​ Basalt is a hard, gray to black extrusive volcanic rock, mafic (rich in iron and magnesium), fine-grained, sometimes displays frozen gas bubbles (vesicular texture), and sometimes contains phenocrysts of feldspar (Ca-rich varieties), hornblende, pyroxene, biotite mica, sometimes quartz or olivine.

Low Viscosity:
Because of it's low silica content (less than about 52 weight percent silica (SiO2). ), basalt has a low viscosity (resistance to flow). Therefore, basaltic lava can flow quickly and easily move more than 20 kilometers from a vent. The low viscosity typically allows volcanic gases to escape without generating enormous eruption columns. Basaltic lava fountains and fissure eruptions, however, still form explosive fountains hundreds of meters tall.

Common Minerals:
Common minerals in basalt include olivine, pyroxene, and plagioclase.

Eruption Temperatures:
Basalt is erupted at temperatures between 1,100 to 1,250 degrees C. (About 2100 degrees F)

Columnar Basalt:
As basalt lava erupts from volcanic vents and cools, it shrinks and cracks. Sometimes vertical columns form. Well developed columns result from homogeneous lava cooling at a uniform rate.  

Flood Basalts:
In some shield-volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.  

Ocean Floors and Earth's Crust:
Most of the ocean floor is made up of basalt and it is the most abundant volcanic rock in the Earth's crust.