File:Basaltic lava flow top 1.jpg

Igneous rocks form by the cooling and 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), and calcium (Ca), and are dominated by the minerals plagioclase feldspar, pyroxene, and sometimes olivine (this example has a greenish olivine component).

Basalts have an aphanitic texture (finely crystalline - all or almost all crystals are less than 1 millimeter in size each), but some are porphyritic or coarsely crystalline if the lava flow took time to cool and solidify (e.g., some Proterozoic basalt lavas in the Keweenaw Peninsula of Michigan's Upper Peninsula).

The sample seen here is a vesicular basalt (best seen in cross-section), an intermediate lithology between basalt and scoria. Scorias are basaltic rocks with numerous small gas bubbles. Vesicular basalts are basaltic rocks having relatively few, relatively large gas bubbles. Vesicular basalt is more dense than scoria (some scorias will float on water; vesicular basalts won’t do that).

This specimen is from the top surface of a lava flow - the linear features were caused by stretching of cooling lava in the direction of flow.