This produces a melt that is intermediate in composition between basalt and granite. Diorite forms if this type of melt crystallizes below the surface.
Diorite is usually composed of sodium-rich plagioclase with lesser amounts of hornblende , biotite , and pyroxene minerals. It usually contains little if any quartz. This makes diorite a coarse-grained rock with a contrasting mix of black and white mineral grains.
Students often use this "salt and pepper" appearance as a clue to the identification of diorite. Igneous rock compositions: This chart illustrates the generalized mineral composition of igneous rocks. It shows that diorites and andesites are composed mainly of plagioclase feldspar, amphiboles, and micas; sometimes with minor amounts of orthoclase, quartz, or pyroxene.
Diorite and andesite are similar rocks. They have the same mineral composition and occur in the same geographic areas. The differences are in their grain sizes and their rates of cooling. Diorite crystallized slowly within the Earth. That slow cooling produced a coarse grain size. Andesite forms when a similar magma crystallizes quickly at Earth's surface.
That rapid cooling produces a rock with small crystals. Polished diorite: This photo shows a sample of diorite as it might appear in a polished countertop, facing stone, or floor tile. It would probably be marketed as "white granite" at a cabinet shop or building supply store. Diorite Ax: Photographs of a Neolithic ax made of diorite that was found in the surroundings of Reims, France.
Creative Commons photographs by Didier Descouens. In areas where diorite occurs near the surface, it is sometimes mined for use as a crushed stone. It has a durability that compares favorably to granite and trap rock. It is used as a base material in the construction of roads, buildings, and parking areas. 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!
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