Basalt from the Jurassic of New Jersey, USA. (public display, Geology Department, Wittenberg University, Springfield, Ohio, USA)

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. It 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. This refers to igneous rocks that are generally dark-colored, have 45-55% silica (= SiO2 chemistry) (it has also been defined as 45 to 52% silica), are rich in iron (Fe), magnesium (Mg), & calcium (Ca), and are dominated by the minerals plagioclase feldspar and pyroxene.

Basalts have an aphanitic texture (finely crystalline; all or almost all crystals are <1 mm in size), the result of relatively quick cooling of lava. Some basalts are porphyritic or coarsely crystalline if the lava flow took time to cool & solidify (e.g., some Proterozoic basalt lavas in the Keweenaw Peninsula of Michigan's Upper Peninsula).

The sample shown here consists of numerous pieces of "spatter" - blebs of basalt lava spewed from an erupting volcanic vent. Some small volcanoes, called "spatter cones", are entirely composed of this material.