Volcano: shapes and sizes

Seen one volcano, seen them all? Not true, say geologists. A volcano's form depends on a) whether its eruption was quiet or explosive (or both), and b) the type of lava it spews. Read the descriptions of the volcano formations below. Then, in Part 2, match each description with a real-life volcano.

CINDER CONE

These volcanoes form by explosive eruptions' that spew different sizes of volcanic material--called tephra--high into the air. Tephra ranges in size from volcanic ash (the smallest particles) to cinders (rock fragments) to larger rocks called volcanic bombs. Combined with gases, tephra has a frothy texture. After an explosion, tephra settles to the ground around the volcano's vent (lava "pipeline"), forming the steep, loosely arranged sides of a cinder cone. Cinder cones typically have large bowl-shaped craters.

COMPOSITE OR STRATOVOLCANO

These volcanoes are built up from alternating layers of rock particles and lava. First, a violent eruption occurs, hurling tephra and gas high into the air. Heavier rocks land near the volcano's opening, while smaller pieces are carried farther away by the wind. Years later, an eruption of quiet lava blankets the rocky material. After several alternating eruptions of tephra-lava-tephra-lava, a large cone-shaped mountain forms.

CALDERA

Some volcanoes have enormous craters, called calderas, which stretch for more than a mile in diameter. A caldera forms when a volcano literally blows its top. Enormous volumes of ash and dust sweep down the slopes in avalanches. Lava erupts, or it may drain from beneath the mountain through underground channels. That weakens the upper part of the mountain. The top collapses inward and forms a deep, bowl-shaped pit--the caldera. Because of their tremendous size and steep walls, calderas often trap rainwater or melted snow and form mountaintop lakes.

SHIELD VOLCANO

Nonexplosive, runny lava flowing over a large area may result in a shield volcano. This lava doesn't "explode" because it contains less dissolved gas than the ash and cinders of a stratovolcano. When the lava hardens in flat layers, a gently sloping, domed-shaped mountain forms. The lava of a shield volcano forms basalt--a black rock made from hardened magma.

LAVA DOME

Domes are formed by the eruption of lava so thick and sticky it hardly flows at all. Instead, the lava bulges up to. the surface and fans outward. It piles up in and around the vent. Eventually, it forms a large rounded mound of lava blocking the vent, like a cork in a soda bottle. As the pressure builds, the volcano may blow its top.

DIAGRAM KEY:

* MAGMA

** LAVA

*** ASH

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