Geology And Geophysics

What is Basalt



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Basalt is an ignaceous rock that is formed after flowing lava has cooled quickly.  It is this cooling process that gives basalt a fine grain appearance.  This greyish-black rock is found on Earth, the Moon, Mars, Venus, and the asteroid Vesta.  "Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface."

Basalt is produced on Earth in three similar environments:  oceanic divergent boundaries, oceanic hotspots, and mantle plumes and hotspots beneath the continents.

Oceanic Divergent Boundaries

"Most of Earth's basalt is produced at divergent plate boundaries on the mid-ocean ridge system.  Here convection currents deliver hot rock from deep in the mantle.  This hot rock melts as the divergent boundary pulls apart and erupts onto the sea floor.  these submarine fissure eeruptions often produce pillow basalts."

Oceanic Hotspots

"Basalt production at these locations begins with an  eruption on the ocean floor.  If the hotspot is sustained, repeated eruptions can build the vocanic cone larger and larger until it becomes high enough to become an island.  All of the islands in the Hawaiian island chain were built up from basalt eruptions on the sea floor."

Plumes and Hotspots Below Continents

"The third baslat-forming environment is a continental environmental where a mantle plume or hotspot delivers enormous amounts of basaltic lava through the continental crust and up to Earth's surface.  These eruptions can be from either vents or fissures.  They have produced the largest basalt flows on the continents.  The eruptions can occur repeatedly over millions of years producing layer after layer of basalt stacked in a vertical sequence."

Like the Hawaiian islands, basalt can been seen in quite a number of locations.  The Columbia River Flood Basalts in Washington, Oregon, and Idaho have extensive flood basalts on the land.  Also in America are the Keweenawan Lavas of the Lake Superior region.  Away from this continent, basalts have arisen in the Emeishan Traps of China, the deccan Traps of India, the Etendeka Basalts of Namibia, the Karroo Basalts of South Africa, and the Siberian Traps of Russia.  By definition, a "trap" is the outcrop profile of these types of layered basalt deposits. 

On other worlds, basalt is rather abundant on the Moon.  In fact, much of it is underlain by basaltic lava flows and flood basalts, also known as lunar maria.  Mars has a shield volcano (Olympus Mars) that, like other volcanic features there, was formed by basaltic lava flows.  Olympus Mars is the "highest mountain on Mars and is the largest known volcano in our solar system."

Other than being just an interesting looking rock, basalt has served it's purpose in a number of functions.  It has been mostly used crushed up for use as an aggregate in construction, as a road base, a concrete aggregate, an asphalt pavement aggregate, a railroad ballast, and a filter stone in drain fields.  It has been cut up for dimension stones.  Thin slabs of it have been sliced and polished to be used as floor tiles, building veneers, and monuments.

*All quotations are taken from www.geology.com.*

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