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The Rock - Basalt

  • Igneous Rock Type: Extrusive volcanic
  • Related to: Scoria, gabbro, diorite
  • Chemistry:  Basic
  • Color: Black, dark gray, greenish black
  • Texture: Aphanitic (crystals too small to see) to porphyritic (a mixture of crystal sizes)
  • Origins: Crustal extension zones such as mid oceanic ridges, hot spot volcanic regions, linear lava flows, lunar mare and martian volcanic regions 
  • Common Minerals: Calcium rich plagioclase feldspars, pyroxenes and olivine
  • Accessory Minerals: Ilmenite, magnetite, biotite, phlogopite and feldspathoids
  • Uses: Decorative stones, lava rocks for grills and ovens
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Basalt is a very common igneous rock. In fact it is the most common rock in the Earth's crust. Almost all oceanic crust is made of basalt and basalt is a common extrusion from many volcanic regions around the world. It forms from the melting of the upper mantle and its chemistry closely resembles the upper mantle's composition. It is generally silica poor and iron and magnesium rich. Basalt originates from "hot spot" volcanoes, massive basalt flows and mid oceanic ridges.

Hot spots around the world have generated large basaltic volcanoes. A hot spot is an unusual upwelling of a deep magma plume generally in the center of tectonic plates. These hot spots can form the largest volcanoes in existence called shield volcanoes. The Hawaiian Islands and Galápagos Islands are all essentially large volcanoes. Shield volcanoes tower over other volcanoes in height and dwarf other volcanoes in width. In fact one of the Hawaiian volcanoes, named Mauna Kea, from its base to its peak is the tallest mountain in the world (Mt Everest is the highest mountain). The largest mountain in the solar system is a basaltic shield volcano named Olympus Mons on the planet Mars.

Basalt also comes from extensive lava flows. Basalt lava flows are not very viscous (a measure of resistance to flow) and so can flow easily and quickly across great distances delivering great volumes of basaltic rock. Some of these extrusions covered huge areas of the Earth and there is still potential for an eruption of this kind to happen again. Basalt lava flows have covered over a million and a half square kilometers of Siberia called the Siberian Trapps. The 65 million year old Deccan Trapps in India, a source of wonderful zeolite specimens, may have originally been just as large and its eruptions may have contributed to the extinction event that killed off the dinosaurs. The Columbia River Basalt Group is another huge basalt flow that could be over 6,000 feet thick. It covers large areas of Washington, Oregon and Idaho. Its soils contribute to the healthy potato crop from this region. Recent historic basalt flows have occurred on Iceland. The mara (black areas) on the moon are made of basalt and probably formed from basalt lava flows. And as might be expected, there also are large basalt flows on Mars.

Basalt makes up most of the oceanic crust and is formed at the plate boundaries as they are pulling apart. The melting of the mantle below the crust as it is spreading apart is call decompressional melting. It is the loss of pressure that causes the melting instead of an increase in temperature. The result is liquid basaltic magma rising and cooling on the edge of spreading tectonic plates forming prominent ridges.  As the plates spread, more basalt attaches to the plate. Dating of oceanic rock shows that newer rock exists closer to the mid oceanic ridges as one would expect. Because basalt has magnetite as a typical component, when the basalt solidifies the magnetite crystals will be locked into place according to their orientation to the Earth's magnetic field. Since the Earth's magnetic field reverses itself from time to time, these magnetic reversals are recorded in the basaltic rocks.

There are several types of basaltic rocks that are characterized by their features. Basalt that has a lot of volatile gases can have numerous vesicles or openings and is called scoria. Basaltic rock that cools slowly in a large thick layer will often form a distinctive hexagonal prismatic columnar jointing that produces "columnar basalt". Basalt that forms from magma that extrudes into ocean water forms a type of basalt called "pillow lava" or "pillow basalt".  

If basaltic magma does not solidify on the surface of the Earth, but cools in its interior it forms an intrusive igneous rock called gabbro.  Although Basalt is recognized as an extrusive igneous rock, sometimes very fine grained intrusive rock with basalt's composition is referred to as a basalt.