The earth's crust in these areas is thinned, stretched, and eventually it breaks along long fractures called faults. As the plates pull apart a rift valley begins to form. As the Earth's crust is thinned, pressure is released on the underlying mantle. This release of pressure causes part of the mantle material to melt. Partial melting results in the generation of large amounts of basaltic magma, which then rises up to the surface and erupts as basaltic lava.
Basaltic lava has a low viscosity and often flows for long distances from the eruption site.
One example of this is the Laki Lava which erupted in 1773 and flowed up to 130 km from its source in the Laki Gigur area . This flow eventually covered an area of about 900 square kilometers. Basalt is also very common as dikes and sills . These are bodies of basalt that cooled near the surface in the lava conduits that once fed the lava flow. Rapid cooling of basalt lava forms a dark glassy rock.