Extrusive

Extrusive describes igneous rocks that are formed on the Earth's surface. These rocks solidify from rapidly cooling lava, as opposed to magma that cools slowly beneath the surface (intrusive rocks). This rapid cooling often results in smaller crystal sizes, giving extrusive rocks a fine-grained or glassy texture. The chemical composition of the lava determines the specific mineral content and color of the extrusive rock. Features such as volcanic vents, lava flows, and ash deposits are associated with extrusive rock formation.

Extrusive meaning with examples

• The Hawaiian Islands are largely composed of extrusive basalt, a dark-colored, fine-grained rock formed from lava flows that erupted from shield volcanoes. These lava flows spread across the landscape, gradually building the islands over millions of years.
• Pumice, an extrusive volcanic rock, is incredibly lightweight due to the numerous gas bubbles trapped within it during rapid cooling. This unique property allows it to float on water, making it a fascinating example of extrusive rock formation.
• Obsidian, a naturally occurring volcanic glass, is a striking example of extrusive rock. Its glassy texture and sharp edges, formed from the very rapid cooling of silica-rich lava, have made it a valuable material for tools and decorative objects.
• The eruption of Mount St. Helens in 1980 produced a vast amount of extrusive volcanic ash, which blanketed the surrounding areas. This ash, a fine-grained material, is a testament to the explosive nature of some extrusive events.