Basaltic Flows

THE WORD basalt is said to have an Ethiopian origin, meaning “black stone.” Basalt is a dark gray to black, dense to finely grained igneous rock that is the result of lava eruptions. Basalts are not only the most abundant lavas, but they are also the most voluminous. Basalt, when exposed to the air, becomes covered with a brown crust, consisting largely of oxides of iron. The continental masses are mainly built up of granite material, but at some places they are injected with or penetrated by material of greater density from the layers below. These lower layers may be the source of the great basaltic extrusions on the Earth’s surface. Basalt flows are noneruptive, voluminous, and characterized by relatively low viscosity. They generally advance less than one mile or kilometer per hour on gentle slopes and may reach more than 30 mi (50 km) from their source or erupting vent.

A lava flow is defined as the product of a single eruption that may be divisible into one or more flow units, which represents a single pulse or surge of lava that flowed away from the eruptive vent and then cooled separately. The length of the lava flow is determined largely by the magma effusion rate. Basalt lava flows that originate from fissures spread for distances that are roughly proportional to the third power of their thickness.

These lava flows are called flood basalts because a large volume of lava erupted in a short period of time. Each flood of lava has its own unique chemical composition. The dark area that forms part of the moon’s face is a flood basalt eruption. Flood basalt regions exist on every continent. In plains basalt provinces such as Snake River and ICELAND flows are much less than 1 cubic mi or km. Basaltic lava flows in HAWAII extend for more than 21 mi (35 km) with an average thickness of 16 ft (5 m). Andesite flows have higher viscosity and few extend more than 9 mi (15 km). One andesite flow of Pleistocene age in the Cascade range is 50 mi (80 km) long. One Icelandic basaltic flow reached 93 mi (150 km).