*Magma which has flowed over the earth's surface. The viscosity of lava depends on its silica content, pressure, and temperature. Temperature is the most important factor. Basic lavas have a low silica content and flow freely (Arculus (1976) GSA Bull. 87, 4); acid lavas are more viscous (Wilson and Grant (2006) GSA Spec. Paper 405). See extrusion.
Subjects: Earth Sciences and Geography.