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Basaltic andesite is a
Basaltic andesite is a fine-grained (
Basaltic andesite is found in
Basaltic andesite is a volcanic rock
Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...
that is intermediate in composition between basalt
Basalt (; ) is an aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the surface of a rocky planet or moon. More than 90 ...
and andesite
Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predo ...
. It is composed predominantly of augite and plagioclase
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more pro ...
. Basaltic andesite can be found in volcanoes around the world, including in Central America
Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...
and the Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
of South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
.
Description
Basaltic andesite is a fine-grained (aphanitic
Aphanites (adj. ''aphanitic''; ) are igneous rocks that are so fine-grained that their component mineral crystals are not visible to the naked eye (in contrast to phanerites, in which the crystals are visible to the unaided eye). This geolog ...
) igneous rock that is moderately low in silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is ...
and low in alkali metal oxide
The alkali metals react with oxygen to form several different compounds: suboxides, oxides, peroxides, sesquioxides, superoxides, and ozonides. They all react violently with water.
Alkali metal suboxides
* Hexarubidium monoxide (Rb6O) h
* No ...
s. It is not separately defined in the QAPF classification
A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition. The acronym QAPF stands for "Quartz, Alkali feldspar, Plagioclase, Feldspathoid (Foid)". These are the mineral groups used for ...
, which is based on the relative percentages of quartz
Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...
, alkali feldspar, plagioclase feldspar
Plagioclase is a series of tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continuous solid solution series, more prop ...
, and feldspathoid
The feldspathoids are a group of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are usually not found in rocks containing prim ...
s, but would fall in the basalt-andesite field. This corresponds to rock in which feldspathoid makes up less than 10% and quartz less than 20% of the total QAPF fraction, and in which at least 65% of the feldspar is plagioclase. Basaltic andesite would be further distinguished from basalt and andesite by a silica content between 52% and 57%.
Although classification by mineral content is preferred by the IUGS, this is impractical for glassy or very fine-grained volcanic rock, and then the chemical TAS classification
The TAS classification can be used to assign names to many common types of volcanic rocks based upon the relationships between the combined alkali content and the silica content. These chemical parameters are useful, because the relative proportio ...
is used. Basaltic andesite is then defined as volcanic rock with a silica content from 52% to 57% and a total alkali content ( K2O plus Na2O) of less than about 6%, corresponding to the O1 field in the TAS diagram.
A basaltic andesite (or andesite) enriched in magnesium
Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...
and depleted in titanium
Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...
, with more than 8% MgO and less than 0.5% TiO2, is termed a boninite.
Occurrence
Basaltic andesite is found in volcanic arc
A volcanic arc (also known as a magmatic arc) is a belt of volcanoes formed above a subducting oceanic tectonic plate,
with the belt arranged in an arc shape as seen from above. Volcanic arcs typically parallel an oceanic trench, with the arc lo ...
s, such as those of Central America
Central America ( es, América Central or ) is a subregion of the Americas. Its boundaries are defined as bordering the United States to the north, Colombia to the south, the Caribbean Sea to the east, and the Pacific Ocean to the west. ...
, the Andes
The Andes, Andes Mountains or Andean Mountains (; ) are the longest continental mountain range in the world, forming a continuous highland along the western edge of South America. The range is long, wide (widest between 18°S – 20°S ...
of South America
South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the sout ...
, and the Cascades of North America.
Basaltic andesite is also common where the Earth's crust is undergoing extension. For example, mid- Cenozoic volcanic sequences in western Mexico
Mexico (Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guatema ...
, southwestern New Mexico
)
, population_demonym = New Mexican ( es, Neomexicano, Neomejicano, Nuevo Mexicano)
, seat = Santa Fe
, LargestCity = Albuquerque
, LargestMetro = Tiguex
, OfficialLang = None
, Languages = English, Spanish ( New Mexican), Navajo, Ke ...
, and southeastern Arizona
Arizona ( ; nv, Hoozdo Hahoodzo ; ood, Alĭ ṣonak ) is a state in the Southwestern United States. It is the 6th largest and the 14th most populous of the 50 states. Its capital and largest city is Phoenix. Arizona is part of the Fou ...
are capped by basaltic andesite of the Southern Cordilleran Basaltic Andesite (SCORBA) suite, which may be the most extensive Cenozoic mafic
A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks incl ...
suite in North America. Mount Mazama
Mount Mazama (''Giiwas'' in the Native American language Klamath) is a complex volcano in the state of Oregon, United States, in a segment of the Cascade Volcanic Arc and Cascade Range. Most of the mountain collapsed following a major erupt ...
, a large composite volcano in south-central Oregon, consists of several overlapping basaltic andesite shields.
Basaltic andesite is a component of back-arc basin basalt suites. The Columbia River Basalt Group
The Columbia River Basalt Group is the youngest, smallest and one of the best-preserved continental flood basalt province on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The basalt grou ...
is composed of 80% basaltic andesites on the basis of silicon content.
Thick sequences of basaltic andesites were erupted in a shallow-marine environment during the Paleoproterozoic
The Paleoproterozoic Era (;, also spelled Palaeoproterozoic), spanning the time period from (2.5–1.6 Ga), is the first of the three sub-divisions ( eras) of the Proterozoic Eon. The Paleoproterozoic is also the longest era of the Earth's ...
as melts formed in the anomalous Kaapval craton lithosphere.
The Mars Global Surveyor
''Mars Global Surveyor'' (MGS) was an American robotic space probe developed by NASA's Jet Propulsion Laboratory and launched November 1996. MGS was a global mapping mission that examined the entire planet, from the ionosphere down through t ...
has found evidence for basaltic andesite on Mars
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, only being larger than Mercury. In the English language, Mars is named for the Roman god of war. Mars is a terrestrial planet with a thin at ...
.
References
{{Volcanoes Volcanic rocks Intermediate rocks