Olivine porphyritic basalt from the Pleistocene of California, USA. (4.0 cm across at its widest)

Igneous rocks form by the cooling & crystallization of hot, molten rock (magma & lava). If this happens at or near the land surface, or on the seafloor, they are extrusive igneous rocks. If this happens deep underground, they are intrusive igneous rocks. Most igneous rocks have a crystalline texture, but some are clastic, vesicular, frothy, or glassy.

Basalt is a common extrusive igneous rock. Basalt is the dominant rock in Earth’s upper oceanic crust. Black lava rocks seen at famous volcanoes such as Mt. Kilauea in Hawaii and Mt. Etna in Sicily are composed of basalt. Basalt has a mafic chemistry. Mafic igneous rocks are generally dark-colored, have 45-52% silica (= SiO2 chemistry) (mafic has also been defined as 45 to 55% silica), are rich in iron (Fe), magnesium (Mg), & calcium (Ca), and are dominated by the minerals plagioclase feldspar and pyroxene.

Many basalts have an aphanitic texture (finely crystalline; all or almost all crystals are <1 mm in size). Porphyritic basalt (see above photo) has the same chemistry & mineralogy as ordinary basalt, but differs in the size range of its crystals. Porphyritic basalts have a mix of large & small crystals. Large crystals in a porphyritic rock are called phenocrysts. The smaller crystals make up the groundmass.

The above sample is an olivine porphyritic basalt. The greenish-yellow crystals are olivine phenocrysts ((Mg,Fe)2SiO4). The dark-colored material is plagioclase feldspar and pyroxene. This rock apparently comes from a Pleistocene lava flow (likely "Qb" of Stinson, 1977 - Geologic Map of the Keeler 15' Quadrangle, Inyo County, California).