Andesite Facts

Andesite Facts
Andesite is an igneous rock that is usually light to dark gray in color. Weathering may cause the color to change to various shades of brown. It is typically found in lava flows produced by stratovolcanoes, which is a composite, conical volcano built up by many layers of hardened lava, tephra, pumice, and volcanic ash. The lava cools rapidly at the surface and are usually composed of small crystals, and are too small to be seen unless one uses a magnifying device. Some of the andesite specimens that cool rapidly may contain a large amount of glass, and others that formed from gas-changed lavas have a vesicular or amygdaloidal texture.
Interesting Andesite Facts:
Quartz and olivine are not usually part of andesite. However, it is rich in plagioclase feldspar minerals, and may include biotite, pyroxene, or amphibole.
Andesite, along with diorite, are commonly found in the continental crust above subduction zones formed after an oceanic plate melts producing a source of magma.
It is a fine-grained igneous rock that forms when the magma erupts onto the surface and then crystalizes quickly.
The composition of andesite is intermediate between basalt and granite. The granite comes from the melting rocks as it is mixed with granitic magma.
The source of the rock's name, andesite, comes from the Andes Mountains of South America as lava flows mixed in with ash and tuff deposits on the flanks of stratovolcanoes.
These type of stratovolcanoes can be found above subduction zones in many locations including Central America, Mexico, Oregon, and Washington, as well as the Aleutian Arc, Indonesia, Japan, the Philippines, the Caribbean, New Zealand, and other places.
Andesite can also form at ocean ridges and hot spots in the ocean from the partial melting of basaltic rocks. It can form during eruptions at continental plate interiors and many other environments.
Sometimes, andesite will contain grains of plagioclase, amphibole, or pyroxene, which are large crystals known as phenocrysts, which begin forming when magma is at cooling depths. This causes the andesite porphyry to grow to visible sizes before the magma erupts.
Andesite porphyry is used as the name for the rocks with the two crystal sizes. Large crystals that formed slowly at depth and small crystals formed quickly at the surface.
When gas-charged andesitic magmas erupt it, they cause many volcanic plumes and ash eruptions. The pressure of the gas causes the eruptions, and then blows large amounts of tiny rock and magma particles into the air and carried long distances by the wind. They may also cause problems for aircraft downwind from the volcano.
Catastrophic eruptions produced by andesitic magmas include Mount St. Helens, Pinatubo, Redoubt, and Novarupta, which resulted in large amounts of dissolved gas under pressure.
Sometimes the formal definition of andesite is problematic since many authors are in perfect agreement to their chemical and mineralogical compositions.
The classification of an igneous rock like andesite may not be precise at first glance, since they may require closer examination and require chemical or mineralogical analysis, which may not be readily available, practical, or affordable.
If a person is not sure a rock is andesite, and only appears as andesite, it may be called an "andesitoid" rock, but could later be proven wrong using closer microscopic examination or chemical testing.

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