Volcanic rock

An igneous rock formed when lava or volcanic fragments cool and solidify at or near Earth’s surface. Rapid cooling commonly produces fine-grained or glassy textures.

Lava: Molten rock that has reached Earth’s surface. Its cooling and solidification produce many volcanic rocks.
Extrusive rock: Igneous rock that solidifies at or near Earth’s surface. It is the principal category that includes volcanic rock.
Basalt: A dark, fine-grained volcanic rock with low silica content, commonly rich in iron and magnesium. It is a widespread product of fluid, low-silica lava.
Pillow lava: Rounded masses of lava formed when magma erupts underwater and cools rapidly. Its shape and chilled margins preserve evidence of underwater eruptions.
Plutonic rock: Igneous rock formed when magma cools and solidifies beneath Earth’s surface. Its slower underground cooling usually produces larger crystals than in volcanic rock.
Magma: Molten or partly molten rock beneath Earth’s surface, containing crystals and dissolved gases. Its composition largely determines the composition of volcanic rock after eruption.
Pyroclastic rock: Rock made from volcanic fragments deposited during explosive eruptions. It forms when erupted fragments accumulate and become consolidated.
Andesite: A fine-grained volcanic rock with an intermediate silica content. It commonly forms in volcanic arcs above subduction zones.
Columnar jointing: A pattern of polygonal columns formed as thick lava or other rock cools and contracts. It reveals cooling fractures in solidified volcanic flows.
Granite: A coarse-grained, silica-rich plutonic rock composed mainly of quartz and feldspar. It contrasts with rhyolite as a slowly cooled, coarse-grained counterpart of similar composition.

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