Where is andesite rock found

It is known as a "host rock" for the popular gemstone, Andesine. The name "Andesite" is derived from the Andes Mountains of South America, where the rock was originally discovered.

However, the rock forms from subduction zones all around the world. Andesite rocks will form in regions above subduction zones.

This formation usually occurs after oceanic plates melt amid the subduction zone, as the melting serves as a source of magma. When such magma erupts to the surface, it creates a fine-grained rock that crystallizes quickly. Quartz is among the most common elements to be found in Andesite. In the modern world, much like Basalt, we use Andesite for many construction purposes, including aggregated gravel for roadbeds. We also see Andesite used in famous statues and monuments around the world.

Many artifacts found in ancient temples and monuments, including the famous "Head of Buddha" found in the Indonesian Mountains, are made from Andesite. This rock also has a resistance against slipping, which makes it an excellent choice for floor tiling work. 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. Hornblende Andesite Porphyry: A specimen of andesite with large visible hornblende phenocrysts. This type of rock could be called an "andesite porphyry" because of its texture.

It could also be called a "hornblende andesite" because of its composition. Photo by NASA. Occasionally, andesites contain large, visible grains of plagioclase, amphibole, or pyroxene. These large crystals are known as "phenocrysts. These high-crystallization-temperature minerals begin forming below the surface and grow to visible sizes before the magma erupts. When the magma erupts onto the Earth's surface, the rest of the melt crystallizes quickly. This produces a rock with two different crystal sizes: large crystals that formed slowly at depth the "phenocrysts" , and small crystals that formed quickly at the surface known as "groundmass".

The name of an abundant phenocryst mineral may be used as an adjective to the rock name. An example is the hornblende andesite porphyry shown in the accompanying photo. Andesite outcrop: Close view of an andesite lava flow at Brokeoff Volcano in California. Photo by the United States Geological Survey.

The best way to learn about rocks is to have specimens available for testing and examination. Some magmas that produce explosive eruptions above subduction zones contain enormous amounts of dissolved gas.

These magmas can contain several percent dissolved gas by weight! This gas can have several origins, examples of which include the following:. At depth, these gases can be dissolved in the magma like carbon dioxide dissolved in a can of cold beer.

If that can of beer is shaken and suddenly depressurized by opening the can, the gas and the beer will erupt from the opening.

A volcano behaves in a similar manner. A rising magma chamber can instantly be depressurized by a landslide, faulting, or other event and an enormous volume of rapidly expanding gas will explode through the overlying rock. Many volcanic plumes and ash eruptions occur when gas-charged andesitic magmas erupt.

The gas pressure that causes the eruption blows large amounts of tiny rock and magma particles into the atmosphere. These particles, known as volcanic ash , can be blown high into the atmosphere and carried long distances by the wind.

They often cause problems for aircraft operating downwind from the volcano. Catastrophic eruptions like Mount St. Helens, Pinatubo, Redoubt , and Novarupta were produced by andesitic magmas with enormous amounts of dissolved gas under high pressure. It is difficult to imagine how a magma can contain enough dissolved gas to produce one of these eruptions.

Magma is Earth's most powerful solvent. Andesite Flow: One of numerous massive andesite flows from the Zarembo Island area of southeastern Alaska.