Igneous Rock
Igneous rock is formed by the cooling and solidification of magma (underground molten rock) or lava (magma that reaches the surface), making it one of the three fundamental rock types in Earth's crust. Intrusive igneous rocks such as granite solidify slowly deep underground, forming coarse-grained crystals, while extrusive igneous rocks such as basalt cool rapidly at the surface, forming fine-grained or glassy textures. Igneous rocks make up about 90% of Earth's crust by volume and are essential raw materials for construction, ceramics, and as indicators of volcanic history.
Common Igneous Rocks and Their Properties
| Rock Name | Formation Type | Grain Size | Composition | Common Use |
|---|---|---|---|---|
| Granite | Intrusive (plutonic) | Coarse (>1 mm) | Quartz, feldspar, mica | Construction, countertops |
| Basalt | Extrusive (volcanic) | Fine (<1 mm) | Pyroxene, olivine, feldspar | Road aggregate, oceanic crust |
| Obsidian | Extrusive (volcanic) | Glassy (amorphous) | Silica-rich glass | Tools (historical), ornamental |
| Pumice | Extrusive (volcanic) | Vesicular (porous) | Frothy silica glass | Abrasive, lightweight fill |
| Gabbro | Intrusive (plutonic) | Coarse (>1 mm) | Pyroxene, feldspar | Tiles, monuments |
| Rhyolite | Extrusive (volcanic) | Fine–glassy | Quartz, feldspar rich | Research, aggregate |
Interactive Tools
Wikimedia Commons, CC BY-SA
Related Terms
Rock Cycle
The rock cycle is a continuous geological process by which rocks are transformed from one type to another through processes such as melting, cooling, erosion, deposition, compaction, cementation, heat, and pressure over geological timescales. The three main rock types — igneous, sedimentary, and metamorphic — are interconnected in this cycle, each capable of being converted into another type under the right conditions. Understanding the rock cycle helps geologists interpret Earth's history, locate mineral resources, and predict geological hazards.
Volcano
A volcano is a geological feature where magma (molten rock) from Earth's mantle or crust reaches the surface through a vent or fissure, releasing lava, ash, gases, and pyroclastic material. Volcanoes form at tectonic plate boundaries where subduction drives magma upward, at divergent boundaries where plates separate, and over hotspots where mantle plumes create stationary magma sources. Volcanic activity plays a vital role in building continents, regulating Earth's atmosphere over geological timescales, and creating fertile soils.
Metamorphic Rock
Metamorphic rock is formed when existing igneous, sedimentary, or other metamorphic rocks are transformed by intense heat, pressure, or chemically active fluids deep within Earth's crust — a process called metamorphism — without the rock melting completely. The transformation alters the mineral composition, texture, and crystal structure of the original rock, often producing distinctive features such as foliation (banding or layering) in rocks like schist and gneiss. Metamorphic rocks are indicators of past geological events such as mountain building, and include economically valuable materials like marble, slate, and quartzite.
From Latin "igneus" (fiery, of fire), derived from "ignis" (fire). The term reflects the origin of these rocks from molten material. The classification of igneous rocks was systematized by geologists in the 19th century; the intrusive/extrusive distinction was formalized by German petrographer Harry Rosenbusch in the 1870s.