1. Cooling Magma: As magma rises towards the surface, it cools. This cooling is a gradual process, not an instant event.
2. Nucleation: As the magma cools, the atoms within it begin to move more slowly. This allows atoms of the same element to bond together and form tiny seed crystals, called nuclei.
3. Crystal Growth: These nuclei act as templates for further crystal growth. As the magma continues to cool, more atoms attach themselves to these nuclei, causing the crystals to grow larger.
4. Fractional Crystallization: Minerals within the magma crystallize at different temperatures. Minerals with higher melting points crystallize first, while those with lower melting points crystallize later. This means that different minerals crystallize at different stages of cooling.
5. Separation of Minerals: As crystals form, they can become denser than the surrounding magma. This causes them to sink to the bottom of the magma chamber, separating from the remaining molten rock. This separation of minerals is called fractional crystallization.
6. Changing Magma Composition: The remaining magma is now depleted of the minerals that crystallized first, changing its overall chemical composition. This can lead to the formation of different types of rocks.
Important Factors Influencing Crystallization:
* Cooling Rate: The rate of cooling influences the size of the crystals. Slower cooling allows for larger crystals to form, while rapid cooling results in smaller crystals.
* Chemical Composition: The chemical composition of the magma influences the types of minerals that will crystallize.
* Pressure: Pressure can also affect crystallization, with higher pressures promoting the formation of denser minerals.
Examples of Minerals Formed by Fractional Crystallization:
* Olivine: One of the first minerals to crystallize from magma, often found in mafic rocks like basalt.
* Pyroxene: Another early-forming mineral found in mafic and intermediate rocks.
* Feldspar: A group of minerals that crystallize at a variety of temperatures and are found in a wide range of igneous rocks.
* Quartz: A silica mineral that often crystallizes late in the process, found in felsic rocks like granite.
By understanding fractional crystallization, we can learn about the history of the magma that formed igneous rocks, including its chemical composition, temperature, and the processes that it underwent.
