Andesite: A Volcanic Rock of Intermediate Composition
Andesite is a type of igneous rock that forms from the cooling and crystallization of magma that erupts onto the surface of the Earth. It is usually light to dark gray in color and has a mineral composition that is intermediate between granite and basalt. Andesite is commonly found in volcanoes that occur above subduction zones, where an oceanic plate sinks beneath a continental plate and melts to produce magma. Andesite is named after the Andes mountain range, where this rock type is abundant.
Andesite is characterized by its fine-grained (aphanitic) texture, which means that the individual mineral grains are too small to be seen with the naked eye. The most common minerals in andesite are plagioclase feldspar and amphibole, with minor amounts of quartz, pyroxene, mica, or olivine. Some andesite may contain glass or vesicles (holes) that result from the rapid cooling or gas escape of the lava. Andesite may also have a porphyritic texture, which means that it contains some larger crystals (phenocrysts) embedded in a finer matrix.
Andesite is an important rock type because it indicates the presence of a subduction zone and the potential for volcanic activity. Andesite is also a major component of the continental crust, as it represents the intermediate stage between basaltic and granitic magmas. Andesite can produce explosive eruptions that generate pyroclastic flows, ash clouds, and lahars (mudflows). Some examples of andesitic volcanoes are Mount St. Helens in Washington, Mount Fuji in Japan, and Cotopaxi in Ecuador.
Formation of Andesite
Andesite forms when a basaltic magma undergoes partial crystallization and fractionation in the crust. The first minerals that crystallize and separate from the magma are olivine and amphibole, which are rich in iron and magnesium. These minerals form mafic cumulates that sink to the bottom of the magma chamber. The remaining magma becomes more enriched in silica and sodium, and less in iron and magnesium. This magma can then erupt onto the surface as andesite lava flows or tephra, or intrude into the crust as diorite dikes or sills.
The source of the basaltic magma that produces andesite is usually an oceanic plate that subducts beneath a continental plate. As the oceanic plate descends into the mantle, it releases water and other volatiles that lower the melting point of the overlying mantle wedge. This causes partial melting of the mantle and the formation of basaltic magma that rises towards the crust. The basaltic magma can then interact with the continental crust, assimilating some of its material and changing its composition. Alternatively, the basaltic magma can mix with more silicic magmas that are already present in the crust, producing intermediate magmas that can erupt as andesite.
Geography of Andesite
Andesite is a common rock type in volcanic arcs, where it occurs along with other intermediate to felsic rocks such as dacite and rhyolite. Andesite is also found in oceanic islands that have a history of subduction, such as Japan, Indonesia, New Zealand, and the Caribbean. Andesite can also form in continental rift zones, where extensional forces cause upwelling of asthenospheric mantle and partial melting of the lower crust.
Andesite is named after the Andes Mountains in South America, where it is a dominant rock type of the volcanic arc that resulted from the subduction of the Nazca Plate beneath the South American Plate. The Andes are home to some of the largest and most active volcanoes in the world, such as Cotopaxi, Llaima, Villarrica, and Nevado del Ruiz. These volcanoes produce andesitic lava flows, pyroclastic flows, lahars, and ash fall that pose hazards to nearby populations and ecosystems.