Igneous Rock Chart  
Cochise College                  
Virtual Geology Museum
Hall of Rocks

Photos of Rocks  
Geology Home Page                       

Roger Weller, geology instructor 
(
wellerr@cochise.edu )

 

 

dark
colors

intermediate
colors

pastel
colors


very fast cooling, glass

 obsidian with no gas
 pumice with gas

 

 


extrusive “volcanic” rocks
 erupted at surface
 cooled fast
 
fine grained
 (aphanitic)


 

       basalt

covers ocean floor

andesite

Andes mountains

       rhyolite
    ignimbrite eruptions

 

 

porphyry has:
large-grained crystals,
dispersed in a fine-grained groundmass.

 

 

intrusive “plutonic” rocks
 cooled beneath surface
 cooled slowly
 
coarse grained
 (phaneritic)

        gabbro

coarse grains

diorite

white granite

granite

often contains pink
feldspar

 

 

 

 

 

 

 

 
pegmatite
has:
 very large crystals of quartz,
 feldspar and mica.

   

 

                                 
chart prepared by Steve Tyminski (10/4/07)