Since most magmas form in the upper mantle, their composition is mostly silica (SiO 2 ) with different amounts of iron, calcium, sodium, potassium, aluminum, magnesium, and other trace elements. The crystallization of these compounds is known, but as the ratios of the elements are different, the crystallization looks different.
In 1912, Bowen performed a series of experiments where he compared the crystallization temperature of compounds with different silica levels. He found that minerals, which crystallized at higher temperatures (calcium-rich plagioclases , olivines , pyroxenes ), were low in silica. These high-temperature minerals were further divided into a ferromagnesian class and plagioclase feldspar. The minerals that crystallized at lower temperatures were usually low in silica. In 1928, Bowen published, The Evolution of Igneous Rocks , where he focused mostly on magma. Bowen became known as the Father of Canadian Geology for his ideas on crystal formation. This mineral crystallization cycle is known as Bowen ’ s Reaction Series . Figure 6-4 shows the minerals of the Bowen Reaction Series.
Fig. 6-4. The Bowen reaction series have continuous and discontinuous paths.
The plagioclase class is a continuous reaction series where some crystals are already formed, while the rest melt. The magma composition changes continuously, but crystals that are already formed don’t change. Remember, the higher the amount of calcium in a crystal, the higher the temperature it takes to melt. As the magma cools, the crystals are constantly reacting with other elements in the melt. When cooling happens quickly, then the series shifts from high calcium-containing crystals to high silica-containing crystals.
The ferromagnesian class goes through a discontinuous reaction series . These elements begin with olivine crystallizing first, and then react with other elements in the magma melt to form pyroxene.
Mg 2 SiO 4 + SiO 2 → 2MgSiO 3
olivine + silica in fluid magma → pyroxene
As the magma cools and the temperature lowers even more, pyroxenes continue to react with elements in the melt. They are then converted to amphiboles . This series of reactions is interrupted (discontinuous) between each formation of different compounds. Some compounds are formed at different temperatures, before they react with the melt elements and possibly form other new compounds with different compositions.
For example, at lower temperatures, pyroxene reacts with an increase in available silica and forms amphibole. The magma continues to cool and amphibole reacts to form biotite , which contains even greater amounts of silica.
Continuous and discontinuous reaction series are separate, but the external environment also has a role. Many times, the final form of the created rock can have unique characteristics that have been brought about by the environment specifics.
Some of the environmental factors that affect crystallization are: pressure, temperature changes, rate of cooling, local deposits of calcium- or sodiumrich minerals, and timing of crystallization during cooling. Magma can also be changed by the type of rock pockets that may be around it.
Today on Education.com
- Coats and Car Seats: A Lethal Combination?
- Kindergarten Sight Words List
- Child Development Theories
- Signs Your Child Might Have Asperger's Syndrome
- 10 Fun Activities for Children with Autism
- Why is Play Important? Social and Emotional Development, Physical Development, Creative Development
- First Grade Sight Words List
- Social Cognitive Theory
- The Homework Debate
- GED Math Practice Test 1