Exercise 6: Igneous Rocks

Some of you may not be taking a laboratory with your lecture course. With this Internet exercise you will have an opportunity to work with igneous rocks. Of course, this is not a substitute for handling the rocks but it beats never trying!

There are two fundamental properties of igneous rocks that are used to assign a name.

1. Texture -- the size of the crystals can be crudely related to the rate of cooling. The higher the rate of cooling the smaller the size of the crystals. The rate of cooling is proportional to the difference in temperature between the bodies in contact with heat flowing from the warmer body to the cooler body.
   • Phaneritic: coarse grain, individual crystals are visible - slow cooling

   • Aphanatic: fine grained, individual crystals are not visible - rapid cooling

   • Porphyritic:a mixture of large and small crystals - perhaps a two-stage cooling history with the large crystals (the phenocrysts) cooling slowly and the small crystals (the groundmass) cooling more quickly.

2. Mineralogy -- N.L. Bowen showed that some minerals (and mineral assembledges) are stable at high temperatures, some at intermediate temperatures and some at low temperatures.
   
   1. With the singular exception of quartz, the other phases present represent solid solution series; that is, their composition varies between fixed limits.

   2. The viscosity (resistance to flow) of a melt (magma/lava) increases with decreasing temperature; the viscosity of a melt is lowered if water is present in the melt.

   3. The complexity (amount of sharing of the oxygens of the silicon-oxygen tetrahedra) increases with decreasing temperature.

   4. The dashed lines are drawn to reflect three mineral assemblages :
      • high temperature - olivine, pyroxene and Ca-rich plagioclase;
        
        
      • intermediate temperature - amphibole, biotite and Na-rich plagiolase; and
        
        
      • low temperature - muscovite, alkali feldspar and quartz.
        

   5. Bowen's Reaction Series points out that there are commonly occurring mineral assemblages (based on similar temperatures of formation/crystallization). For example, quartz and olivine (at least the magnesium-rich variety) are not expected to occur together as an equilibrium assemblage.

   6. Several of the minerals (amphibole, biotite and muscovite) must contain (OH)^- or one of the Halogens (F^-, or Cl^-). Thus, these minerals will only crystallize from a wet melt.

   7. The following table illustrated one way (admittedly quite over-simplified) that a geologist could choose to classify igneous rocks. The two dimensions are texture (cooling rate) and mineralogy (a minimum estimate of highest temperature reached by the melt). Note that the feldspars represent a wide range of temperatures -- Calcium-rich Plagioclase, Sodium-rich Plagioclase and Alkali Feldspar. Quartz occurs in both granites and rhyolites.

   						
   Texture                         Alkali Feldspar   Sodium Plagioclase   Calcium Plagioclase
   
   Phaneritic		Granite	                         Diorite	             Gabbro
   
   Aphanitic		Rhyolite                      Andesite                   Basalt
   
   

3. Note that a granite is the coarse grained equivalent of a rhyolite. That is, if the rate of cooling is very slow a granite will form. The same melt, cooled rapidly, will form a rhyolite.

4. The relationship between temperature and mineralogy allows another over-simplification that relates igneous rock type to plate tectionic setting:

   1. Grabbro/Basalt -- Divergent Margins

   2. Diorite/Andesite -- Convergent Magrins - Subduction Zones

   3. Granite/Rhyolity -- Convergent Margins - Continent/Continent Collision

1. The rock to the right has a (an) ______ texture

   phaneritic
   porphyritic
   aphanitic
   

2. This rock cooled:

   with a two-stage cooling history
   rapidly
   slowly
   

   
   

   
3. The rock to the right has a (an) ______ texture

   phaneritic
   aphanitic
   porphyritic
   

4. This rock cooled:

   rapidly
   slowly
   with a two-stage cooling history
   

   
   

   
5. The rock to the right is a (an): andesite porphyry
   basalt
   rhyolite
   diorite porphyry
   diorite
   

   
   

6. The rock to the right may have formed in which of the following tectonic settings?

   spreading center
   continent/continent collision zone
   subduction zone
   

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