Volcano

Harry Truman (not the former president of the same name) lived at the Mount St. Helens Lodge on the shore of Spirit Lake at the foot of Mount St. Helens. He had run the Lodge since 1928. Even though he was warned that the mountain could erupt at any time, he refused to leave. Some people consider him a hero and symbol of personal freedom and independence. Others consider him just an ecentric old man who refused to heed appropriate warnings. It no longer matters, because Mount St. Helens erupted with enormous destructive force on May 18, 1980, obliterating both Mount St. Helens Lodge and parts of Spirit Lake. Harry Truman's body was never found.

Mount St. Helens was a dormant volcano that sprang back to life after a long quiet period. A volcano is an opening, or vent, in Earth's surface through which lava, ash (very small pieces of lava or rock), gas, and rock fragments escape. Below the vent is a passageway, called a pipe, that leads down to a reservoir of molten rock, or magma, below the surface. When a volcano erupts, the magma and gases it contains emerge from underground. An eruption may look like curtains of fire, rivers of red-hot lava (magma above ground is called "lava"), or exploding clouds of rock fragments, ash, and dust.

The material ejected by volcanoes forms hills or mountains around the vent. Those landmasses, which are usually cone-shaped and may rise to 19,000 feet (5,800 meters) or higher, are the objects most people think of as volcanoes. Some volcanoes exist singly, looming over a flat landscape, while others exist in groups and form mountain ranges. Volcanic eruptions throughout history have played a major role in shaping Earth's surface. Most volcanoes, however, are not visible because they exist beneath the sea.

Volcanoes may be active, dormant, or extinct. An active volcano is one that has erupted, or shown signs of erupting, in recent times and is likely to erupt in the future. A dormant volcano is one that has not erupted for many years, but may still roar back to life. It is not unusual for volcanoes to go one hundred years between eruptions. An extinct volcano is one that has not erupted since the beginning of recorded history and that scientists are reasonably sure will never erupt again. Extinct-volcano cones are gradually eroded by the action of wind, rain, and ice.

There are about six hundred active volcanoes on Earth today. Some fifty volcanoes worldwide erupt every year; at least one of those eruptions disrupts human lives and settlements. During the decade from 1980 to 1990, about 26,000 people were killed and nearly 450,000 had to flee their homes due to volcanic activity.

The eruption of Mount St. Helens

Mount St. Helens, located in southwestern Washington State in the Cascade mountain range, erupted on May 18, 1980. It was one of the most violent eruptions in modern history, equivalent in its power to the

WORDS TO KNOW

active volcano:

a volcano that continues to erupt regularly.

ash:

very small, fine fragments of lava or rock that are blasted into the air during volcanic explosions.

basalt:

a type of rock that forms from hardened lava.

caldera:

a large depression, usually circular or oval shaped, left behind when a volcano's summit collapses.

cinder:

a small piece of material thrown from a volcano during an eruption.

cinder cone:

a volcanic cone made of lava fragments.

composite volcano:

a volcano with steep sides made of layers of lava and ash.

cone:

the sloping outer sides of a volcano (not all volcanoes have cones).

conelet:

a small cone on the side of a large volcano.

continental drift:

the geologic theory that all continents were originally part of a single landmass before they slowly separated and drifted apart.

crater:

the bowl-shaped area around the opening at the top of a volcano.

crust:

the outermost layer of Earth, varying in thickness from 3.5 miles (5 kilometers) under the ocean to 50 miles (80 kilometers) thick under the continents.

dormant volcano:

a volcano that has not erupted for many years.

effusive eruption:

the type of eruption in which lava spills over the side of a crater.

eruption:

the release of pressure that sends lava, rocks, ash, and gases out of a volcano.

extinct volcano:

a volcano that is never expected to erupt again.

fissure:

a crack in Earth's surface through which volcanic materials can escape.

flood basalt:

high temperature basaltic lava that flows from a fissure in Earth's crust and covers large areas of the landscape. Also known as plateau basalt.

fumarole:

a vent in Earth's surface that releases steam and other gases, but generally no lava.

geologist:

a scientist who studies the origin, history, and structure of Earth.

geyser:

a regular spray of hot water and steam from underground into the air.

hot spot:

an area beneath Earth's crust where magma currents rise.

igneous rock:

rock made of solidified molten material that made its way from the interior of the planet to the surface.

lahar:

a mudflow of volcanic ash and water that sometimes occurs after a volcanic eruption.

lava:

molten rock that erupts from a fissure or a vent (see magma).

lava domes:

volcanic formations built up from layers of viscous lava, which does not flow far from its source.

lava tube:

a tube formed when an outer layer of lava is cooled by the air and hardens and molten lava then flows out of the middle of the tube, leaving it hollow.

largest hydrogen bomb ever exploded. The eruption destroyed 212 square miles (550 square kilometers) of land and claimed sixty-two human lives. Mount St. Helens was the first volcano to erupt in the continental United States (excluding Alaska and Hawaii) since California's Lassen Peak in 1917, and the first eruption in recorded history to claim a life in the continental United States.

Mount St. Helens is classified as a composite volcano (a steep-sided, often symmetrical cone constructed of alternating layers of lava flows, ash, and other volcanic debris). Composite volcanoes, other examples of which include Mount Vesuvius and Krakatau, tend to erupt explosively. Mount St. Helens was no exception.

Prelude to the eruption

After being dormant since 1857, Mount St. Helens came alive in March 1980. In the two months before the eruption, Mount St. Helens experienced more than ten thousand small earthquakes and hundreds of small eruptions involving blasts of steam. Also during that time, the north slope of the volcano bulged outward more than 260 feet (80 meters). On March 27 there was an explosion that created a 200-foot (60-meter) hole in the mountainside. The next day a plume of steam, ash, and gas erupted from the volcano and rose 4 miles (6.4 kilometers) into the atmosphere.

Evacuation advised

Scientists advised people in the area to evacuate, warning that a larger eruption was likely to occur. Park officials closed Mount St. Helens to campers, hikers, bikers, loggers, fishermen, and others drawn to the popular recreation spot. They established an area called the Red Zone, which they declared was in the greatest danger should the volcano fully erupt, and placed roadblocks around the perimeter. A small number of backpackers and loggers, however, continued to enter the area through old logging roads. One group whose presence at Mount St. Helens was sanctioned by authorities was a scientific team of the United States Geological Survey (USGS). More than twenty-four volcano-watchers were assigned to monitor the mountain.

Eruption triggered by earthquake

At 8:27 on the morning of May 18 there was a magnitude 5.1 earthquake directly below the north slope. (The Richter scale of earthquake magnitudes is a measure of the damage caused by an earthquake.) This weakened the crater rim (the edge of the bowl-shaped area around the opening), and about ten seconds later the bulging and cracked north face of the volcano collapsed and slid downward at 110 to 155 miles (175 to 250 kilometers) per hour in the largest landslide in recorded history. As the landslide swept across Spirit Lake, it displaced all of the water in the lake and created waves 600 feet (180 meters) high that crashed into a ridge north of the lake. When the water rushed back into the lake basin, it dragged with it thousands of trees that had just been knocked down by the explosion.

As the cone crumbled, gas was released and the magma was exposed. The avalanche of rock that exposed the magma removed the last restraint to eruption, and the magma exploded outward in a lateral blast of hot gas, steam, ash, and rock fragments. The blast was heard all the way in Vancouver, Canada, some 200 miles (300 kilometers) away. Instead of moving straight upward, as material in explosive eruptions usually does, the material in this case traveled sideways to the north and overtook the landslide where the crater rim had been destroyed. The 572°F (300°C) pyroclastic flow (flow of hot ash and gas) raced across the ground at speeds approaching 680 miles (1,100 kilometers) per hour and burned everything it touched. The flying debris, gas, and heat killed millions of animals including deer, elk, birds, and other animals. The windstorm produced by the eruption flattened trees. It was powerful enough to pick up and toss around logging trucks and bulldozers several miles (kilometers) from the vent.

Famous volcano eruptions

Volcano eruptions have had a major impact on world cultures since the beginning of recorded history. Here are a few of the most famous eruptions:

• 79—Vesuvius, Italy. The cities of Pompeii and Herculaneum were destroyed by this eruption.
• 1783—Laki, Iceland. This eruption lasted eight months and killed most of the area livestock. The animals died after eating plants contaminated by gases and rain poisoned by the eruption. A quarter of the residents of the surrounding area died in the famine caused by the eruption.
• 1792—Unzen, Japan. The collapse of a lava dome in this volcano caused a tsunami that killed nearly 15,000 people. This was Japan's worst volcano-related disaster. The volcano was dormant for nearly 200 years after this event, but began showing signs of activity again in the early 1990s.
• 1883—Krakatau, Indonesia. This eruption destroyed the uninhabited volcanic island of Krakatau, triggering massive tsunamis that killed more than 35,000 in coastal communities in the vicinity.
• 1995—Soufrière, Montserrat. The volcano on this Caribbean island had not erupted in all of recorded history, but began showing signs of activity in the 1960s. In 1995, it began producing destructive pyroclastic flows, and continued to do so for two years. About 7,000 of the island's 10,000 residents were forced to evacuate.

The heat from the eruption melted snow and ice on the volcano. This water combined with ash and chunks of ice to form mudflows called lahars (pronounced LAH-hahrs). The lahars filled riverbeds and lakes, and buried houses, roads, and bridges. Millions of trout and salmon died in the mud-choked rivers. The flow of the Columbia River, a main thoroughfare to the Pacific Ocean, was blocked by mud. Thirty ocean-bound vessels were trapped behind the mud dam.

Ash travels far and wide/

The eruption, which lasted nine hours, sent a cloud containing millions of tons of dust and ash 15 miles (24 kilometers) into the air. The cloud spread across 22,000 square miles (57,000 square kilometers) of western U.S. skies, blocking out the Sun. Residents of Yakima, Washington, 85 miles (137 kilometers) from Mount St. Helens, experienced complete darkness throughout the day of the eruption and had to wear face masks when venturing outside. Ash fell to the ground over the next few days, blanketing crops in parts of Washington, Oregon, and Idaho.

Fifty-seven lives lost

In all, fifty-seven lives were lost in the Mount St. Helens eruption. Most of the victims choked to death on volcanic ash. People as far as 18 miles (29 kilometers) to the north of the volcano were killed by heat, ash, showering rocks, and lethal gases in the eruption. It had been determined that 16 miles (26 kilometers) from the volcano was a safe distance, yet experts never predicted the volcano would spew its stream sideways. One of those who lost their lives was David A. Johnston, a thirty-year-old member of the USGS volcano-monitoring team. Johnston had been stationed 6 miles (9.7 kilometers) north of the volcano. His final words by radio to the local USGS office in Vancouver, Washington, were, "Vancouver! Vancouver! This is it!" Johnston's body was never found.

One person who narrowly made it out alive, after stumbling through the dark landscape for nine hours, was a television cameraman, David Crockett. He captured the fear of the moment on audio tape. "I can hear the mountain rumbling behind me," Crockett said in the recording. "I feel the ash in my eyes. Oh dear God, this is hell … it's a black hell, totally pitch black…. Dear God, help me breathe. I can't see a thing…. I honest to God believe I'm dead."

Changes to the mountain

Before the eruption, Mount St. Helens had stood at 9,420 feet (2,870 meters) and was the fifth-tallest mountain in Washington. The mountain lost its uppermost 1,300 feet (400 meters) in the eruption, dropping it to the fifteenth-tallest peak in the state. The pre-eruption Mount St. Helens had been a nearly perfect cone shape, picturesque with its cap of snow and ice. During the eruption, however, the crater and north side of the mountain, including the crater rim, were destroyed, leaving the crater clearly visible. After the eruption, the forests that formerly adorned the mountainside lay in ruins, wildlife was nowhere to be seen, lakes had become pools of mud, and a grayish layer of muck covered everything.

WORDS TO KNOW

magma:

molten rock containing dissolved gas and crystals that originates deep within Earth. When it reaches the surface it is called lava.

magma chamber:

a reservoir of magma beneath Earth's surface.

mantle:

the thick, dense layer of rock that lies beneath Earth's crust. The mantle is about 1,800 miles (2,900 kilometers) thick and accounts for about 84 percent of Earth's volume.

pipe:

a narrow passageway that leads from a magma reservoir to a vent.

plate:

a large intact section of Earth's crust.

plate tectonics:

the geologic theory that Earth's crust is composed of rigid plates that are in constant motion with respect to each other, creating the major geologic features on the planet's surface.

Plinian eruption:

a volcanic eruption that releases a deadly cloud of gas, dust, and ash.

pumice:

volcanic rock formed during the explosive eruption of magma; it has numerous gas bubbles and may float on water.

pyroclastic flow:

a rapid flow of hot material consisting of ash, pumice, other rock fragments, and gas ejected by an explosive eruption.

Ring of Fire:

the name given to the geologically active belt that surrounds the Pacific Ocean and is home to more than 75 percent of the world's volcanoes.

shield volcano:

a volcano with long, gentle slopes, built primarily by lava flows.

steam eruption:

a violent eruption that occurs when water comes in contact with magma, rapidly turns to steam, and causes the mixture to explode.

subduction zone:

a region where two plates come together and the edge of one plate slides beneath the other.

vent:

an opening in the surface of Earth through which molten rock, lava, ash, and gases escape.

volcano:

an opening in the surface of Earth (vent) through which molten rock, lava, ashes, and gases escape; it is also the name for the mountain or hill that is formed by the lava and other erupted material.

Just a few months after the eruption plants and animals began to reappear on Mount St. Helens. First, thistles that had survived the eruption emerged through the layer of ash. The rains washed away the ash and exposed the soil. Seeds blew in from surrounding areas or were dropped by birds. Four months after the eruption, fireweed began

blooming on the edges of the eruption area. Other plants began to reappear and, slowly but surely, wildlife returned to the region.

Mount Vesuvius: Destruction preserved

In 79 ce, Mount Vesuvius shook the Roman Empire with a mighty eruption—perhaps the most famous eruption in history. The volcano, near Naples, Italy, had been quiet for hundreds of years. In the years 63 ce through 79 ce Vesuvius awoke with a series of earthquakes. On August 24, 79 ce, an enormous blast jolted the ground. Ash and lava blew out of the top of the volcano and rained down on the mountainside for hours, setting fire to vineyards and fig orchards. Ash clouds also rose and blocked out the sun.

Some people in the prosperous town of Pompeii (population twenty thousand) just south of the mountain, fled at the first signs of eruption and escaped with their lives. Others were trapped in the city and suffocated in the ash cloud. Before long, pyroclastic flows descended on all sides of the mountain, engulfing the smaller towns of Herculaneum and Stabiae, as well as Pompeii. In all, more than two thousand people (possibly many more) were killed by flows of hot gas, ash, and mud.

A detailed written record of the event was made by an eighteen-year-old scholar named Pliny the Younger. Pliny and his mother watched the eruption from 20 miles (32 kilometers) away, in the town of Misenum across the Bay of Naples. Pliny's uncle, Pliny the Elder, a celebrated natural historian, was at the foot of Vesuvius during the eruption and suffocated from the poisonous gases.

Pompeii lay buried under 23 feet (7 meters) of rock, ash, and dust, undisturbed for more than fifteen centuries. Then, in 1595, workers digging a tunnel in the area encountered the ruins of Pompeii. They found buildings, vases, statues, and other artifacts. A systematic excavation of Pompeii did not begin until 1860. The Italian government in that year began overseeing an effort to unearth the entire city—an effort that continues today.

What was uncovered was an archaeologist's treasure trove. Pompeii's streets, houses, stores, theaters, and sports stadium stood as they were when buried. Paintings were still on walls. Intact loaves of bread and bowls of figs and walnuts were recovered; all had had been carbonized (turned to charcoal).

Most remarkable were the remains of people trying to flee when overtaken by poisonous gas or hot ash. While the body tissues had disintegrated, the ash covering the bodies had formed cement-like molds around the spaces where their bodies had been. Hollow areas with only skeletons inside were excavated. These casts represented people in the positions they had assumed at the moment of death. Many were covering their faces, protecting their children, or clutching their chests as if trying to breathe. The historians and scientists directing the excavation had the molds turned into statues by pouring plaster into the hollow areas. The ruins of Pompeii remain a popular tourist attraction.

There was a repeat of the Mount Vesuvius tragedy in 1631, when an eruption took the lives of some three thousand people. Since that time, Vesuvius has not erupted explosively. While the region continues to be heavily populated, residents are now evacuated when signs of volcanic activity appear.

Continuing volcanic activity

After the May 18 blast, a new vent opened in Mount St. Helens. Volcanic activity, in the form of puffs of steam and ash, continued for months. Thick, pasty lava emerged from the vent and formed a new cone in the crater. By 1986 the cone had grown to a height of 850 feet (260 meters) and had a diameter of about 3,600 feet (1,100 meters). Mount St. Helens is expected to erupt again sometime in the future, but no one can say when.

Mount St. Helens remains active and has experienced several eruptive events since 1980. On October 1, 2004, the mountain ejected a plume of steam and ash to about 9,700 feet (2,950 meters). There were several more steam and ash eruptions over the next few days, along with low-frequency seismic tremors that may indicate movement of magma. One of the most significant recent eruptive events occurred on March 8, 2005, when a 36,000-foot (11-kilometer) plume of steam and ash was witnessed emerging from the volcano, accompanied by a tremor that measured about 2.5 on the Richter scale. If expansion continues at its present rate, the lava dome that has been growing inside the crater will replace all of the material lost in the 1980 eruption within another forty to fifty years.

Despite the continued activity, most trails in Mount St. Helen's National Monument are now open to visitors. It is even possible to climb a summit trail and look down into the crater and view the growing lava dome.

Dangerous science: How volcanoes work

The source of volcanic activity is many miles (kilometers) below ground, in Earth's upper mantle. The interior of Earth is divided into layers, distinguished by their material composition. The core, at the planet's center, which is some 3,975 miles (6,395 kilometers) from the surface, has a solid inner portion and a liquid outer portion. Temperatures in the core are believed to exceed 9,900°F (5,480°C). Surrounding the core is the thick, dense layer of rock called the mantle.

The mantle is divided into two sections: the upper and the lower. It is in the upper mantle, the 40-mile-thick (65-kilometer-thick) section farthest from the core, that magma originates. In the upper mantle, magma exists as a solid mass that is held together by tremendous pressure. The pressure there is about forty thousand times greater than it is on the planet's surface.

The legend of Atlantis

Most people are familiar with the legend of Atlantis from books and movies. This "lost continent" was mentioned in the writing of the ancient Greek philosopher Plato. Plato recounted the story that Solon, a Greek statesman of the seventh century bce, supposedly heard from priests at an Egyptian temple. These priests told Solon of a mythic island inhabited by an advanced race about 9,000 years before Plato's time, or nearly 10,000 bce.

According to legend, the island suffered a cataclysm of some sort and disappeared into the sea. There had been little evidence to support this legend until it was hypothesized that a translation or math error might have caused the date of the disappearance of Atlantis to be placed 9,000 years in the past instead of 900 years in the past. Interestingly enough, 900 years before Plato's time, there was a catastrophic volcanic eruption on the island of Santorini, destroying the island and triggering massive tsunamis. Archaeological evidence now suggests that a highly developed civilization had inhabited Santorini about 3,800 years ago. This has led some historians to speculate that the fate of Santorini might be the foundation for the "Atlantis" legend.

The layer adjacent to the upper mantle is the crust. The crust, which forms Earth's surface, is the thinnest layer. It ranges in thickness from 3 to 35 miles (5 to 50 kilometers) It is thinnest under the ocean basins and thickest under the continents. The crust is divided into several large, interlocking pieces, called tectonic plates (or simply "plates"), which "float" on the mantle. There are seven major and eight minor plates. The plates move very slowly, about 2 to 4 inches (5 to 10 centimeters) per year (about the speed at which fingernails grow), in response to slow-moving convection currents in the mantle beneath them.

The majority of the world's volcanoes are found along the boundaries between plates; specifically, plates that are moving toward each other. When two plates come together, one plate may slide beneath the other, forming a region called a subduction zone. As one plate slowly plunges beneath another, the rock at the leading edge of the buried plate melts. That molten rock, which is less dense than the surrounding crust, then rises toward the surface. Driven by pressure created by trapped gas, the magma forces its way through the weakened layers of rock at the plate boundary and emerges through cracks at the surface. Plates can also pull apart. This is happening along the center ridge of the Atlantic Ocean in a process called seafloor spreading. This process can allow molten magma to burst through the ocean floor creating volcanoes. Iceland is part of this ridge and is also the location of several volcanoes.

When magma reaches a crack or hole in the surface, tremendous pressure is released. The magma, which is about 3,270°F (1,810°C), becomes fluid and runs over the ground. At this point it is called lava. Lava may glow brightly, the color of fire, or may be coated with a layer of blackened rock. The lava cools as it travels along the surface and, in time, hardens.

Anatomy of a volcano

The cone-shaped structure we typically think of as a volcano is actually piled-up material (lava, ash, and rocks) that has been ejected during previous eruptions. This landform is called a cone. Smaller cones, called conelets, often form on the slopes of main cones. One of the world's most active volcanoes, Kilauea in Hawaii, for instance, is a conelet on the larger Mauna Loa volcano.

On the shelves: Fire in the Sea

Published in 2000, this examination of the effects of the eruption of Santorini in about 1650 bce offers possible links between Santorini and the legend of the "lost continent" of Atlantis. Written by Walter L. Friedrich, the book is a carefully constructed case study of one of the biggest natural disasters of the Bronze Age.

The volcano itself is the opening in the ground through which material erupts. That opening is called a vent. The vent is typically at the center of a crater (a wide, round opening), which is situated at the top of the cone. When the volcano is inactive, the crater may fill with water, snow, ice, or rocks. Some craters continuously hold pools of bubbling molten rock.

Below the vent is a passageway, called a pipe. The pipe leads down to the magma chamber, a pool of magma in the upper mantle. In some eruptions, magma forces its way through the pipe and out of the vent. In others, magma flows upward and sideways between layers of rock until it finds a weak area at the surface and breaks through. Magma may also reach the surface by melting its way through layers of rock in the crust, near the volcano, or along the sides of the volcano.

Types of eruptions

When volcanoes erupt, they spew out gases such as carbon dioxide, nitrogen, and sulfur dioxide. (Poisonous gases cause more deaths than any other substance emitted by volcanoes.) Other materials that may be ejected include lava, steam, and fiery rock fragments (known as pyroclastics) of various sizes. The escaping gases have so much force that they blast hot rock into billions of minuscule pieces, forming a choking cloud of ash and dust. Boulders called volcanic bombs and weighing up to 100 tons, may be thrown out during eruptions. Large rocks usually land near the vent, but are sometimes tossed several miles. Eruptions commonly deposit deep layers of pumice (rock that contains numerous air holes and floats on water) and burned lava pieces.

Volcanic eruptions range in intensity from little puffs of ash and gas to explosions that produce glowing clouds of ash and dust and spew material far across the land. The intensity of the eruption depends upon the density and composition of the magma, the shape and size of the vent, and the presence or absence of water. The most explosive eruptions occur in volcanoes that sit on subduction zones, such as the Cascade Mountains.

Volcanic eruptions in which lava spills from the vent into the crater, sometimes overflowing the rim and running down the sides, are called effusive eruptions. Effusive eruptions occur when magma is runny, has a high gas content, and has low concentrations of a mineral called silica. (Silica is the mineral that forms quartz, sand, and flint.) The volcanic gas does not explode during eruptions because it can escape from the lava relatively easily. Sometimes escaping gas causes lava to spray high into the air, creating brilliant displays of fire. The volcanoes of Hawaii, for example, experience effusive eruptions.

The volcanoes of the Cascade Mountain Range in the northwestern United States, in contrast, undergo explosive, violent eruptions. The magma in the Cascade volcanoes is thick and viscous. It tends to trap volcanic gases. As the pressure on the magma is released, the gases expand within the magma. Eventually, the gases burst through the surface of the magma, with a great explosion. The magma is shattered into tiny pieces and thrown high into the atmosphere. The tiniest fragments are volcanic ash. They can remain suspended in the air for days or weeks. The larger fragments rain down on the cone and flow down its sides at tremendous speeds. The river of fragments can mix with heated gas and air to form hot and dangerous pyroclastic flows. If this mixture encounters water, destructive lahars are the result.

A steam eruption can be another violent form of eruption. This outburst occurs when water, either from underground rocks or the sea, comes in contact with magma. The water turns to steam, expands outward rapidly, and the mixture explodes. An example of a steam eruption occurred in 1883 at Indonesia's Krakatau volcano.

A particularly deadly type of eruption is the Plinian eruption. In this kind of eruption the volcano releases a suffocating cloud of gas, dust, and ash that can cover great distances. The eruption of Mount Vesuvius in 79 CE, in which some two thousand people died, is a famous example of a Plinian eruption. The name "Plinian" comes from the Roman writer Pliny the Younger, who provided a written record of the Vesuvius eruption.

Some explosive eruptions are due to the presence of hardened lava or boulders over the vent. Material that blocks the vent is called a cap or a plug. The cap prevents magma from breaking through to the surface for a while. When enough pressure builds up beneath the plug or cap, the magma pushes away the cap and bursts through explosively. Some of the most powerful eruptions in history have occurred on volcanoes that have been blocked.

In some very violent eruptions, large portions of the top or side of the cone are destroyed, and the sides of the crater collapse. This destruction occurs because the reservoir of magma beneath the crater becomes mostly or totally emptied. Where the cone used to stand, a shallow, wide, steep-walled, circular depression called a caldera is all that remains. This phenomenon typically occurs when there is a large pool of magma beneath the crater. The magma erupts forcefully, sending lava outward and emptying the crater. The walls of the crater then collapse.

In Oregon, for example, a volcano called Mount Mazama existed until destroyed by a violent eruption sixty-six hundred years ago. All that remains today is a caldera that has filled with water and is called Crater Lake. A small cone called Wizard Island has formed in the center of the lake, but the volcano is now extinct.

Cones that are destroyed in one eruption may be rebuilt in successive eruptions. For example, Indonesia's Krakatau volcano collapsed during its massive eruption in 1883, and a new volcano, called the Anak Krakatau, has since formed in its place.

Types of cones

There are four categories of cones, based on appearance and composition: cinder cones, composite cones, shield cones, and lava domes. Cinder cones are the steepest cones, with slopes of 30 to 40 degrees, and are seldom taller than 1,640 feet (500 meters). They are created by mildly explosive eruptions that send hot rocks into the air. When the rocks rain back down on Earth, they pile up in a steep cone around the vent. Ash and lava also contribute to the formation of cinder cones. Sunset Crater in Arizona, Stromboli in the Mediterranean Sea, and Paricutín in Mexico are examples of cinder cone volcanoes.

Composite cones are made of intermittent layers of lava and layers of fragmented rock and ash. This physical makeup exists because these volcanoes alternate between nonexplosive, flowing-lava eruptions and violently explosive eruptions. Composites have the most symmetrical cone shapes of any volcanoes. They are not as steep as cinder cones, having slopes less than 30 degrees at the summit and tapering off to 5 degrees at the base. Mount Vesuvius in Italy, Mount Fujiyama (also called Mount Fuji) in Japan, Mount St. Helens in Washington, and Mount Mahon in the Philippines are examples of composite cones.

Shield cones are formed mainly by runny, fast lava flows. They have broad, gentle slopes, seldom more than 10 degrees at the summit and 2 degrees at the base, and resemble warrior shields lying flat with the curved face up. The Hawaiian Islands, including Mauna Loa, the world's largest active volcano, are comprised of shield volcanoes.

The fourth type of volcano landform, the lava dome, is made of hardened, thick, pasty layers of lava that have been squeezed like toothpaste from a tube. Lava domes take on a variety of strange shapes. Examples of lava domes include Lassen Peak and Mono Dome in California.

Where volcanoes occur

More than half of the world's active volcanoes above sea level are located in a geographic belt called the "Ring of Fire" or "Circle of Fire." This belt surrounds the Pacific Ocean basin. It marks the boundary between the Pacific plate (the tectonic plate, or large piece of Earth's crust, underlying the Pacific Ocean) and the surrounding plates.

The Ring of Fire follows the west coast of the Americas from Chile to Alaska. It runs through the Andes Mountains, Central America, Mexico, California, the Cascade Mountains, and the Aleutian Islands. It continues down the east coast of Asia from Siberia to New Zealand, through Kamchatka, the Kurile Islands, Japan, the Philippines, Celebes, New Guinea, the Solomon Islands, and New Caledonia.

Most of the rest of the world's volcanoes are located in the oceans; some protrude above the water as islands and others lie entirely beneath the surface. Volcanoes are found in the middle of the Pacific Ocean (the Hawaiian Islands, for example), in the northern Atlantic Ocean (Iceland and nearby islands), in the mid-Atlantic between Maine and Spain, in the southern Atlantic between eastern South America and western Africa, and in the southern Pacific east of Australia.

The volcanoes in the Atlantic follow undersea ridges where sections of four different plates come together: the Eurasian plate, the North American plate, the African plate, and the South American plate. The volcanoes east of Australia follow the border between the Pacific plate and the Indo-Australian plate.

The volcanoes in the middle of the Pacific, however, exist on a single plate—in the absence of plate boundaries. Those volcanoes occur above hot spots, places where magma rises in currents from the upper mantle toward the crust. There is no definitive answer as to why hot spots form where they do, however some geologists (scientists who study the origin, history, and structure of Earth) think hot spots are related to the former positions of plate boundaries. Another theory is that hot spots mark the site where a plate may break apart in the future. At hot spots, the magma melts or otherwise forces its way through the layers of rock leading up to

Volcanoes in the United States

In the United States, the Pacific Northwest, Hawaii, and Alaska are home to several active volcanoes. Historically, the United States trails behind only Indonesia and Japan for having had the greatest number of eruptions in recorded history. About 10 percent of the eruptions that have occurred throughout the world over the last ten thousand years, including some of the largest and most destructive eruptions, have been in the United States.

The largest active volcano—not only in the United States, but in the world—is Hawaii's Mauna Loa. This volcano is 29,500 feet (9,000 meters) tall from its base on the ocean floor to its peak, making it taller than Mount Everest, although the bottom 19,000 feet (5,800 meters) are underwater. The volcano is 62 miles (97 kilometers) wide at its base on the ocean floor. Since 1900, Mauna Loa has erupted fifteen times; eruptions have ranged in duration from one day to one hundred and forty-five days. Mauna Loa's eruptions are nonexplosive. Instead, they cause great quantities of lava to flow over the ground in thin sheets.

Kilauea (pronounced key-lou-AYE-ah) is a volcano that formed on the side of Mauna Loa. Kilauea has been erupting almost continuously since January 1983, making it the longest-erupting volcano in recorded history. Kilauea also erupts nonexplosively, giving scientists an opportunity to make careful studies of its behavior in relative safety. The National Park Service provides a viewing area for tourists to observe Kilauea's activity from a safe distance, earning it the nickname of the "drive-in volcano."

The Cascade Mountain Range, which runs through northern California, Oregon, and Washington, and on into western Canada, is home to several volcanoes that erupt infrequently but explosively. The eruption of Mount St. Helens occurred in a manner typical of Cascade volcanoes. Another volcano in the Cascades of concern to area residents is Washington's Mount Baker. This volcano, which periodically (but not since 1976) expels puffs of smoke and ash, is covered with ice and snow. The fear persists that an eruption by Mount Baker would cause tremendous avalanches and lahars. Similarly, Mount Rainier, a glacier-covered volcano in Washington that has been quiet for five hundred years, is also a source of concern in the region. If the volcano were to become active again, melting ice would trigger massive lahars.

Another region of the United States that is a hotbed of volcanic activity is the Alaskan Panhandle and Aleutian Islands (the land that forms a tail leading into the Bering Sea, in southwestern Alaska). The area is home to about 80 major volcanoes and experiences one or two eruptions per year. One of those eruptions, which occurred in 1912 at Novarupta (in what is now the Katmai National Monument), was the world's largest of the twentieth century. Novarupta ejected some 3.6 cubic miles (15 cubic kilometers) of magma—about 30 times more material than ejected by Mount St.

Helens—over a 60-hour period and caused the collapse of Mount Katmai, a volcano 6 miles (10 kilometers) away. A group of scientists who visited the site four years after the eruption discovered that a valley 15 miles (24 kilometers) from Novarupta, covered by ash from the eruption, still emitted jets of steam and was hot enough in places to boil water. They named the area the Valley of Ten Thousand Smokes.

the surface and forms volcanoes. The world's largest volcanoes are located above hot spots.

Consequences of volcanic eruptions

Volcanic eruptions burn and blacken the landscape. They clear an area of plant and animal life and blanket it in a layer of ash. The lethal gases, dust, and ash suffocate people and wildlife. The ash, rocks, and lava that cover the ground, sometimes in deep layers, may smolder for weeks, months, and in some cases for years. Homes, farms, and entire communities may be buried under flows of material ejected from a volcano.

One of the most hazardous elements of a volcanic eruption is a mudflow called a lahar. The lahar is made of volcanic ash and water and may be very hot. Lahars flow for great distances, burying everything they encounter. Another deadly feature of eruptions is a pyroclastic flow, also called pyroclastic surge, a fast-moving mixture of hot gas and ash. Volcanoes near the oceans may trigger tsunamis (pronounced tsoo-NAH-mees), huge ocean waves (also called tidal waves) that result when a large quantity of ocean water is displaced. Tsunamis can destroy coastal towns and cities.

Gradual recovery following an eruption

After an eruption, new life emerges from the ashes. This process may take months or generations, depending on the depth of the ash-and-lava layer and the climate type. (Rebound takes longer in harsher climates.) Rain and wind eventually break down the hardened lava into soil in a process called weathering. Other factors that promote weathering are the heat during the day (which makes lava expand) and the cooling at night (which makes lava contract).

Once the soil is reestablished, plant life reappears. Moss and lichens are the first plants to spread across the landscape. Flowering plants, the seeds of which are brought to the area by wind or birds, take hold next. Animals then reestablish their presence on the volcano. It generally takes at least a century before trees attain their pre-eruption size.

After the eruption of Mount St. Helens, Spirit Lake was a poisonous mess still bubbling with volcanic gases seeping from the lake bed. A month after the eruption the lake was completely lacking dissolved oxygen in the water. Scientists predicted that the lake would take many decades to recover. However, they were pleasantly surprised to discover that only three years after the Mount St. Helens eruption Spirit Lake was rich with biological activity. In 1993 scientists were even more surprised to see healthy fish in Spirit Lake. Even though the lake was devastated by Mount St. Helens it has rebounded significantly and is on the way to recovery.

Eruptions produce rich soil

For all their hazards, there is at least one positive thing about volcanoes; they create fertile soil. Some of the world's best farmland is located on the slopes and around the base of volcanoes. The soil is formed from broken down volcanic rocks mixed with decaying vegetation. Those rocks, and the layer of soil they form, have large concentrations of minerals that are necessary for plant growth. The fertility of the soil is what draws people to farm the slopes of Mount Vesuvius, Mount Etna, and other famous volcanoes around the world, despite the danger of possible eruptions. In Japan and Indonesia, farmers excavate slopes to create flat areas, or terraces, on which they grow crops. Volcanic ash blown over thousands of square miles has added nutrients to the soil in forests and farmland.

Precious gems and useful rocks

Volcanoes are good sources of diamonds; several diamond mines have been established at the sites of ancient volcanoes. Geologists believe that diamonds, which are a form of carbon that has been subjected to high heat and pressure, are created in the pipes leading from magma chambers to volcanic vents. Diamonds have been found in these pipes, as well as in sand and gravel in streams near volcanoes. The diamonds found in streams are thought to have originated in pipes that have eroded over time. While a pipe can be destroyed by wind and water, diamonds, which are among the hardest substances on Earth, survive and erode out of the rock.

Sulfur, marble, tin, and copper are also found in the vicinity of volcanoes. Sulfur is a pale-yellow mineral that is used in making matches, gunpowder, and rubber. It is also used in some medicines. Tin and copper are formed by the intense heat and pressure in volcanoes. When in a liquid state, these metals run between layers of rock. When hardened, they can be seen as veins, or lodes, in the rock. Tin is a soft, silvery metal used to coat other metals in order to prevent corrosion. Copper is a flexible, reddish-brown metal that conducts electricity and is used for electrical wiring.

Another useful by-product of volcanoes is a soft white clay called china clay or kaolin. China clay is produced by the action of steam (from underground water) on feldspar, which is a type of crystal in the rock. China clay is used in the production of ceramics, china plates and cups, paper, paint, medicine, and electronic devices.

Effects on weather

Volcanic eruptions place large amounts of gases, ash, and dust into the air; those substances may affect the weather in the short term and even influence global climate for long periods of time. While large particles spewed out of volcanoes rain down on the ground, the smallest particles make their way upward, high into the atmosphere. Those particles are carried by upper-air winds for hundreds of miles (kilometers), even circling the globe. They remain suspended in the air, sometimes for months, until they fall to Earth or are washed down with the rain. The presence of volcanic material in the sky is most noticeable at sunset, when the sunlight reflects off the particles and turns the sky red.

The primary effect that airborne volcanic dust has on the weather is increasing the amount of rain. That happens because the specks of dust serve as condensation nuclei—tiny solid particles around which water vapor condenses, or changes to liquid form. The added number of condensation nuclei in the air cause higher-than-normal rates of condensation and, consequently, more rain.

A fascinating side effect of volcanic eruptions is lightning. Lightning flashes occur above an erupting volcano because the particles of dust and ash rub against each other, creating a build-up of static electricity. When the charge builds to a certain point, it is released as lightning.

Effects on climate and the environment

Volcanic eruptions have very likely brought about periods of glaciation, extremely cold times when glaciers covered large portions of Earth. In the early stages of Earth's history, thousands of volcanoes dotted the surface. These volcanoes underwent frequent, large eruptions that had a great impact on the climate. In addition to releasing gases that rose up and formed Earth's

atmosphere, the eruptions sometimes spewed out ash and dust so thick that they blocked out the sun.

Volcanic eruptions today are far fewer in number and intensity than they once were. A very large volcanic eruption today only affects global climate for a few years. In 1815, for example, the Indonesian volcano Mount Tambora erupted. The Tambora event, together with smaller eruptions of other volcanoes over the preceding four years, led to a short-term decrease in global temperature. There was a severe cold spell in 1816, causing the year to be called "the year without a summer."

Some eruptions discharge large amounts of sulfur gases into the air. Even after the ash and dust clears from the atmosphere, sulfur continues to react with water vapor to produce sulfuric acid particles. These particles collect and form a heavy layer of haze. This layer can remain in the upper atmosphere for years, reflecting away a portion of the incoming sunlight. The result is a decrease in temperature around the world.

Sulfuric acid in the air has another consequence: acid rain. Acid rain, which is rain with an unusually high acidity, is formed when sulfuric acid (or nitric acid, from car exhaust systems or factory smokestacks) mixes with rainwater. Acid rain raises the acidity of lakes and rivers, making them inhospitable to many animal species. It also kills trees and has been recently shown to endanger human health.

Technology connection

Volcanologists (scientists who study volcanoes and volcanic phenomena) use several methods to predict when volcanoes may erupt. The purpose of predicting eruptions is to give people in the vicinity time to evacuate. In most cases, volcanic eruptions can be predicted a few days to a few weeks before they occur. It is difficult to foretell, however, just how a volcano will behave and what it will eject. The United States Geological Survey operates three observatories for the study of active volcanoes, in Hawaii, the Cascade Mountains, and Alaska.

One way that volcanologists and other geologists (scientists who study the origin, history, and structure of Earth) learn about the behavior of volcanoes is by studying past eruptions. They analyze volcanic rock on and around the cone to determine the composition of the materials ejected from the volcano. They also study satellite photographs to create maps of previous lava flows, and read written reports of past eruptions. They put together a timeline that tells when the volcano has erupted in the past and how many years have passed between eruptions. All of this information helps scientists predict when the volcano might erupt next, what materials might be cast out, the probable nature of the eruption, and where the materials will likely travel.

Volcanologists also learn about volcanic behavior by traveling to volcanoes that are showing signs of activity. They use gas detectors to sample and identify the gases being emitted and thermometers to measure the temperature of rocks around the crater. Increased rock temperature is a sign that magma is nearing the surface, because the approaching magma heats the rocks.

Another tool used by volcanologists to predict when a volcano will erupt is a tiltmeter. A tiltmeter measures changes in the shape and angle of a cone's slope. The formation of a bulge on a slope indicates the swelling of the magma chamber beneath the surface and the rising of magma toward the vent. Scientists also use seismographs or seismometers, instruments that detect ground movements. These tools can even pick up movements that are too slight to be felt by humans. The movements may be caused by a swelling of magma or by an earthquake. The swelling of magma is an indicator that a mountain is ready to erupt. Earthquakes, which produce cracks in the ground, often trigger volcanic eruptions.

Controlling volcanic flows

While there is no way to stop a volcano from erupting, there are ways to lessen the damage caused by eruptions. People

who live in the vicinity of active volcanoes attempt to control flows of volcanic materials in several ways. Examples include spraying cold water on the lava to cool it and slow it down, constructing dams to prevent the flow from entering certain areas, and digging trenches that direct the flow away from human settlements.

In Iceland, a country situated in an arc of volcanoes, residents had success battling the eruption of the Helgafell volcano in 1973. Aiming to protect the town of Heimaey, people sprayed cold water, pumped from the ocean, on the flowing lava. The method seemed to have achieved some success as the lava slowed down and hardened before it reached the town.

People living near Sicily's Mount Etna are also knowledgeable about volcano damage control. Sicilians have been able to direct lava flows by blasting holes in the sides of hardening lava tubes (hollow tubes formed when the outer layer of lava hardens, while molten lava continues running through the tube). Some of the lava then runs out through the holes and establishes new routes instead of all continuing to flow through the tube.

The Sicilians also bulldoze rocky ground to form walls that keep the lava from flowing into towns. They sometimes fortify those walls with boulders dropped by helicopters. In light of Mount Etna's history of erupting and burying homes and farms, controlling the lava flow is essential to the survival and livelihood of villagers near the volcano.

A matter of survival

For people living in the vicinity of a volcano, even a volcano that has not been active for many years, the best strategy for survival is adequate preparation. (Note that volcanoes that lay dormant for long periods of time generally erupt with greater explosiveness than do volcanoes that have frequent eruptions.) It is imperative to understand what may happen during an eruption and to be familiar with your community's warning systems and emergency plans. Learn your evacuation route and develop an emergency communication plan so that members of your family, if at different places during the day, can find each other during or after an eruption.

People living in volcano danger zones should have a pair of goggles and a breathing mask for each member of the family (to be used if ash is falling). Every family should also have a disaster supply kit with the following items: flashlight and extra batteries, first aid kit, bottled water and nonperishable food, manual can opener, and necessary medicines.

American Red Cross guidelines state that if a volcano erupts, people in the danger zone should evacuate when ordered to do so by authorities. People who cannot get away and are caught indoors should close all doors and windows. People trapped outdoors should try to seek shelter indoors. If that is impossible, they should move as far as possible from the volcano and stay upslope from rivers or streams (to avoid mudflows). Do not try to approach the volcano to watch the eruption—such a move can be deadly.

Even people living several miles from a volcano may experience a shower of volcanic ash during an eruption. If you have to go outdoors during an ashfall, wear long pants, a long-sleeved shirt, and goggles. Breathe through a dust mask or a damp cloth. Do not attempt to drive, as the ash may harm your vehicle's engine and the moving vehicle will stir up ash, worsening the situation.

Stay tuned to a battery-powered radio or television for emergency information. After the eruption, try to avoid areas where ash has fallen. People with asthma or other respiratory ailments, in particular, should

avoid breathing ash as much as possible. Offer assistance to those neighbors who are elderly or who have disabilities. If you have been ordered to evacuate, do not return to your home until given the all-clear by authorities.

Bizarre lava formations

Hardened lava creates some of the world's most unusual land formations. Two of these formations are common in Hawaii and have Hawaiian names: pahoehoe (pronounced pa-HO-eh-HO-eh) and aa (pronounced ah-ah). Pahoehoe, which resembles coiled rope when cooled, is made from fluid lava with high concentrations of gas. A hard skin forms on this lava as its surface cools. Meanwhile, hot lava continues flowing beneath the skin. As the lava moves below, it wrinkles the skin, forming coiled-rope patterns. Lava may continue flowing for miles beneath a hardened pahoehoe skin, which is thick enough to walk on. Hot lava that drips from the roof of pahoehoe skin creates icicle-like rock formations.

Slow-moving, thicker lava, in contrast, hardens to form blocks called aa. Aa formations have sharp, jagged rocky edges and are difficult to walk on. About 99 percent of the island of Hawaii is composed of aa and pahoehoe.

Another distinctive lava formation is the lava tube. When lava flows as a narrow stream, the outer layer cools and hardens first. Meanwhile, this hardened crust traps the heat inside it and hot lava continues to flow through it. When the lava flow ceases, a hollow tube, called a lava tube, is left.

One of the world's most remarkable lava formations is found in Antrim, in Northern Ireland. The Giant's Causeway, as the formation is called, is comprised of tall, six-sided columns. The columns were formed by flood basalt, which is lava that

flowed from cracks in the ground and buried everything in its path. When the lava cooled and hardened, it cracked in patterns similar to parched ground. When the cooling was complete, the basalt (a type of rock formed from lava) had separated into six-sided pillars. Another stunning display of these seemingly unnatural pillars can be seen at Devil's Postpile in California.

[See AlsoAvalanche; Earthquake; Landslide; Tsunami ]

For More Information

BOOKS

Gates, Alexander E., and David Ritchie. Encyclopedia of Earthquakes and Volcanoes (Facts on File Science Library). 3rd ed. New York: Facts on File, 2006.

Marti, Joan, and Gerald Ernest. Volcanoes and the Environment. Cambridge, UK: Cambridge University Press, 2005.

O'Meara, Donna. Into the Volcano: A Volcano Researcher at Work. Tonawanda, NY: Kids Can Press, 2007.

WEB SITES

"Cascades Volcano Observatory." United States Geological Survey. 〈http://vulcan.wr.usgs.gov/〉 (accessed March 24, 2007).

Jaffe, Eric. "Volcanic Lightning." Smithsonian Science and Technology. 〈http://www.smithsonianmag.com/issues/2007/february/augustine.php〉 (accessed March 24, 2007).

"Hawaiian Volcano Observatory." United States Geological Survey. 〈http://hvo.wr.usgs.gov/〉 (accessed March 24, 2007).

Helmuth, Laura. "Antarctica Erupts!" Smithsonian Science and Technology. 〈http://www.smithsonianmag.com/issues/2006/december/antarctica.php〉 (accessed March 24, 2007).