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Unformatted text preview: Magma and Igneous Rocks Magma Formation
Magma Partial melting in crust / upper mantle, mostly at
divergent plate boundaries.
divergent Melting is from… Pressure release.
Heat Magma Formation
gradient – Earth is
depth. Magma Formation Heat transfer. Rising magma
carries mantle heat.
This raises T in
crustal Addition of Volatiles
Addition of Volatiles cause rocks to melt at much lower T. Water.
Carbon dioxide. Adding volatiles to hot, dry rocks initiates melting. Magma Compositions
Magma 4 major magma types based on % silica (SiO2). Felsic (Feldspar and silica)
Mafic (Mg and Fe-rich)
Ultramafic 66 to 76% silica.
52 to 66% silica.
45 to 52% silica.
38 to 45% silica.
38 Magma Compositions
Magma Composition controls density and viscosity. Most important is the content of silica (SiO2). Silica-rich magmas are thick and viscous. Silica-poor magmas and thin and “runny.” These characteristics govern eruptive style.
Type Density Viscosity Felsic Very low Very High: Explosive eruptions. Intermediate Low High: Explosive eruptions. Mafic High Low: Thin, hot runny eruptions. Ultramafic Very high Very low Magma Variation
Magma Why are there different magma compositions?
Magmas vary chemically due to…
Magmas Initial source rock compositions.
Fractional crystallization. Bowen’s Reaction Series Partial Melting
Partial Upon heating, silica-rich minerals melt first.
Partial melting, then, yields a silica-rich magma.
Creates: Felsic magma.
Mafic residue. Assimilation Magma melts the country
rock it passes through.
composition. Magma Mixing
Magma Different magmas may blend in a magma chamber.
The result combines the characteristics of the two.
Often magma mixing is incomplete, resulting in
blobs of one rock type suspended within the other.
blobs Fractional Crystallization
(magmatic differentiation) As magma cools, early crystals settle by gravity.
Melt composition changes as a result. Fe, Mg, and Ca is removed in early settled solids.
Si, Al, Na, and K remain in melt and increase. ...
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- Fall '07