Igneous rock
Igneous rock forms when molten rock cools and solidifies. It includes intrusive rocks that crystallize below the surface, extrusive volcanic rocks that cool at or near the surface, and many textures that record cooling rate, gas content, mineral composition, and eruption or intrusion history.
What igneous rock is
Igneous rock is one of the three broad rock groups, along with sedimentary and metamorphic rock. It forms from molten rock: magma below the surface or lava after eruption. Once molten material cools enough, minerals crystallize or glass forms, locking in clues about where and how the rock formed.
Magma, lava, and cooling
Magma is molten or partly molten rock below Earthโ€s surface. When magma erupts, it is called lava. Cooling rate matters. Slow underground cooling gives crystals time to grow, while rapid cooling at the surface can make tiny crystals, volcanic glass, or frothy rock full of gas bubbles.
Intrusive igneous rocks
Intrusive, or plutonic, igneous rocks crystallize below the surface inside magma chambers, dikes, sills, batholiths, and other bodies. Because they cool slowly, they often have visible interlocking crystals. Granite, diorite, and gabbro are common intrusive examples. Erosion may later expose them at the surface.
Extrusive igneous rocks
Extrusive, or volcanic, igneous rocks form when lava or volcanic fragments cool at or near the surface. Basalt, rhyolite, andesite, obsidian, pumice, scoria, and volcanic ash are examples. Some form from flowing lava, while others form from explosive eruptions that throw ash and rock fragments into the air.
Composition and minerals
Igneous rocks are often described by silica content and mineral composition. Mafic rocks such as basalt and gabbro are rich in iron and magnesium minerals. Felsic rocks such as rhyolite and granite contain more silica-rich minerals such as quartz and feldspar. Intermediate rocks fall between these end members.
Textures as evidence
Texture records the physical history of cooling and eruption. Aphanitic rocks have crystals too small to see easily. Phaneritic rocks have visible crystals. Porphyritic rocks contain large crystals set in a finer groundmass, showing two cooling stages. Vesicular rocks preserve gas bubbles, and glassy rocks cooled too quickly for many crystals to form.
Plate tectonic settings
Igneous rocks form in several tectonic settings. Mid-ocean ridges produce basaltic ocean crust. Subduction zones generate volcanic arcs and many intermediate to felsic magmas. Hot spots can build volcanic islands and flood basalts. Continental rifts and intrusions create their own igneous records of extension, heat, and magma movement.
Why it matters
Igneous rocks matter because they build ocean crust, volcanoes, plutons, lava fields, and parts of continents. They record mantle melting, eruptions, plate boundaries, magma chemistry, and cooling histories. They also host mineral resources and help geologists date events because many igneous minerals can be used in radiometric dating.