Basaltic andesite is an extrusive igneous rock that is compositionally intermediate between basalt and andesite. It represents a common rock type found in arc volcanism and is part of the continuous spectrum of volcanic rocks from mafic to felsic compositions.
Composition and Classification: Basaltic andesite typically contains 53–57% silicon dioxide (SiO₂) by weight, placing it above basalt (typically <52% SiO₂) and below andesite (typically 57–63% SiO₂). Mineralogically, it is characterized by the presence of both calcic plagioclase feldspar and pyroxene (orthopyroxene and/or clinopyroxene), often with minor amounts of olivine or amphibole. Quartz is generally absent or present only in trace amounts. The specific classification often depends on the total alkali content relative to silica, as defined by the TAS (Total Alkali vs. Silica) diagram.
Characteristics:
- Color: Often dark gray to black, reflecting its relatively high content of mafic minerals (iron and magnesium-rich minerals) compared to more felsic rocks.
- Texture: Typically aphanitic (fine-grained) due to rapid cooling at the Earth's surface. Porphyritic textures, where larger crystals (phenocrysts) are embedded in a finer-grained groundmass, are common. Phenocrysts frequently include plagioclase, pyroxene, and sometimes olivine or hornblende.
- Eruptive Style: Volcanic eruptions producing basaltic andesite can range from effusive (lava flows) to moderately explosive, depending on gas content and viscosity. Its viscosity is higher than basalt but lower than more silicic andesite or dacite.
Formation and Occurrence: Basaltic andesites are most commonly found in volcanic arcs associated with subduction zones, both in oceanic island arcs and continental margins. They are believed to form through several processes, often in combination:
- Fractional Crystallization: Basaltic magmas originating from the mantle can undergo fractional crystallization as they ascend through the crust, removing early-formed mafic minerals and thus enriching the remaining melt in silica.
- Magma Mixing: Mixing of more mafic (basaltic) and more felsic (andesitic/dacitic) magmas can produce intermediate compositions.
- Assimilation: Basaltic magmas can assimilate surrounding crustal rocks as they pass through, leading to an increase in silica content.
- Partial Melting: In some cases, partial melting of previously hydrated mantle or subducted oceanic crust can directly produce basaltic andesite melts.
Examples: Basaltic andesite is a common rock type in many active volcanic arcs globally, including the Andes Mountains, the Cascade Range (North America), the Japanese archipelago, and various Pacific island arcs.