extrusief

Extrusief, or extrusive in English, is a geological term describing igneous rocks that form when lava erupts onto the surface or erupts at very shallow depths, cooling rapidly to form fine-grained or glassy textures. By contrast, intrusief (intrusive) rocks crystallize below the surface. Extrusive rocks commonly have an aphanitic texture, with crystals too small to see without magnification; some are porphyritic, containing larger crystals in a fine groundmass; others are glassy, such as obsidian, from rapid quenching of lava. Vesicular textures occur when gas bubbles are preserved, as in pumice or scoria.

Typical extrusive rocks include basalt, andesite, rhyolite, and dacite, as well as volcanic glass (obsidian), pumice, and scoria. They originate from lava eruptions at volcanoes, fissures, or lava domes. Magma composition affects viscosity and eruption style, which in turn influences crystal size and texture. Eruptive products can be pyroclastic, forming tuffs, tephra layers, and volcanic ash deposits.

Geologists distinguish extrusive rocks by field observations and petrographic analysis. Radiometric dating can determine their ages. The Dutch term extrusief corresponds to extrusive in English, commonly used to describe rocks formed at or near the surface through volcanic activity.