Which magma is andesite formed from

Glossary - Andesite. Andesite is a gray to black volcanic rock with between about 52 and 63 weight percent silica SiO2. Andesites contain crystals composed primarily of plagioclase feldspar and one or more of the minerals pyroxene clinopyroxene and orthopyroxene and lesser amounts of hornblende. At the lower end of the silica range, andesite lava may also contain olivine.

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Skip to content Study with The Open University. Search for free courses, interactives, videos and more! Free Learning from The Open University. Featured content. Free courses. Small amounts of mica will be present as biotite or muscovite. Andesite usually does not contain olivine. The accompanying chart titled "Generalized Composition Ranges of Common Igneous Rocks" illustrates the mineral composition of andesite. Stratovolcanoes: Pavlof Volcano right and Pavlof Sister Volcano left are a pair of symmetrical stratovolcanoes built of andesite flows and tephra on the Alaska Peninsula.

Pavlof Volcano is one of the most active volcanoes in Alaska. Photo by T. Miller, United States Geological Survey. 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 mineral crystals in the rock. Andesite is a fine-grained rock that forms when the magma erupts onto the surface and crystallizes quickly. Andesite and diorite have a composition that is intermediate between basalt and granite.

This is because their parent magmas formed from the partial melting of a basaltic oceanic plate. This magma may have received a granitic contribution by melting granitic rocks as it ascended or mixed with granitic magma. Pavlof Volcano - plate tectonics: Simplified plate tectonics cross-section showing how Pavlof Volcano is located in the Aleutian Island chain, above a subduction zone, where basaltic crust of the Pacific Plate is being partially melted at depth.

The ascending magma then passes through continental crust of the North America Plate, where it might mix with other magmas or be altered by melting rocks of different composition. 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 can also form away from the subduction zone environment. For example, it can form at ocean ridges and oceanic hot spots from partial melting of basaltic rocks. It can also form during eruptions at continental plate interiors where deep-source magma melts continental crust or mixes with continental magmas.

There are many other environments where andesite might form. Big Obsidian Flow. As demonstrated above, felsic magmas will either 1 erupt explosively to produce extensive deposits of tephra, or 2 nonexplosively to produce degassed, viscous lava domes, coulees, or obsidian flows which advance only short distances from their vents. There has been a significant amount of controversy, therefore, over rare rhyolite lavas that appear to occur as large-volume flows cubic kilometers.

Most such flows occur near continental hotspots. The best known examples are those associated with 1 the Yellowstone hotspot track near the Idaho-Oregon border, and 2 the Ethiopian hotspot in northeastern Africa.

These large-volume felsic volcanic rocks have outcrop, hand specimen, and thin section characteristics typical of lava flows. However, many volcanologists suspect that they are not lava flows at all, but rather rheomorphic ignimbrites. These are densely welded pyroclastic flows of pumice and ash, which were thick and hot enough to flow downslope and obliterate primary pyroclastic structures.

They suggest that the original pumice and ash fragments have been streaked out like toffee strands so that the pyroclastic nature of the flow becomes unrecognizable.