As with earthquakes, volcanic activity is linked to plate-tectonic processes. Most of the world's active above-sea volcanoes are located near convergent plate boundaries where subduction is occurring, particularly around the Pacific basin. However, much more volcanism -- producing about three quarters of all lava erupted on Earth -- takes place unseen beneath the ocean, mostly along the oceanic spreading centers, such as the Mid-Atlantic Ridge and the East Pacific Rise.

Subduction-zone volcanoes like Mount St. Helens (in Washington State) and Mount Pinatubo (Luzon, Philippines), are called composite cones and typically erupt with explosive force, because the magma is too stiff to allow easy escape of volcanic gases. As a consequence, tremendous internal pressures mount as the trapped gases expand during ascent, before the pent-up pressure is suddenly released in a violent eruption. Such an explosive process can be compared to putting your thumb over an opened bottle of a carbonated drink, shaking it vigorously, and then quickly removing the thumb. The shaking action separates the gases from the liquid to form bubbles, increasing the internal pressure. Quick release of the thumb allows the gases and liquid to gush out with explosive speed and force.

In 1991, two volcanoes on the western edge of the Philippine Plate produced major eruptions. On June 15, Mount Pinatubo spewed ash 40 km into the air and produced huge ash flows (also called pyroclastic flows) and mudflows that devastated a large area around the volcano. Pinatubo, located 90 km from Manila, had been dormant for 600 years before the 1991 eruption, which ranks as one of the largest eruptions in this century. Also in 1991, Japan's Unzen Volcano, located on the Island of Kyushu about 40 km east of Nagasaki, awakened from its 200-year slumber to produce a new lava dome at its summit. Beginning in June, repeated collapses of this active dome generated destructive ash flows that swept down its slopes at speeds as high as 200 km per hour. Unzen is one of more than 75 active volcanoes in Japan; its eruption in 1792 killed more than 15,000 people--the worst volcanic disaster in the country's history.

While the Unzen eruptions have caused deaths and considerable local damage, the impact of the June 1991 eruption of Mount Pinatubo was global. Slightly cooler than usual temperatures recorded worldwide and the brilliant sunsets and sunrises have been attributed to this eruption that sent fine ash and gases high into the stratosphere, forming a large volcanic cloud that drifted around the world. The sulfur dioxide (SO2) in this cloud -- about 22 million tons -- combined with water to form droplets of sulfuric acid, blocking some of the sunlight from reaching the Earth and thereby cooling temperatures in some regions by as much as 0.5 °C. An eruption the size of Mount Pinatubo could affect the weather for a few years. A similar phenomenon occurred in April of 1815 with the cataclysmic eruption of Tambora Volcano in Indonesia, the most powerful eruption in recorded history. Tambora's volcanic cloud lowered global temperatures by as much as 3 °C. Even a year after the eruption, most of the northern hemisphere experienced sharply cooler temperatures during the summer months. In part of Europe and in North America, 1816 was known as "the year without a summer."

Apart from possibly affecting climate, volcanic clouds from explosive eruptions also pose a hazard to aviation safety. During the past two decades, more than 60 airplanes, mostly commercial jetliners, have been damaged by in-flight encounters with volcanic ash. Some of these encounters have resulted in the power loss of all engines, necessitating emergency landings. Luckily, to date no crashes have happened be-cause of jet aircraft flying into volcanic ash.

Since the year A.D. 1600, nearly 300,000 people have been killed by volcanic eruptions. Most deaths were caused by pyroclastic flows and mudflows, deadly hazards which often accompany explosive eruptions of subduction-zone volcanoes. Pyroclastic flows are fast-moving, avalanche-like, ground-hugging incandescent mixtures of hot volcanic debris, ash, and gases that can travel at speeds in excess of 150 kilometers per hour. 

Mudflows (also called debris flows or lahars, an Indonesian term for volcanic mudflows) are mixtures of volcanic debris and water. The water usually comes from two sources: rainfall or the melting of snow and ice by hot volcanic debris. Depending on the proportion of water to volcanic material, mudflows can range from soupy floods to thick flows that have the consistency of wet cement. As mudflows sweep down the steep sides of composite volcanoes, they have the strength and speed to flatten or bury everything in their paths. 

Eruptions of Hawaiian and most other mid-plate volcanoes differ greatly from those of composite cones. Mauna Loa and Kilauea, on the island of Hawaii, are known as shield volcanoes, because they resemble the wide, rounded shape of an ancient warrior's shield. Shield volcanoes tend to erupt non-explosively, mainly pouring out huge volumes of fluid lava. Hawaiian-type eruptions are rarely life threatening because the lava advances slowly enough to allow safe evacuation of people, but large lava flows can cause considerable economic loss by destroying property and agricultural lands. For example, lava from the ongoing eruption of Kilauea, which began in January 1983, has destroyed more than 200 structures, buried kilometers of highways, and disrupted the daily lives of local residents. Because Hawaiian volcanoes erupt frequently and pose little danger to humans, they provide an ideal natural laboratory to safely study volcanic phenomena at close range

In recorded history, explosive eruptions at subduction-zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations. Yet scientists have estimated that about three quarters of the material erupted on Earth each year originates at spreading mid-ocean ridges. However, no deep submarine eruption has yet been observed "live" by scientists. Because the great water depths preclude easy observation, few detailed studies have been made of the numerous possible eruption sites along the tremendous length (50,000 km) of the global mid-oceanic ridge system. Recently however, repeated surveys of specific sites along the Juan de Fuca Ridge, off the coast of the Oregon and Washington, have mapped deposits of fresh lava, which must have been erupted sometime between the surveys.

Iceland, where the Mid-Atlantic Ridge is exposed on land, is a different story. It is easy to see many Icelandic volcanoes erupt non-explosively from fissure vents, in similar fashion to typical Hawaiian eruptions; others, like Hekla Volcano, erupt explosively.

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03/02/01, Lyn Topinka