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All rights reserved. The Open University is authorised and regulated by the Financial Conduct Authority in relation to its secondary activity of credit broking. 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. All content. Andesite Updated Thursday, 28th September A brief description of the nature of andesite.
Copyright: The Open University How was it formed? Andesite is the fine-grained extrusive equivalent of diorite.
Get closer to geology. Geology toolkit Discover the unique landscape of the British Isles with our interactive Geology Toolkit, featuring a geology timeline, rock analyser, rock cycle, landscape features and safety tips. Take part now Geology toolkit. Activity Level: 1 Introductory. Copyright information.
Publication details Originally published : Wednesday, 27th September 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.
They are gray pyroxene and feldspar porphyrys that weather to maroon or green. Photo by USGS. The formal definition of andesite is problematic. Many authors have classified igneous rocks based upon their chemical and mineralogical compositions. However, none of these classifications are in perfect agreement. For a fine-grained rock like andesite, these classifications are impossible to use precisely when in the field or the classroom.
They require chemical or mineralogical analyses that are usually not available, affordable, or practical. If you examine a rock that appears to be andesite, but you are not confident that it meets the mineralogical or chemical classification of andesite, you might properly call it an "andesitoid" rock. What does that mean? It means that the rock looks like andesite, but, a microscopic examination or chemical analysis might prove you wrong! Find Other Topics on Geology.
Maps Volcanoes World Maps. What Is Andesite? Rock, Mineral and Fossil Collections. Hardness Picks. Flint, Chert, and Jasper. Tumbled Stones.
Fluorescent Minerals.
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