Volcano: shapes and sizes. (Chart-Reading/Diagram 1).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 teph·ra n. Solid matter that is ejected into the air by an erupting volcano. [Greek tephr ranges in size from volcanic ash (the smallest particles) to cinders cin·der n. 1. a. A burned or partly burned substance, such as coal, that is not reduced to ashes but is incapable of further combustion. b. A partly charred substance that can burn further but without flame. (rock fragments) to larger rocks called volcanic bombs. Combined with gases, tephra has a frothy froth·y adj. froth·i·er, froth·i·est 1. Made of, covered with, or resembling froth; foamy. 2. Playfully frivolous in character or content: a frothy French farce. 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 strat·o·vol·ca·no n. pl. strat·o·vol·ca·nos A volcano composed of alternating layers of lava and ash. [strat(um) + volcano. 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 caldera: see crater. caldera Large, bowl-shaped volcanic depression that forms when the top of a volcanic cone collapses into the space left after magma is ejected during a violent volcanic eruption. The term is Spanish for “caldron. 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 moun·tain·top n. The summit of a mountain. lakes. SHIELD VOLCANO Nonexplosive non·ex·plo·sive adj. That will not explode: a nonexplosive fuel; nonexplosive gases. non , runny run·ny adj. run·ni·er, run·ni·est Inclined to run or flow: runny icing; a runny nose. runny Adjective [-nier, -niest 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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