IGNEOUS ROCKS
Igneous rocks are aggregates of minerals that crystallize from molten material that is generated within the earth's mantle. The heat to generate this molten material comes from within the earth. The type of igneous rock formed depends on a number of factors, including the original composition of the magma, the rate of cooling, and the reactions that occurred within the magma as cooling took place.
Igneous rocks are widely studied for many reasons. First, many people live close enough to volcanoes to be killed by explosive eruptions or huge landslides triggered by eruptions. Geologists study the ancient deposits of volcanoes to understand their likely future eruptive style. Second, the bulk of the Earth's crust, both continental and oceanic, is made of igneous rocks. Third, igneous rocks commonly form at tectonic plate boundaries; hence, the study of ancient igneous rocks tells us a great deal about the history of the Earth. Finally, hot magmas drive circulation of a lot of hot water. These hot fluids pick up a number of important metals and can deposit them to create meta ore deposits.
Magma forms under conditions that are strongly connected to movements of lithospheric plates. These movements control where rocks of the crust and upper mantle melt and whether they will be intruded or extruded. (From "Understanding Earth", press et al., 2003).
(For Discussion Section)
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Modern classifications group igneous rocks according to their relative proportions of silicate minerals - quartz, feldspar, muscovite, biotite, amphibole, pyroxene and olivine. Felsic (= Feldspar-Silica) minerals are high in silica; mafic (=Magnesium-Ferric) minerals are low in silica. The Latin adjectives felsic and mafic are applied to both minerals and rocks that have high contents of these minerals. Mafic minerals crystallize at higher temperatures- that is, earlier in the cooling of a magma- than do felsic minerals. The figure below shows the classification model of igneous rocks. The vertical axis measures the mineral composition of a given rock as a percentage of its volume. The horizontal axis is a scale of silica content by weight.
(From Understanding Earth, 4th Edition)
Bases for classification of igneous rocks
Classification of igneous rocks is based on a combination of two parameters:
(1) Texture: the visual overall aspect imparted by the sizes, shapes, and arrangement of the rock-forming minerals.
(2) Composition: relative amounts of minerals present.
The common igneous rocks can be identified using the following flowchart:
For more details:
Schematic igneous rock textures
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