GL 107 Earth Systems Science

K. Hannula

Discussion questions for 9/14/07

1.  List the order of crystallization of minerals predicted by Bowen's reaction series.

Discontinuous series (mostly dark-colored minerals): 1st olivine, then pyroxene, then hornblende, then biotite, then (lighter-colored minerals) muscovite and quartz.

Continuous series (mostly feldspars): 1st Ca-plagioclase, then Na-plagioclase, then potassium feldspar.

2.  How does Bowen's reaction series explain which minerals are found together in mafic, intermediate, and felsic rocks?

Minerals in mafic rocks are the first minerals in Bowen's reaction series: olivine, pyroxene, and plagioclase.

Minerals in intermediate rocks are the middle minerals in Bowen's reaction series: hornblende, plagioclase (most commonly).

Minerals in felsic rocks are the last minerals in Bowen's reaction series: quartz, potassium feldspar, and (in smaller quantities) Na-plagioclase and biotite.

3.  How does Bowen's reaction series explain how an intermediate composition rock can form from an originally mafic magma?

The first minerals to form (olivine, Ca-plagioclase) contain less Si and more Mg, Fe, and Ca than the magma as a whole does.  If those minerals are removed from the magma after they form, the remaining magma contains proportionally more Si, K, and Na and less Fe, Mg, and Ca.

4.  List three processes that can cause solid rock to begin to melt.

Increased temperature (least common reason), decreased pressure, and addition of water or other volatiles.

5.  What causes melting at divergent plate margins?

Decrease in pressure as asthenosphere rises.