2.2.7 Differentiation of magmas : 2.2.7 Differentiation of magmas Temperature ° Geothermal Gradient, temperature increases with depth under the surfacePressure ° Increases with depthWater content ° Can change the melting point of rocks ° More water = lower melting pointMineral composition ° Different minerals have different melting points : Temperature ° Geothermal Gradient, temperature increases with depth under the surfacePressure ° Increases with depthWater content ° Can change the melting point of rocks ° More water = lower melting pointMineral composition ° Different minerals have different melting points Factors that Affect Magma Formation How Rocks Melt : How Rocks Melt Partial Melting
Different minerals have different melting points
This process is called partial melting
As magma cools it cools in the reverse order of partial melting
Different minerals form at different temperatures; this process is called fractional crystallization Crystallisation of Magma : Crystallisation of Magma Ions are arranged into orderly patterns
Crystal size is determined by the rate of cooling
Slow rate forms large crystals
Fast rate forms microscopic crystals
Very fast rate forms glass Magmatic Differentiation : Magmatic Differentiation When a magma cools, chemical reactions occur that create a series of different minerals. This process of differentiation occurs along two branches: discontinuous and continuous.
The discontinuous branch
form minerals that are ferromagnesian (contain high percentages of iron and magnesium, dark green to black colour).
called discontinuous because the minerals form at discrete temperatures and not continuously during cooling.
The first mineral to crystallize is olivine, followed by pyroxene, amphibole, and biotite. Continuous Series : Continuous Series The continuous branch is made up of the plagioclase feldspars.
The calcium/sodium ratio in this mineral type changes continuously as the magma cools.
The first feldspars to form contain the highest amounts of calcium; subsequent feldspars have progressively less calcium and more sodium.
These minerals tend to be pink, tan, brown, or whitish.
Any magma left over after all these reactions have been completed crystallizes at the lowest temperature as quartz.
The progression in the series explains why the first lavas from a volcanic vent are rich in iron, magnesium, and calcium, are low in quartz, and are dark green to black and why the later lavas are lighter coloured and contain more quartz. Bowen’s Reaction Series : Bowen’s Reaction Series As magma cools, minerals form in predictable patterns
Bowen discovered two main patterns or branches of crystallisation
Minerals crystallize in a sequential pattern known as Bowen’s Reaction Series Bowen’s Reaction Series : Bowen’s Reaction Series Silicic Classification of Igneous Rocks : Classification of Igneous Rocks