The Magma Chamber, part b

The kind of rock produced by a given magma depends on the magma's composition and on what happens during the cooling. As the temperature drops in a magma chamber, minerals form and react chemically with each other and with the remaining magma. The basic minerals tend to crystallize first ( high temperatures) and the more silica-rich ones last (lower temperatures). If fractionation does not occur, the first minerals to crystallize are olivine and calcium-rich feldspars (plagioclase). These crystals remain in the melt and react again with the hot fluid forming pyroxenes. Eventually the whole mass cools and a basalt (or gabbro) is formed. The chemical process which describes this phenomena is called solid solution.

With fractionation, or the removal of parts from the system, the sequence changes. The olivines are denser than the melt and sink to the bottom of the chamber. This stops the reaction of olivine with the melt so that the melt evolves differently. Removing the cations from the system makes the melt become more and more basic as a whole. Feldspars, being less dense than the melt will float to the top of the chamber and form felspar-rich rocks there, again changing how the melt evolves. If fractionation continues, the last stages of crystallization will produce quartz. In a planetary environment wherein there is much volcanism and continued remelting, fractionation causes more and more lower density material to be put forth onto the surface. (Hence the importance of granite in the earth's crust.)


This is a view of the Earth.
Click on image for full size version (40K GIF)
Image from: NOAA

Go to a listing of Rocks by mineral group


The Magma Chamber, part b

The kind of rock produced by a given magma depends on the chemical content of the magma, and on what happens during the cooling. As the temperature drops inside a magma chamber, minerals form and react chemically with each other and with the remaining magma. The basic minerals tend to crystallize first (those that form at high temperatures) and the more silica-rich ones last (lower temperatures).

The first minerals to crystallize are olivine and feldspar. These crystals remain in the melt and react again with the hot fluid forming pyroxenes. Eventually the whole mass cools and either a basalt or a gabbro is formed.


This is a view of the Earth.
Click on image for full size version (40K GIF)
Image from: NOAA

Go to a listing of Rocks by mineral group


The Magma Chamber, part b

The kind of rock produced by a given magma depends on the magma's composition and on what happens during the cooling. As the temperature drops in a magma chamber, minerals form and react chemically with each other and with the remaining magma. The first minerals to crystallize are olivine and calcium-rich feldspars (plagioclase). These crystals remain in the melt and react again with the hot fluid forming pyroxenes. Eventually the whole mass cools and a basalt (or gabbro) is formed.


This is a view of the Earth.
Click on image for full size version (40K GIF)
Image from: NOAA

Go to a listing of Rocks by mineral group



Last modified November 15, 1997 by the Windows Team

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