Lava

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Pyroclastic Eruptions

Pyroclasts are fragments of hot lava or rock shot into the air when gas-rich lava erupts. Gases easily dissolve in liquids under pressure and come out of solution when the pressure is released. Magma deep underground is under many tons of pressure from the overlying rock. As the magma rises, the pressure from the overlying rocks drops because less weight is pressing down on the magma. Just as the rapid release of bubbles can force a fountain of soda to be ejected from a shaken soda bottle, the rapid release of gas can propel the explosive release of lava.

Pyroclasts come in a wide range of sizes, shapes, and textures. Pieces smaller than peas are called ash. Cinders are pea sized to walnut sized, and anything larger are lava bombs.

Cinders and bombs tend to fall to earth fairly close to where they are ejected, but in very strong eruptions they can travel farther. Lava bombs as large as 100 tons have been found 10 km (6 mi) from the volcano that ejected them. When cinders and bombs accumulate around a volcanic vent, they form a cinder cone. Although the fragments of lava cool rapidly during their brief flight through the air, they are usually still hot and sticky when they land. The sticky cinders weld together to form a rock called tuff.

Ash, because it is so much smaller than cinders, can stay suspended in the air for hours or weeks and travel great distances. The ash from the 1980 eruption of Mount Saint Helens in the state of Washington circled the earth twice.

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Many volcanoes have both lava eruptions and pyroclastic eruptions. The resulting volcano is composed of alternating layers of lava and pyroclastic material. These volcanoes are called composite volcanoes or stratovolcanoes. With slopes of 15° to 20°, they are steeper than the gently sloped shield volcanoes. Many stratovolcanoes, such as the picturesque Mount Fuji in Japan, have convex slopes that get steeper closer to the top.

Pyroclastic materials that accumulate on the steep upper slopes of stratovolcanoes often slide down the mountain in huge landslides. If the volcano is still erupting and the loose pyroclastic material is still hot, the resulting slide is called a pyroclastic flow or nuée ardente (French for “glowing cloud”). The flow contains trapped hot gases that suspend the ash and cinders, enabling the flow to travel at great speed. Such flows have temperatures of 800° C (1500° F) and often travel in excess of 150 km/h (100 mph). One such pyroclastic flow killed 30,000 people in the city of Saint-Pierre on the Caribbean island of Martinique in 1902. Only one person in the whole town survived. He was in a basement jail cell.

Loose accumulations of pyroclastic material on steep slopes pose a danger long after the eruption is over. Heavy rains or melting snows can turn the material into mud and set off a catastrophic mudflow called a lahar. In 1985 a small pyroclastic eruption on Nevado del Ruiz, a volcano in Colombia, melted snowfields near the summit. The melted snow, mixed with new and old pyroclastic material, rushed down the mountain as a wall of mud 40 m (140 ft) tall. One hour later, it smashed into the town of Armero 55 km (35 mi) away, killing 23,000 people.

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Explosive Eruptions

Rhyolitic lava, because it is so viscous, and because it contains so much gas, is prone to cataclysmic eruption. The small amount of lava that does emerge from the vent is too thick to spread. Instead it forms a dome that often caps the vent and prevents the further release of lava or gas. Gas and pressure can build up inside the volcano until the mountaintop blows apart. Such an eruption occurred on Mount Saint Helens in 1980, blowing off the top 400 m (1,300 ft) of the mountain.

Other catastrophic eruptions, called phreatic explosions, occur when rising magma reaches underground water. The water rapidly turns to steam which powers the explosion. One of the most destructive phreatic explosions of recorded history was the 1883 explosion of Krakatau, in the strait between the Indonesian islands of Java and Sumatra. It destroyed most of the island of Krakatau. The island was uninhabited, so no one died in the actual explosion. However, the explosion caused tsunamis (giant ocean waves) that reached an estimated height of 30 m (100 ft) and hit the nearby islands of Sumatra and Java, destroying 295 coastal towns and killing about 34,000 people. The noise from the explosion was heard nearly 2,000 km (1,200 mi) away in Australia.

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