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Because many of the properties of a magma (such as its viscosity and temperature) are observed to correlate with silica content, silicate magmas are divided into four chemical types based on silica content: felsic, intermediate, mafic, and ultramafic. [20]
In Vulcanian eruptions, intermediate viscous magma within the volcano make it difficult for vesiculate gases to escape. Similar to Strombolian eruptions, this leads to the buildup of high gas pressure , eventually popping the cap holding the magma down and resulting in an explosive eruption.
The lava flowing from stratovolcanoes typically cools and solidifies before spreading far, due to high viscosity. The magma forming this lava is often felsic, having high to intermediate levels of silica (as in rhyolite, dacite, or andesite), with lesser amounts of less viscous mafic magma. [2]
Intermediate composition magma, such as andesite, tends to form cinder cones of intermingled ash, tuff and lava, and may have a viscosity similar to thick, cold molasses or even rubber when erupted. Felsic magma, such as rhyolite , is usually erupted at low temperature and is up to 10,000 times as viscous as basalt.
Magmatic water. Magmatic water, also known as juvenile water, is an aqueous phase in equilibrium with minerals that have been dissolved by magma deep within the Earth's crust and is released to the atmosphere during a volcanic eruption. It plays a key role in assessing the crystallization of igneous rocks, particularly silicates, as well as the ...
Andesite is an aphanitic (fine-grained) to porphyritic (coarse-grained) igneous rock that is intermediate in its content of silica and low in alkali metals. It has less than 20% quartz and 10% feldspathoid by volume, with at least 65% of the feldspar in the rock consisting of plagioclase. This places andesite in the basalt /andesite field of ...
Rhyolite is an extrusive igneous rock, formed from magma rich in silica that is extruded from a volcanic vent to cool quickly on the surface rather than slowly in the subsurface. It is generally light in color due to its low content of mafic minerals, and it is typically very fine-grained ( aphanitic ) or glassy .
Volcano. Sabancaya volcano erupting, Peru in 2017. A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. The process that forms volcanoes is called volcanism . On Earth, volcanoes are most often found where tectonic plates ...