1. adj. 
Pertaining to one of three main classes of rocks (igneous, metamorphic and sedimentary). Igneous rocks crystallize from molten rock, or magma, with interlocking mineral crystals. Igneous rocks that crystallize slowly, typically below the surface of the Earth, are plutonic igneous rocks and have large crystals (large enough to see with the naked eye). Volcanic igneous rocks crystallize quickly at the Earth's surface and have small crystals (usually too small to see without magnification). Common examples include granite (plutonic) and rhyolite (volcanic), diorite (plutonic) and andesite (volcanic), and gabbro (plutonic) and basalt (volcanic). Igneous rocks typically comprise the minerals quartz, mica, feldspar, amphibole, pyroxene and olivine.
See: basement, diapir, dike, felsic, geomagnetic polarity reversal, groundwater, hard rock, hydrothermal, hydrothermal alteration, mafic, mantle, marker bed, massif, midoceanic ridge, montmorillonite, nonconformity, orogeny, plate tectonics, reservoir, sedimentary, soft rock, unconformity, vesicle, vesicular porosity, volcano