Volcanoes

HOW VOLCANOES FORM

Three types of plate boundaries have worked together to create the oceans and continents: divergent boundaries, convergent boundaries, and transform boundaries.

*Divergent boundaries:* The crustal plates move partly because molten material wells up along some of their boundaries. This material pushes the plates apart, forming new crust. These plate boundaries are called divergent (spreading apart) boundaries and occur along mid-ocean ridges and continental rift zones. The Mid-Atlantic Ridge is one such boundary.
*Convergent boundaries:* At the opposite edges of the plates, the motion causes them to collide with other plates and either slide under them or ride up over them. These collisions cause volcanoes and earthquakes and create mountains. The Cascade Range in the northwestern U.S. is a volcanic mountain chain caused by subduction activity.
*Transform boundaries:* The third type of boundary, a transform boundary, occurs when the edges of plates slide against each other. Earthquakes are also common along transform boundaries such as the San Andreas Fault in California.

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A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle.

The Mid-Atlantic Ridge has volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates coming together.

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DEFINITIONS

Magma - Molten rock beneath Earth's surface.

Parasitic Cone - A small cone-shaped volcano formed by an accumulation of volcanic debris.

Sill - A flat piece of rock formed when magma hardens in a crack in a volcano.

Vent - An opening in Earth's surface through which volcanic materials escape.

Flank - The side of a volcano.

Lava - Molten rock that erupts from a volcano that solidifies as it cools.

Crater - Mouth of a volcano - surrounds a volcanic vent.

Conduit - An underground passage magma travels through.

Summit - Highest point; apex

Throat - Entrance of a volcano. The part of the conduit that ejects lava and volcanic ash.

Ash - Fragments of lava or rock smaller than 2 mm in size that are blasted into the air by volcanic explosions.

Ash Cloud - A cloud of ash formed by volcanic explosions.

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