Bowen's reaction series explains the order in which common silicate minerals crystallize as magma cools. This reference helps students connect mineral formation to magma temperature, igneous rock composition, and mineral stability at Earth's surface. It is useful for identifying rock-forming minerals and predicting how magma changes during crystallization.
Key Facts
- Bowen's reaction series shows that high-temperature minerals crystallize first and low-temperature minerals crystallize last as magma cools.
- The discontinuous branch follows the order olivine, pyroxene, amphibole, and biotite as temperature decreases.
- The continuous branch shows plagioclase feldspar changing from calcium-rich at high temperature to sodium-rich at lower temperature.
- Potassium feldspar, muscovite, and quartz crystallize at the lowest temperatures in the series.
- High-temperature minerals such as olivine are less stable at Earth's surface and weather more easily than quartz.
- Mafic magma is rich in iron and magnesium, forms minerals like olivine and pyroxene, and crystallizes at higher temperatures.
- Felsic magma is rich in silica, sodium, and potassium, forms minerals like quartz and potassium feldspar, and crystallizes at lower temperatures.
- Fractional crystallization changes magma composition because early crystals are removed, leaving the remaining melt enriched in silica.
Vocabulary
- Bowen's reaction series
- A model that shows the sequence in which silicate minerals crystallize from cooling magma.
- Discontinuous branch
- The side of Bowen's reaction series where one mineral reacts with magma to form a different mineral as temperature drops.
- Continuous branch
- The side of Bowen's reaction series where plagioclase feldspar changes composition gradually from calcium-rich to sodium-rich.
- Fractional crystallization
- The process in which early-formed crystals separate from magma, changing the composition of the remaining melt.
- Mafic
- A term for igneous materials rich in iron and magnesium and usually lower in silica.
- Felsic
- A term for igneous materials rich in silica, potassium, and sodium and usually lighter in color.
Common Mistakes to Avoid
- Reversing the crystallization order is wrong because olivine forms at high temperature while quartz forms at low temperature.
- Thinking both branches show the same type of change is wrong because the discontinuous branch changes mineral type while the continuous branch changes plagioclase composition.
- Assuming high-temperature minerals are most stable at Earth's surface is wrong because minerals that form far from surface conditions usually weather faster.
- Confusing mafic with felsic composition is wrong because mafic rocks are iron and magnesium rich, while felsic rocks are silica rich.
- Ignoring crystal removal during fractional crystallization is wrong because removing early mafic minerals makes the remaining magma more silica rich.
Practice Questions
- 1 A magma cools from 1200°C to 700°C. Which minerals from the discontinuous branch would likely crystallize in order?
- 2 A rock contains abundant quartz and potassium feldspar. Is it more likely mafic or felsic, and what does that suggest about its crystallization temperature?
- 3 During fractional crystallization, olivine and pyroxene crystals settle out of a magma chamber. How does the remaining magma composition change?
- 4 Why does quartz usually resist chemical weathering better than olivine, based on Bowen's reaction series?
Understanding Bowen's Reaction Series Reference
The series is really a model of which crystal structures can exist in a melt at different conditions. Silicon and oxygen form the basic framework of most igneous minerals. At very high temperatures, the melt has enough energy to hold iron, magnesium, calcium, sodium, potassium, and silica in a mixed liquid.
As energy is lost, some groups of atoms fit together more easily than others. Minerals with simpler silica structures can form first.
Later minerals have more linked silica units in their structures. This structural change helps explain why minerals formed from the same original melt can look and behave very differently.
A magma chamber does not always turn into one uniform rock. Once crystals begin to form, they may stay suspended in the melt, stick to the chamber walls, rise, or sink. Dense iron and magnesium rich crystals commonly settle downward if the magma remains liquid long enough.
If these early crystals are separated from the melt, their elements leave with them. The liquid left behind has a different chemical balance.
It can produce lighter colored minerals and rocks than the original magma. This process helps account for layered igneous bodies, where different mineral bands formed as crystals collected at different levels.
The reaction series describes an ideal pattern, but natural magmas have complications. Cooling rate matters. Slow cooling gives atoms time to move into ordered crystal lattices, so minerals can grow large.
Fast cooling may trap the melt as volcanic glass or produce tiny crystals that are difficult to identify. Water and other dissolved gases lower melting temperatures and change which minerals are stable. Pressure changes deep underground can affect the sequence too.
A mineral may react with remaining melt and become a new mineral if conditions change gradually. If cooling is rapid, that reaction may be incomplete. Rocks can therefore contain minerals that record several stages of cooling.
Students often use this idea when classifying igneous rocks and interpreting thin sections. Dark minerals usually point toward more iron and magnesium, while pale minerals usually indicate more silica, sodium, or potassium. Color alone is not enough, since grain size, impurities, and weathering can mislead.
Look for mineral shape, cleavage, hardness, and the way crystals fit together. The series is useful outside the classroom because it connects rock type to soil formation and landscape change. Minerals that formed under hot conditions are often less suited to cool, wet surface conditions.
They break down into clay minerals, iron oxides, and dissolved ions. Quartz commonly persists as sand because its structure is relatively stable near Earth's surface. This is why a granite and a basalt can weather into soils with different textures and nutrient supplies.