Extrusive igneous rocks form when magma rises to or erupts at the Earth’s surface, then cools and solidifies rapidly, producing fine-grained or glassy textures. This contrasts with intrusive rocks, which crystallize below the surface more slowly and tend to be coarser. Key points

• Source of magma: Most extrusive rocks originate from magma generated in the mantle or lower crust, which migrates upward through the crust until it erupts (volcanoes) or reaches near the surface (lava flows) [extrusive: rapid cooling on or near the surface].

• Cooling rate and texture: Rapid cooling results in small crystals or a glassy texture; slower cooling yields somewhat larger crystals but still finer than most intrusive rocks. Obsidian is a classic glassy extrusive rock, while rhyolite, andesite, basalt, and andesite porphyry are fine-grained volcanic rocks common at the surface.

• Common processes: Extrusive rocks can form via lava flows that spread across the surface, explosive eruptions that eject volcanic ash and pumice, or shallow-subvolcanic activity where magma erupts and cools quickly near the surface.

• Examples: Obsidian (glassy extrusive), rhyolite (fine-grained felsic), and basalt (fine-grained mafic) are typical extrusive rocks; volcanic ash and pumice are also extrusive products of rapid cooling after eruption.

• Texture details: Because crystals have little time to grow, mineral grains in extrusive rocks are usually microscopic or glassy, though some may show vesicles from gas bubbles trapped during rapid solidification.

If you’d like, I can tailor this to a specific grade level or provide a quick comparison table of extrusive vs intrusive rocks with example minerals and textures.