1. Cooling and Crystallization:
* As magma cools, different minerals begin to crystallize at different temperatures.
* Minerals with higher melting points (like olivine and pyroxene) crystallize first, while minerals with lower melting points (like quartz and feldspar) crystallize later.
2. Removal of Crystals:
* The denser crystals that form early tend to settle to the bottom of the magma chamber.
* This separation of crystals from the remaining liquid magma is called crystal settling.
* In some cases, the crystals may be physically removed from the magma chamber by processes like eruption.
3. Changing Magma Composition:
* As crystals are removed, the remaining liquid magma becomes enriched in the elements that were not used to form the crystals.
* This change in the chemical composition of the magma leads to the formation of different minerals and ultimately, different igneous rocks.
Examples of Fractional Crystallization:
* Ultramafic Rocks: Formed from the earliest crystallizing minerals (olivine and pyroxene).
* Mafic Rocks: Formed from the magma remaining after the ultramafic minerals crystallize out.
* Intermediate Rocks: Formed from the magma enriched in silica after mafic minerals crystallize.
* Felsic Rocks: Formed from the final, silica-rich magma remaining after all other minerals have crystallized.
Key Factors Influencing Fractional Crystallization:
* Cooling Rate: Slower cooling allows for more complete crystallization and greater separation of minerals.
* Magma Composition: Initial composition of the magma dictates the order of mineral crystallization.
* Pressure: Higher pressure favors crystallization of minerals with lower melting points.
In Summary:
Fractional crystallization allows a single magma to form a variety of igneous rocks by systematically removing minerals as the magma cools. The process changes the chemical composition of the magma, leading to the formation of rocks with different textures, mineral content, and compositions.
