About Extrusive Igneous Rocks

Lava flows at Kilauea volcano in Hawaii
Emerging pahoehoe lava flow at Kilauea volcano in Hawaii
Artist interpretation of volcanism in the Ancestral Cascades, in North West United States.
Map of the western United States showing the inferred extent of Cenozoic ancestral and modern Cascades magmatic arcs in Washington, Oregon, California, and Nevada.
NMNH geologist Ben Andrews stands in front of basalt columns in Washington, USA, which were produced during the time of the Ancestral Cascades.
Pyroclasitc flow at Mayon Volcano, Spet. 15, 1984. Philippines.
The Giant's Causeway, in Northern Ireland, is a large and well-known example of columnar jointing. (Giant's Causeway HDR, 2009 Bill Strong via Flick.)
This dam in Washington, USA, is constructed primarily from basalt from a quarry that mines the Ancestral Cascades.
Volcanic rock quarry in Washington. Dark gray basalt covered by volcanic ash and soil. Location: White River Quarry, Enumclaw, WA.
Volcanic rocks used to protect harbor in Washington, USA.
Artist interpretation of rifting along the Eastern United States, leading to the creating of the Atlantic Ocean and rift basins that filled with sediments and volcanic rock.
Contact between igneous diabase (left) and metamorphic hornfels (right), which used to be sedimentary siltstone. Location: Manassas Quarry, Manassas, VA.

When molten rock (magma) reaches Earth's surface, it solidifies or hardens. Scientists call the resulting solid rocks "extrusive" igneous rocks. Extrusion is the process of pushing material out to the surface of the Earth's crust. At some volcanoes, the extrusive rock flows as lava across the ground before it hardens; the ripples in the lava may freeze in place. Hot, rapidly expanding gases within other volcanoes' vents can force the magma out explosively, forming pumice: low-density rock full of vesicles, or frozen bubbles. Extrusive igneous rocks are easy to find near many volcanoes, such as Mount St. Helens in Washington state. Hawai`i Volcanoes National Park, home of two active volcanoes, contains lava flows that cooled only a few decades, or minutes, ago.