Rule: Silica-poor minerals crystallize before (...at higher temperatures than...) silica-rich minerals.

Example: Basaltic magma
Composition (future minerals):
 

First mineral:
2nd and 3rd:
Last:
 

Example: Rhyolitic magma
Composition (future minerals):
 

First mineral:
2nd and 3rd:
Last:
 
 

 Quiz Question: explain cooling history of a gabbro with each olivine crystal having an abraded rim encrusted with pyroxene crystals.
 
 
 
 
 
 
 
 

Quiz Question: explain cooling history of a diorite with “zoned plagioclase” crystals.
 (Ca-rich plagioclase in the middle, surrounded by Na-rich plag on the outside).
 
 
 
 
 
 
 
 
 
 
 

Hint-- plagioclase: CaAl₂Si₂O₈ <==> NaAlSi₃O₈.
(which is more silica-poor?)

Quiz Question:   Does a magma become more silica-rich or more silica-poor during crystallization?
 
 
 
 
 
 
 
 

A guy named Bowen did some experiments about 70 years ago to figure this all out.  How do you tell (in a lab) which minerals form first from a magma as it cools?
 Bowen’s Reaction Series:
 

 
 
 

 
 Temp?  Si-content?  Color?  Ferromag vs. Non-ferrromag?
Basaltic vs Andesitic vs Rhyolitic?  Magma viscosity?

Let’s get tricky now.....
- Magma of one composition can yield igneous rocks of several compositions
- Magma of different compositions can be derived from melting one parent rock type

* Fractional Crystallization *.
(draw one volcano with linked magma chambers)
 
 
 
 
 
 
 
 
 
 
 
 
 
 

Basaltic magma, partially crystallizes: __________
Leftover magma => enriched in Si, depleted in Fe+Mg = an Andesitic magma.
This Fractional Crystallization process creates a new magma that is more Silica-rich than the original.
 

 * Partial Melting * - Bowen’s Backwards!
Oceanic crust:
Minerals:
First to melt:
 
 
 
 
 

Andes Mountains:
Minerals:
First to melt:
 
 
 
 
 

Mantle Peridotite:
First to melt:
 

This Partial Melting process creates a new magma that is more Silica-rich than the original rock.

 *** Know This (or be able to figure it out):
Products of Typical 10-30% partial melting...

- Partially melt a Mantle Peridotite (ol + px)  =>
 Basalt (Hawaii, MOR)
 

- Partially melt a Wet Basalt (px + pg + am)  =>
 Andesite (Andes, Cascades)
 

- Partially melt an Andesite (pg+px+am+bio) =>
 Granite (Yellowstone)

Repeat Summary:
Fractional Crystallization makes magma more silica rich than before.
Partial Melting makes magma more silica rich than parent rock.
 

for thought:
At what type of plate boundary is there lots of Basaltic magma forming?
At what type of plate boundary is there lots of Andesitic magma forming?
What rocks must be partially melting at those places?

Global Distribution of Volcanism:

Mid-ocean ridges
- P.M. of Peridotite => Basaltic magma

Ocean Hot spots (upwelling plumes of mantle peridotite)
- P.M. of Peridotite => Basaltic magma

Subduction of Ocean Lithosphere:
Oceanic Island Arcs
- P.M. of wet basalt + seds (at 100 km) => andesite
- some basalt, too

Continental Arcs
- P.M. of wet basalt => andesite
- some basalt and rhyolite, too

Continental Rifting
- P.M. of Peridotite => Basaltic magma
- some andesite and rhyolite, too
 

Yellowstone National Park = Continental Hot Spot
Magma type?  Why?