10 Common Igneous Rocks
Cochise College   
Virtual Geology Museum           

Geology Home Page

Roger Weller, geology instructor
wellerr@cochise.edu
copyright 2007-R.Weller



Igneous Rock Table  (shows relationships)

10 igneous rocks


 

1. obsidian

2. pumice

3. rhyolite

4.andesite

5. basalt

6. granite

7. diorite

8. gabbro

9. porphyry

10. pegmatite


These rocks can be identified by their textures, mineral content, and color. 
Refer to Igneous Rocks Photos for more visual examples of each of these igneous rock types.


glassy texture
1. obsidian
     Obsidian is volcanic glass without gas bubbles.  It is usually black or dark brown in color
and breaks with a
conchoidal (shell-like) fracture. Be careful not to cut yourself on the sharp
edges. A variety of obsidian with white to light gray crystallized patches surrounded by black
glass is known as
snowflake obsidian.


2. pumice
   
 Pumice is volcanic glass filled with gas bubble holes (vesicles). It may be thought of as
a glass foam.  Because of the large number of holes, pumice is very light-weight; it will
float on water. Pumice comes in
many colors, but the most common color is gray.


fine-grained (aphanitic texture)
3. rhyolite
    
Rhyolite is a high-silica, fine-grained rock.  You cannot see the mineral grains with the
naked eye.  Its colors are
gray, light brown, tan, pale yellow, pink, and other earth-colors. 
Sometimes there may be a
sprinkling of small crystals, but most of the rock is fine-grained. 
Using food terms, it resembles baloney
(unidentifiable components).  Rhyolite has the same
chemical and mineral content as granite.


4. andesite
     Andesite is the name of fine-grained igneous rocks that are midway in color and mineral
composition between rhyolite and basalt. Andesites are commonly gray or some shade of

medium brownCommonly they have a porphyritic texture; there are larger visible crystals
surrounded by the gray or brown andesite.


5. basalt
    
Basalt is a fine-grained igneous rock rich in iron that gives it a black to brown color
Fluid lava flows, such as those in Hawaii, produce basalt. If basalt has a large number of
gas bubble holes it is called
vesicular basalt or scoria. Basalt that has been exposed to air
and water for a long time may oxidize to

a red color.


coarse-grained (phaneritic) texture
6. granite
     Granite is a coarse-grained igneous rock often with a pink to reddish color.  A large
portion of the granite is made of small crystals of
orthoclase feldspar which give the rock
the pink or reddish color
Other minerals present are quartz (usually gray). albite feldspar
(white) and either white mica (muscovite) or black mica (biotite). The word granite means
grain-rock; it it weathers, it crumbles into loose grains.


7. diorite
     Diorite is a coarse-grained igneous rock intermediate in composition between granite
and gabbro.  It can sometimes be described as a
"white granite" because of the abundance
of albite, a white feldspar.
 Depending upon the amount of iron rich minerals present,
diorite can range from
nearly white to quite darkDiorite has the same mineral content
as andesite.


8. gabbro
     Gabbro is a dark, coarse-grained igneous rock.  It has the same mineral content as
basalt, but the grains in
gabbro are visible to the naked eye.


mixed grain sizes (large and small)
9. porphyry
   
 The term porphyry simply refers to the two distinctly different grain sizes present in an
igneous rock.  The larger crystals are called phenocrysts and the finer crystals are the
groundmass.
  The groundmass can be rhyolite, andesite, or basalt and even, rarely, granite
The phenocrysts are often
feldspar crystals or hornblende crystals.

very large grain size (larger than 1/2 inch)
10. pegmatite

     Pegmatite is very coarsely crystallized.  Some of the largest crystals in the world have
been found in pegmatites.  Pegmatites often have the same mineral composition of granites
with large crystals of
mica and feldspar.  Gem minerals, such as tourmaline and beryl are
found in pegmatites.