The reason for this "stepped" evolution of minerals is that with dropping temperature we we have decreasing thermal vibration of molecules, and that allows silica to form more complex structures.
Thus, olivine with its isolated silica tetrahedrons forms at the highest temperatures, and as temperatures drop silica tetrahedrons first manage to join together in chains pyroxenes , then in ribbons amphiboles , and then sheets micas.
Finally, at the lowest temperatures the two branches merge and we get the minerals that are common to felsic rocks - muscovite mica, orthoclase feldspar,and quartz 3D frameworks. Magmatic Differentiation: With this term we describe the process of separating the magma into several batches separating crystals from melt as it evolves and migrates upwards in the earth's crust.
The remaining melt would crystallize pyroxene, hornblende, and plagioclase with intermediate Ca content andesine , and might erupt from a volcano and form andesite.
Or, if the material remained buried, the crystal fraction would form a diorite from which in turn a granitic melt would rise.
The parental magma that Bowen envisioned was a "primitive" basalt with high Fe and Mg contents and fairly low SiO2 contents. This gradual change in mineral composition during differentiation is accompanied by a change in color mafic rocks dark, felsic rocks light and density of the respective rocks, and is the basis for most of the accepted classification schemes of igneous rocks.
The original composition of the magma of course determines the variety of igneous rocks we can derive from it. As with everything else in geology, there are exceptions to this rule see Strickler's 1st Law of GeoFantasy , but for the most part it works. Bowen determined that specific minerals form at specific temperatures as a magma cools. At the higher temperatures associated with mafic and intermediate magmas, the general progression can be separated into two branches.
The continuous branch describes the evolution of the plagioclase feldspars as they evolve from being calcium-rich to more sodium-rich. The discontinuous branch describes the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. List of Partners vendors.
Share Flipboard Email. Andrew Alden. Geology Expert. Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U. Geological Survey. Updated February 03, Featured Video. Cite this Article Format. Alden, Andrew. The Bowen Reaction Series in Geology. Rhyolite Rock Facts: Geology and Uses. Your Privacy Rights.
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