Basalt (pronounced: bass-alt)

Iceland is the only place in the world where a mid-oceanic ridge is exposed above sea level. Because of this, Iceland is an unusual place where we can see many types of basalt, a rock that makes up most of the world's ocean floor. More than ninety percent of all rocks in Iceland are some form of basalt.

Basalt is a fine-grained, extrusive igneous rock with the same minerals as its intrusive equivalent, gabbro. Basalt is either aphanitic and black, or grey. Many basalts contain phenocrysts. This rock type is called a porphyritic basalt.

Basalt is formed in areas where tectonic plates move apart (diverge). 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. Basaltic lava flows are usually very thin (less than 10-30 m thick) and solidify as either as a ropey pahoehoe lava or as a blocky aa lava. 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. This occurs at contacts such as along the edges of dikes or in pillow lavas. Lava bombs are also glassy because they cool quickly while falling through the air after being blown out of a basaltic volcano. Explosive marine or subglacial eruptions as well as magma fountain activity lead to large quantities of very dark and glassy pumice and ash.

Information provided by Patrick M. Colgan, Northeastern University
http://www.casdn.neu.edu