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Glossary beginning with B

Volcano Term Definitions

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Basalt

Volcanic rock (or lava) that characteristically is dark in color, contains 45% to 54% silica, and generally is rich in iron and magnesium.

Basement

The undifferentiated rocks that underlie the rocks of interest in an area.

Basic

A descriptive term applied to igneous rocks (basalt and gabbro) with silica (SiO2) between 44% and 52%.

Bench

The unstable, newly-formed front of a lava delta.

Blister

A swelling of the crust of a lava flow formed by the puffing-up of gas or vapor beneath the flow. Blisters are about 1 meter in diameter and hollow.

Block

Angular chunk of solid rock ejected during an eruption.

Bomb

Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.