Basalt is a hard, black volcanic rock with less than 53 weight percent silica (SiO2). Because of basalt's low silica content, it has a low viscosity (resistance to flow). Therefore, basaltic lava flows can move quickly and often travel many kilometers from their vent. The low viscosity also aids gas bubbles in escaping without causing explosions. Common minerals in basalt include olivine, pyroxene, and plagioclase. Depending on how it is erupted, basalt can be hard and massive or crumbly and full of bubbles. Basalt is erupted at temperatures of 1100 to 1250° Centigrade.
Basalt, Etna, Sicily. Photo: J. Alean