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Examples of aphanitic igneous rock include basalt, andesite, and rhyolite. Glassy or vitreous textures occur during some volcanic eruptions when the lava is quenched so rapidly that crystallization cannot occur. The result is a natural amorphous glass with few or no crystals. Examples include obsidian.
Andesite is usually light to dark grey in colour, due to its content of hornblende or pyroxene minerals. [2] but can exhibit a wide range of shading. Darker andesite can be challenging to distinguish from basalt, but a common rule of thumb, used away from the laboratory, is that andesite has a color index less than 35. [9]
The following is a list of rock types recognized by geologists.There is no agreed number of specific types of rock. Any unique combination of chemical composition, mineralogy, grain size, texture, or other distinguishing characteristics can describe a rock type.
Rhyolite is common along convergent plate boundaries, where a slab of oceanic lithosphere is being subducted into the Earth's mantle beneath overriding oceanic or continental lithosphere. It can sometimes be the predominant igneous rock type in these settings. Rhyolite is more common when the overriding lithosphere is continental rather than ...
Examples of this type of dacite outcrop are found in northwestern Montana and northeastern Bulgaria. Nevertheless, because of the moderately high silica content, dacitic magma is quite viscous [9] and therefore prone to explosive eruption. A notorious example of this is Mount St. Helens in which dacite domes formed from previous eruptions.
Basaltic andesite from Parícutin volcano in Mexico. Basaltic andesite is a volcanic rock that is intermediate in composition between basalt and andesite. It is composed predominantly of augite and plagioclase. Basaltic andesite can be found in volcanoes around the world, including in Central America [1] and the Andes of South America. [2]
Arches of the "Mirador of Yanahuara" in Arequipa are made of sillar. Sillar is a variety of rhyolite, which is a type of volcanic rock.Although sillar is of rhyolitic composition, it has been erupted from volcanoes which mostly erupt andesite lava, and sillar contains small fragments of andesite.
Resurgent calderas can refill with an eruption of rhyolitic magma to form the extrusive rock rhyolite like the Yellowstone Caldera. [2] Submarine volcanoes erupt on the ocean floor and produce the extrusive rock pumice. [2] Pumice is a light-weight glass with a vesicular texture that differs from scoria in its silicic composition and therefore ...