk?
ERUPTING volcanoes, spewing forth hot ash and flows of incandescent lava, produce some of the most spectacular displays of natural energy on earth. Perhaps you have not personally witnessed such an event, but you may have enjoyed bathing in volcanic hot springs or relished eating food grown in fertile soil composed of volcanic ash. Some even receive the benefits of geothermal energy in their homes.

Recently, however, many living near active volcanoes have witnessed death and destruction wrought by volcanic disasters. Since the violent blast of Mount St. Helens in southwestern Washington State, U.S.A., on May 18, 1980, various parts of the world have been subjected to a seemingly relentless barrage of lethal volcanic eruptions. Loss of life during this time period has exceeded the combined total recorded for the previous seven decades, and property damage has amounted to hundreds of millions of dollars. Near disasters have occurred when airborne volcanic ash has caused airplanes to lose power, forcing emergency landings.

Most destructive have been the eruptions and subsequent mudflows of Mount Pinatubo, in the Philippines, which wiped out tens of thousands of homes, and Nevado del Ruiz, in Colombia, which killed more than 22,000 people. More disasters are possible. Volcano experts Robert Tilling and Peter Lipman, of the U.S. Geological Survey, state that “by the year 2000, the population at risk from volcano hazards is likely to increase to at least 500 million.”

You may therefore find it wise to ask: ‘Do I live near an active, or potentially active, volcano? What kinds of eruptions are the most dangerous, and can they lead to even deadlier threats of other kinds? If I live in a volcanically hazardous area, what can I do to minimize the danger?’

Active Volcanoes—Where Are They?

You may be surprised if you learn that you live near a sleeping volcano and that you would be unavoidably affected if it should happen to reawaken. Scientists who study volcanoes (known as volcanologists) have been successful in recent decades not only in identifying active and dormant volcanoes but also in understanding why volcanoes tend to occur in certain places.

Elsewhere, geysers, fumaroles, and hot springs betray the presence of other dormant volcanoes; these also have potential for future activity. More than half the active volcanoes are strung along the margins of the Pacific, comprising what is known as the Ring of Fire. Some of these volcanoes occur on the continents, such as in the Cascade Mountains of North America and the Andes Mountains of South America, whereas others form island chains in the ocean, such as the Aleutian Islands, Japan, the Philippines, and southern Indonesia. Volcanoes are also common in and near the Mediterranean.

Scientists have determined that these volcanoes occur along the boundaries of huge, moving slabs of crust, or plates, particularly where an ocean plate is plunging beneath a continental plate. This process is called subduction. Heat generated by this process yields magma (molten rock) that rises to the surface. In addition, sudden movements between the plates cause strong earthquakes in many of the same areas that experience volcanic eruptions.

Volcanoes may also form where ocean plates are spreading apart. Many of these eruptions take place on the ocean floor and are not seen by man. However, if you live in the island country of Iceland, you are perched atop the Reykjanes Ridge, which connects to the Mid-Atlantic Ridge, where the plates that include North and South America are moving apart from the ones that include Europe and Africa. In a few other instances, isolated hot spots beneath crustal plates have generated large volcanoes in Hawaii and on the African continent.

What Are the Dangers?

The hazard level posed by a volcano is governed by its recent history of activity, including the magnitude of volcanic eruptions and associated hazards. The risk is a measure of the size and preparedness of the population living in a hazardous zone. First, let us examine the hazards.

Generally, more dangerous explosive eruptions are produced by magma rich in silica. This kind of magma has a stiff consistency, and it can temporarily clog a volcano until the gases build up sufficient pressure to blast open the volcano. Silica-rich magma solidifies into light-colored rocks and is common for volcanoes along plate margins. Explosions can also result when rising magma encounters water and flashes it to steam. Hot ash produced by explosive eruptions can be deadly—three volcanoes in the Caribbean-Central America region killed more than 36,000 people in a six-month period in 1902.

On the other hand, oceanic hot-spot and plate-spreading volcanoes, and many others, are largely made up of dark basalt, which is low in silica but rich in iron and magnesium. Basalt magma is fluid and generally results in mildly explosive or nonexplosive eruptions and also slow-moving lava flows that are relatively easy for people to avoid. Yet, these eruptions may be long-lived—Kilauea volcano on the island of Hawaii has been erupting continually since January 1983. Although extensive property damage has resulted from such eruptions, they rarely result in injury or death.

Some eruptions deposit huge amounts of loose ash on a volcano’s flank, which can result in landslides or, when mixed with a large volume of snow, ice, or water, can form heavy slurries that can quickly wash down valleys. Such mudflows (also known as lahars, from the Indonesian term for lava) can reach many miles from a volcano, perhaps long after eruptions have ceased.

Particularly far-reaching, but historically rare, are tsunamis—giant ocean waves produced by an eruption in the ocean or by a submarine landslide along the flank of a bulging volcano. These energetic waves can travel at speeds of hundreds of miles per hour. Although tsunamis are very low over the deep ocean, actually posing no threat to passing ships, they rapidly build in height near land. These waves crest higher than the tops of houses and many buildings. In 1883 when Krakatau exploded, 36,000 lost their lives as tsunamis slammed the coastlines of Java and Sumatra.

Other volcanic hazards that can harm or disrupt life include falling volcanic ash and fragments, atmospheric shock waves produced by explosive blasts, poisonous fumes, acid rain, and earthquakes. With scores of high-risk volcanoes identified around the world and a myriad of potential dangers, meaningful evaluation of volcanic hazards is indeed a complex, challenging task.

Can You Minimize the Risk?

As the world population grows, more people live in potentially dangerous volcanic areas. For this reason, as well as the recent increase in volcanic activity worldwide, volcanologists have been intensifying their efforts to mitigate volcanic risk. In some cases, eruption forecasts and predictions have been successful, and lives have been saved. What forms the basis for such predictions?

Eruptions are commonly preceded by earthquakes in the volcano or its underlying plumbing system, signaling the upward movement of magma. As magma accumulates high in a volcano, pressure builds up. Gases are released, and groundwater may increase in temperature and acidity. Minor eruptions may also occur before a big one. All of these activities can be monitored.

Well in advance of an eruption, geologists can get an idea of potential risks by looking at the rock record. Many times the types of volcanic flows and secondary hazards repeat themselves, or eruptions mimic those of other studied volcanoes. Based on such data, maps showing areas of greatest risk have been drawn for many volcanoes.

Keys to saving lives from volcanic hazards therefore include hazard assessment and volcano monitoring by volcanologists as well as early warning of impending disaster by local authorities. In contrast with earthquakes, which still largely elude prediction, many erupting volcanoes can be monitored closely enough that people in harm’s way may be evacuated prior to a devastating event. Leaving the danger area is crucial, for man-made structures generally offer little or no protection against the fury and heat of volcanic flows and blasts and the destructive forces of landslides, mudflows, and tsunamis.

Although commendable efforts are being made to reduce the human toll taken by volcanic eruptions and associated hazards, man remains unable to predict eruptions and related catastrophic activity with complete accuracy so as to ensure total safety from volcanic hazards. Even some of those who monitor volcanoes have been killed because of being caught in an unexpected eruption. However, if you live near a potentially active volcano, you should heed any warnings made by local authorities. By doing so, you will greatly increase your chances of surviving a volcanic disaster.—Contributed by an astrogeologist.

Forecasting Volcanic Eruptions From Space?

Imagine measuring the surface movements of volcanoes to an accuracy of one half inch [1 cm] from satellites 12,000 miles [20,000 km] above the earth—traveling at three miles [5 km] per second, no less! This has become possible through the Global Positioning System (GPS), consisting of a set of satellites in addition to strategically placed radio receivers on earth. For each measurement, the positions of at least four satellites are precisely tracked. Time is measured with atomic clocks, which are extremely accurate. These measurements, possible in most weather conditions, have several advantages over ground-based surveying methods. GPS soundings may significantly improve forecasting of volcanic eruptions, which may be preceded by years of expansion of the volcano. This technology has already been employed for volcanoes in Iceland, Italy, Japan, and the United States.

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