Bowen’s Reaction Series Describes the formation of igneous rocks from different starting temperatures and magma compositions. Fractional Crystallization.

Bowen’s Reaction Series• Describes the formation of igneous rocks from

different starting temperatures and magma compositions.

• Fractional Crystallization – process by which minerals are formed and removed from the molten rock

Discontinuous Reaction Series• On left• Earlier-formed crystals react with the

remaining magma in a series of steps to form a new mineral

• From an initial basaltic magma, the first formed mineral is olivine (Mg2SiO4).

• removal of olivine increases the silica content and decreases the iron and magnesium in the parent magma

• As temperatures lower, a new chain silicate mineral, pyroxene (CaMg(SiO3)2 is formed.

• As temperatures drop further (1100C - 850C), the magma becomes even richer in silicates.

• Amphibole begins to form • The original basaltic magma becomes

andesitic magma

• As temperatures cool to 800C and silica increases further, the sheet silicate biotite forms.

• Andesitic magma now evolves into rhyolitic magma.

• Quartz forms last at low temperatures. • Muscovite mica and potassium feldspar also

form at this temperature• All of these minerals are poor in mafic

components (Fe, Mg) which were removed from the melt by minerals that formed earlier.

Continuous Reaction Series• On right• Earlier formed minerals slowly and steadily

change into new minerals• Crystallization of plagioclase feldspar depletes

calcium from the parent magma and replaces the missing element with sodium as new minerals are formed

• Feldspar minerals form continually at the same time as minerals shown opposite in the Discontinuous Series

Bowens and Weathering• Minerals that crystallize at high temperatures

generally break down more quickly on the surface on Earth

• Minerals that crystallize at low temperatures are more abundant on the surface because they are more resistant to weathering

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