extrusive

Extrusive refers to igneous rocks formed from magma that erupts onto the Earth's surface or intrudes only briefly before cooling. Rapid cooling at or near the surface produces fine-grained textures, where individual crystals are often too small to see (aphanitic). If cooling is extremely fast, the rock may become glassy, as in obsidian. Many eruptions also yield vesicular rocks, such as pumice and scoria, where gas bubbles are preserved as pores.

Extrusive rocks originate from several surface processes, including lava flows that spread across the landscape, lava domes that push upward, and pyroclastic deposits composed of ash, pumice, and volcanic bombs. They span a wide range of chemical compositions from mafic to felsic. Common examples include basalt and andesite (mafic to intermediate), rhyolite and rhyodacite (felsic), as well as obsidian, pumice, and tuff.

Textures in extrusive rocks reflect cooling history. Some rocks are porphyritic, containing larger phenocrysts crystallized during a slower initial cooling, embedded in a finer-grained groundmass that formed during rapid final cooling. The extrusive setting contrasts with intrusive (plutonic) rocks, which crystallize slowly underground to form coarse-grained textures such as granite, diorite, and gabbro.

Studying extrusive rocks helps geologists infer past volcanic activity, magma evolution, and plate tectonic processes, including the distribution of volcanic arcs, hot spots, and spreading centers.