by Claire Lu, Troy High School, Fullerton CA
Igneous rocks are one of the three major subcategories of rocks in the earth. Fittingly derived from the Latin term, ignis, for ‘fire’, they form from the solidification of molten rock, referred to as magma. In a typical garden, you might stumble upon an aggregate of lustrous crystals such as granite, or a mosaic patterned silicate called diorite. In contrast, cdeep into a volcano, you may encounter the candied volcanic glass of obsidian, or finely ground quartz suffused within rhyolite.
The variation of pyretic textures depends on their crystallization as a result of the cooling process each rock undergoes. Below Earth’s crust lies the mantle, one of the layers subject to higher temperatures. Consequently, the mantle becomes less rigid and undergoes convection, generating magma. From magma, igneous rocks almost always produce substances in a crystalized, solidified condition. The form assumed from individual crystallizations produces the subsequent texture of the rock. Igneous rocks are typically divided into two categories: plutonic or volcanic.
The coarsely grained rocks containing visible minerals, like the garden-variety granite or
diorite, develop from slow cooling magma solidifying within Earth’s crust over the course of thousands of years. They are classified as intrusive (or plutonic) rocks, and make up the majority of igneous rocks. Commonly, the visible crystals are referred to as ‘phaneritic’, the typical description of rocks with an intrusive origin. Over time, plutonic rocks are exposed to the surface through erosion.
On the other hand, obsidian, rhyolite, and other similar mineraloids are produced in a process of rapidly cooling magma. This process can take mere weeks or days, preventing the formation of larger crystals. The magma rises to the surface as lava, and quickly cools to form extrusive (or volcanic) rocks. In some situations, magma reaches the surface as volcanic ejecta. The viscosity of the erupted magma varies, as the surrounding conditions can be volatile in temperature and composition. The viscosity of their liquified state is dependent on temperature; viscosity increases as temperature decreases. Often, the more viscous molten rock will yield the most complex silicate structures. Igneous rocks with individual crystals too tiny to be seen are referred to as ‘aphanitic’. Although volcanic rocks are already fine grained via their process, rapid enough cooling can produce a glassy resultant rock, such as the common extrusive example of obsidian.
These rocks hold tremendous geological significance, supplying mineral deposits and terraforming the planet. Composing approximately 95% of the upper portion of Earth’s crust, their abundance has yielded over seven hundred types of (mostly plutonic) igneous rocks being classified. Additionally, their mineral composition provides further information about the Earth’s mantle and its components. More information concerning the geographical features of Earth, such as temperature and pressure conditions at a given time, as well as specific tectonic processes, is able to be extracted from the study of igneous rocks.
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