Reading Guide 4 -- Magmas and Igneous Rocks

The main questions to guide your reading are:

How are igneous rocks catagorized?
What causes the wide diversity of igneous rocks we observe?
Why are there so many kinds of volcanic eruptions?
Where do magmas come from?

Read pages 77-80 carefully. The interaction between composition (to what are we referring when we say "compositional effects"?), temperature (what is a typical range of magmatic temperatures?), gas content (what are the dominant gases in magma?), and viscosity (the resistance to flow) is what governs the type of eruption and the apearance of volcanic deposits. You will want to understand these concepts fully.

What does the term polymerization mean?

Pages 81-94.
How do composition, gas content, and viscosity interact to produce an explosive eruption?
..To produce a lava flow?

Think of your reading this way: you should look at the photographs and read the text until you feel confident that given a photograph of a volcano, you could identify:

The type of volcano, i.e., sheild, composite, dome, etc...
The gas content of the lava in terms of high, medium, or low
The most likely silica content and associated rock name, i.e., basalt, andesite, dacite, or rhyolite.
The viscosity of the erupted material.
The very generalized volcanic hazard

Pages 94-101.
Figure 3.25 is important. Know the rock and mineral names given in it.
Worry more about the names of the rocks than the names of the various plutons.

Pages 102-109 -- the Origin of Magma
Read these sections carefully. I will expand upon them in class.
Figure 3.33 is terrible. There are no flame-throwers beneath the midocean ridges and the subducting slab does not melt to form convergent-margin andesitic stratovolcanoes.
The section on Distribution of volcanoes provides a link between volcanism and tectonics.
What is the significance of the Andesite Line?

Figure B3.3 and B3.4 are very important. In B3.3, the word "albite" could be replace with the words "peridotite mantle" with some slight changes in the temperature and pressure scales. The shapes of the ruves would remain basically the same. In B3.4, realize the the mantle never completely melts; it only partially melts. All of the action takes place between the "Beginning of melting" line and the "25% Melt" line. Recognize too, that melt can be produced by decreasing the temperature as well as increasing the temperature. They don't make this really clear, but it is a decrease in pressure beneath mid-ocean ridges and hot spots that causes most of the mantle melting on this planet. This process is called decompression melting.

Do not read the section on the origin of andesitic magma. In fact, you can take your favorite black indelible marking pen and cross it all out. This description of the origin of andesite is older than you are and has been widely discredited, largly by researchers on this campus.

The origin of rhyolite and granite batholiths is one of the most fundamental unsolved problems in the Earth Sciences. Maybe you can figure it out!

Fractional Crystallization (pp. 106-108) is probably more important than implied by Skinner and Porter. Understand how the removal of a a mineral phase can change the composition of a magma. Also note the similarity in mineral sequence between Bowen's reaction series and fig. 3.25.