extrusive rock

Igneous rock formed on the surface of the Earth by volcanic activity (as opposed to intrusive, or plutonic, rocks that solidify below the Earth's surface). Magma (molten rock) erupted from volcanoes cools and solidifies quickly on the surface. The crystals that form do not have time to grow very large, so most extrusive rocks are finely grained. The term includes fine-grained crystalline or glassy rocks formed from hot lava quenched at or near Earth's surface, and those made of welded fragments of ash and glass ejected into the air during a volcanic eruption. The formation of extrusive igneous rock is part of the rock cycle.

Large amounts of extrusive rock called basalt form at the Earth's ocean ridges from lava that fills the void formed when two tectonic plates spread apart. Explosive volcanoes that deposit pyroclastics generally occur where one tectonic plate descends beneath another. Andesite is often formed by explosive volcanoes. Magmas that give rise to pyroclastic extrusive rocks are explosive because they are viscous. The island of Montserrat, West Indies, is an example of an explosive volcano that spews pyroclastics of andesite composition. Magmas that produce crystalline or glassy volcanic rocks upon cooling are less viscous. The low viscosity allows the extruding lava to flow easily. Fluid-like lavas that flow from the volcanoes of the Hawaiian Islands have low viscosity and cool to form basalt.