Tholeiite (or Tholeiitic basalt) is a type of basalt rock that is olivine-poor, and dominated by clinopyroxene, plagioclase, and iron ore.[1] Tholeiites are formed from tholeiitic magma and are the most common volcanic rocks on Earth by virtue of submarine volcanism, as contrasted to alkali olivine basalts.(AGI, 41),[2] Tholeiitic rocks are usually characterized by a fine, glassy groundmass.(AGI, 41) Basalt Basalt is a common gray to black volcanic rock. ... Olivine The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). ... Figure 1:Mantle-peridotite xenolith with green peridot olivine and black pyroxene crystals from San Carlos Indian Reservation, Gila Co. ... Lunar Ferroan Anorthosite #60025 (Plagioclase Feldspar). ... General Name, Symbol, Number iron, Fe, 26 Chemical series transition metals Group, Period, Block 8, 4, d Appearance lustrous metallic with a grayish tinge Atomic mass 55. ... Magma is molten rock located beneath the surface of the Earth (or any other rocky planet), and which often collects in a magma chamber. ... A volcano is a geological landform usually generated by the eruption through a planets surface of magma, molten rock welling up from the planets interior. ... Earth (often referred to as The Earth) is the third planet in the solar system in terms of distance from the Sun, and the fifth in order of size. ... Submarine volcanoes and volcanic vents are common features on certain zones of the ocean floor. ... The matrix or groundmass of an igneous rock consists of fine grained often microscopic crystals in which larger crystals (phenocrysts) are embedded. ...

References:

• "Polarized Light Microscopy Digital Image Gallery: Tholeiitic Basalt" (Accessed 4/1/06)

• American Geological Institute. Dictionary of Geological Terms. New York: Dolphin Books, 1962.