10 Common Igneous Rocks
Cochise College
Virtual Geology
Museum
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 brown. Commonly 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 dark. Diorite 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.