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Basalt on Moon and Mars
Basalt is also an abundant rock on the Moon. Much of the Moon’s surface is underlain by basaltic lava flows and flood basalts. These areas of the Moon are known as “lunar maria.” Large areas of the Moon have been resurfaced by extensive basalt flows which may have been triggered by major impact events. The…
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Earth’s Most Abundant Bedrock
Basalt underlies more of Earth’s surface than any other rock type. Most areas within Earth’s ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth’s land surface. Basalt is a very important rock.
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What is Basalt?
Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is…
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The Elusive Definition of Andesite
The formal definition of andesite is problematic. Many authors have classified igneous rocks based upon their chemical and mineralogical compositions. However, none of these classifications are in perfect agreement. For a fine-grained rock like andesite, these classifications are impossible to use precisely when in the field or the classroom. They require chemical or mineralogical analyses…
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Dissolved Gas and Explosive Eruptions
Some magmas that produce explosive eruptions above subduction zones contain enormous amounts of dissolved gas. These magmas can contain several percent dissolved gas by weight! This gas can have several origins, examples of which include the following: At depth, these gases can be dissolved in the magma like carbon dioxide dissolved in a can of cold beer. If that…
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Andesite Porphyry
Occasionally, andesites contain large, visible grains of plagioclase, amphibole, or pyroxene. These large crystals are known as “phenocrysts.” They begin forming when a magma, which is cooling at depth, approaches the crystallization temperature of some of its minerals. These high-crystallization-temperature minerals begin forming below the surface and grow to visible sizes before the magma erupts.…
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The Geography of Andesite
Andesite derives its name from the Andes Mountains of South America. In the Andes it occurs as lava flows interbedded with ash and tuff deposits on the steep flanks of stratovolcanoes. Andesite stratovolcanoes are also found above subduction zones in Central America, Mexico, Washington, Oregon, the Aleutian Arc of Alaska, Japan, Indonesia, the Philippines, the Caribbean, and New Zealand, among other locations. Andesite can also form away from the subduction…
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Where Does Andesite Form?
Andesite and diorite are common rocks of the continental crust above subduction zones. They generally form after an oceanic plate melts during its descent into the subduction zone to produce a source of magma. Diorite is a coarse-grained igneous rock that forms when the magma remains below Earth’s surface and cools slowly. Slow cooling facilitates the growth of large…
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Mineral Composition
Andesite often weathers to various shades of brown. In the field, weathered specimens must be broken to properly see their color and their mineral composition. Classroom specimens usually do not require breakage. Andesite is rich in plagioclase feldspar and amphibole minerals. Quartz and pyroxene minerals may be absent or present in small quantities. Small amounts of mica will be present as biotite or muscovite. Andesite usually does…
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An Extrusive Igneous Rock
Andesite is typically found in lava flows produced by stratovolcanoes above subduction zones. Because these lavas cooled rapidly at the surface, they are generally composed of small crystals. The mineral grains are usually so small that they cannot be seen clearly without the use of a hand lens or other magnifying device. Some specimens that cooled rapidly contain a…