l2_magmas - GLY 421: Igneous & Metamorphic Petrology...

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El-Shazly, 2004 1 Magmas and Lavas Definitions: Magma : a mixture of a melt (predominantly silicate) ± crystals ± volatiles which occurs at depths and has the ability to migrate to shallower levels where it either crystallizes at depth giving rise to igneous intrusions, or erupts at the surface to form volcanic rocks. Lava : Is erupted molten material that can flow on the surface of the earth. A lava may therefore be considered a magma that has lost its gases (to the atmosphere upon eruption). Chemical composition of magmas (types of magmas): It is clear from our discussion of the layered structure of the earth that although magmas are generated by partial melting in the upper mantle or lower crust, such a process occurs over a range of depths. Accordingly, not all magmas have the same composition. This is evidenced by the variety of igneous rocks that occur at the surface of the earth or at depth. Volcanic eruptions also show that lavas have different viscosities, which in turn are due to their different chemical compositions and/or temperatures. By carefully studying the chemistry of the different types of igneous rocks, and their associations with each other, petrologists were able to classify magmas into four main chemical groups: 1- Acidic : rich in SiO 2 , Na 2 O and K 2 O. Rocks produced from such magmas have between 66 and 77.5% by weight SiO 2 . " Granite " is an example of an acidic rock, and many acidic magmas are broadly known as " granitic ". 2 - Intermediate: rich in SiO 2 , Na 2 O, K 2 O as well as CaO and Al 2 O 3 . Rocks produced from such magmas have SiO 2 values in the range 52 to 66% by weight. Andesite is a good example of a rock formed by the crystallization of an intermediate lava. 3- Basic : rich in CaO, MgO and FeO. Rocks of this type have SiO 2 values of 45 - 52% by weight. Basalt is an example of a basic rock, and many basic magmas are broadly known as " basaltic ". 4- Ultrabasic : Are magmas poor in SiO 2 (< 45%) but with large amounts of FeO and MgO. Ultrabasic rocks may have SiO 2 values as low as 38% by weight. Peridotite is a good example of an ultrabasic rock. Note that ultrabasic lavas are almost nonexistent, being restricted to Precambrian terranes! (Can you guess why?) Table 1 lists the chemical compositions of some igneous rocks belonging to these four types. Technically speaking, a rock can only be considered basic, acidic or intermediate if its chemical composition (particularly its SiO 2 content) is known. In the absence of such information, it is better to use the more general and descriptive terms: "ultramafic, mafic and felsic" which are based on the mineralogy of the rock. The term ultramafic is used to describe rocks that have very little or no feldspar, and which are very rich in dark- coloured ferromagnesian minerals as olivine and pyroxene. Mafic is used for rocks rich in ferromagnesian minerals, but which also contain some feldspars. Felsic rocks are those rich in feldspars and quartz. Because ultrabasic and basic rocks are dark in colour
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This note was uploaded on 02/28/2012 for the course GLY 421 taught by Professor Staff during the Fall '11 term at Marshall.

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l2_magmas - GLY 421: Igneous & Metamorphic Petrology...

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