Volcano! Hollywood’s most recent attraction, active volcanoes are found worldwide, but notably NOT in Los Angeles. Most active volcanos are located around the rim of the Pacific Ocean, known as the “Ring of Fire”. The volcanoes here erupt explosively, throwing huge volumes of ash high into the atmosphere, like Mt. St. Helens in 1980 and Mt. Pinatubo in 1991. These volcanoes are known as stratocone or composite-cone volcanoes, because they erupt large volumes of both ash and lavas. The other major class of volcano are known as shield volcanoes because they build larger volcanoes with more broad slopes, like the Hawaiian Islands. Shield volcanoes erupt mostly lavas that flow in long thin sheets because they are very hot and fluid. Thus, shield volcanoes are the largest volcanoes in the world.
Where to?
The rocks produced by a volcano are tied to the plate-tectonic setting in which the volcano forms. Volcanoes that form where hot magma rises from deep within the Earth, such as along the mid-ocean ridges or at hot spots like Hawaii, produce very fluid lavas because the temperatures of the lavas are so high.
Volcanic Glass
When lava is erupted from the volcano and cools so rapidly that mineral crystals cannot form, volcanic glass forms instead. These kinds of volcanic rocks are called obsidian, and they display the fracture so typical of broken glass. The edges of these rocks are often sharp enough to cut paper or fingers.
Mineral Composition and Color
The mineral composition of the lava is reflected by its color. Most volcanic rocks are rich in silicate minerals, but their color reflects the amount of iron- and magnesium-bearing minerals they contain. Iron and/or magnesium-rich silicate minerals are called mafic minerals. Rocks that contain large percentages of mafic minerals tend to be black in color. Rocks that contain lesser quantities of iron and magnesium have relatively higher amounts of minerals like quartz and feldspar; these are called felsic minerals. Rocks that contain large percentages of felsic minerals tend to be light in color. Rocks with about equal amounts of mafic and felsic minerals are said to be intermediate in composition; these tend to be grayish in color.
Vesicular Rocks
Volcanic rocks sometimes display abundant air bubbles, called vesicles. Vesicles form as large volumes of gas escape from the cooling lava. There’s often so much gas that the texture is frothy, like bubbles blown on top of a milkshake. Some volcanic rock have so many vesicles that they are extremely light for their size; if the composition of these rocks is mafic, they are called scoria, and if the composition is felsic, the rocks are called pumice.
Pyroclastic rocks
Some volcanoes erupt with such force that they blow some of the surrounding mountain away with them, incorporating fragments of rock into the lava or ash. These kinds of eruptions are more typical of composite-cone volcanoes than shield volcanoes, and are called pyroclastic (pyro = fire, and clastic = block) eruptions. The rocks that result are called pyroclastic rocks.
Porphyritic Rocks
Some volcanic rocks show evidence of a slower journey to the Earth’s surface than others. Since all lavas originate below ground, it is not unusual for the magma to begin to cool and crystallize before it reaches the Earth’s surface. Crystallization of a magma is a relatively slow process, and different minerals crystalize at different rates. Thus, it is possible for some minerals to become quite large before others even begin to form. If magma containing some larger crystals should erupt, the still-molten portion of the magma will cool more quickly, producing smaller crystals. The rock that then forms has larger crystals that appear to “float” in the finer lava. This texture is called porphyritic, and it indicates a two-phase cooling history, where part of the magma cooled below ground and the rest cooled at the surface. The large crystals are called phenocrysts and the surrounding finer crystals are called the groundmass.
Composite-cone volcanoes form above subduction zones—areas where one lithospheric plate is pulled down under another. They usually erupt explosively because the magmas that form as the downgoing plate melts are not as hot as those that form at mid-ocean ridges or hot spots. In addition to the temperature variation, the mineral composition of these magmas is also different. Since they form essentially from the recycling of surficial materials, the composition of these magmas is enriched in felsic minerals relative to mafic minerals. Felsic minerals have a more complex structure than mafic ones, and thus, are more resistant to flow. The final variable contributing to explosive eruptions of composite-cone volcanoes is the amount of water in the magma. Since these magmas form from surficial materials, they tend to incorporate water into the melt. As the water is heated up, it turns to steam. When the magma comes to the surface, the release of pressure causes the water vapor to explode. It’s similar to the “pop” that occurs when the carbon dioxide pressure is released when you open a can of soda.
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