Igneous Rocks:
Igneous rock (derived from the Latin word ignis meaning fire) is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock is formed through the cooling and solidification of magma or lava. Igneous rock may form with or without crystallization, either below the surface as intrusive (plutonic) rocks or on the surface as extrusive (volcanic) rocks.
There are two basic types: 1) intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and 2) extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface.
Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende and sometimes quartz. The specimen shown above is about two inches (five centimeters) across. Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene.
The molecules in the magma have time to arrange themselves into crystal formations before the magma hardens. Intrusive rocks have large crystals that can be seen with the naked eye. A common example of an intrusive igneous rock is granite. Extrusive igneous rocks come from lava.
Classifications-
Igneous rocks are classified according to mode of occurrence, texture, mineralogy, chemical composition, and the geometry of the igneous body.
The classification of the many types of different igneous rocks can provide us with important information about the conditions under which they formed. Two important variables used for the classification of igneous rocks are particle size, which largely depends on the cooling history, and the mineral composition of the rock. Feldspars, quartz or felspathoids, olivine, pyroxenes, amphiboles, and micas are all important minerals in the formation of almost all igneous rocks, and they are basic to the classification of these rocks. All other minerals present are regarded as nonessential in almost all igneous rocks and are called accessory minerals. Types of igneous rocks with other essential minerals are very rare, and these rare rocks include those with essential carbonates.
In a simplified classification, igneous rock types are separated on the basis of the type of feldspar present, the presence or absence of quartz, and in rocks with no feldspar or quartz, the type of iron or magnesium minerals present. Rocks containing quartz (silica in composition) are silica-oversaturated. Rocks with felspathoids are silica-under saturated, because felspathoids cannot coexist in a stable association with quartz.
Igneous rocks that have crystals large enough to be seen by the naked eye are called phaneritic; those with crystals too small to be seen are called aphanitic. Generally speaking, phaneritic implies an intrusive origin; aphanitic an extrusive one.
An igneous rock with larger, clearly discernible crystals embedded in a finer-grained matrix is termed porphyry. Porphyritic texture develops when some of the crystals grow to considerable size before the main mass of the magma crystallizes as finer-grained, uniform material.
BasaltBasalt is a common extrusive igneous rock; it is formed from the rapid cooling of basaltic lava at the surface of a planet. Basalt is Late Latin which is from the word basaltes which means very hard stone, which was imported from Ancient Greek (basanites).
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ObsidianObsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly. Obsidian is mostly found within margins of rhyolitic lava flows or known as obsidian flows.
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