|
Igneous Rocks
Igneous rocks are one of the three major rock types and are basically
rocks that solidified from magma. Magma is the term for liquid or melted
rock and is usually quite hot. After all, the term igneous comes from
the Latin word for "of fire". Most people have seen pictures or video
of magma that flows out of a volcano. When magma flows onto the surface
of the Earth it is called lava. When the lava cools and forms a rock,
that rock is an igneous rock. But there are also rocks that form from
magma that does not reach the surface of the Earth. These rocks form in
what are often called plutons and are also types of igneous rocks. These
two types of igneous rocks are called extrusive
(because it extruded or came out of the Earth) and intrusive (because it intruded and stayed inside
the Earth). They are also referred to respectively as volcanic and plutonic.
Classifying igneous rocks into these two main types is easy and logical
for the most part. If the rock formed from lava spilling onto the surface
of the rock then it is an extrusive or volcanic igneous rock. If the rock
never made it to the surface before it cooled into a rock, then it is an
intrusive or plutonic igneous rock. Easy right? Well, further classification
of igneous rocks is a bit more complicated.
There are several classification techniques classifying igneous
rocks that apply different methods and derive different results. One
technique applies chemical composition as the major distinguishing factor.
This technique provides the petrologist (or rock scientist) with a good
objective method of determining the true origins of the rock from a plate
tectonics aspect. The chemistry of igneous rocks and plate tectonics are
closely related and therefore the origin of a rock's chemistry and not
its minerals or methods of formation is important to petrologists who
use this method of classification. A question that a plate tectonics scientist
would want to know is, did the rock originate in a high silica, low iron
and high hydrous magma that may have resulted from the subduction of certain
types of crustal rock? They are less interested in the minerals that
might have formed there or whether the rock is glassy or fine grained.
This type of classification does not lend itself well to the understanding
of minerals in the rock or to easy identification by rock hounds.
Terms used in this classification technique include acidic, intermediate,
basic and ultrabasic. Acidic rocks contain a lot of silica, SiO2. Basic rocks have around
50% or less silica and a lot of magnesium and iron. Further classification
depends on other chemical analysis. Terms derived from this classification
technique are often used to characterize rocks that are classified from
other methods.
Another classification technique uses the mineral components of
the rock and is called the modal method of classification. Identification
of the rock's minerals is of course important and is easy to do when the
minerals are found in large crystals and can easily be identified. This method
has "official" approval and is generally what petrologists use to classify
rock samples. When the crystals are too small to be seen without a microscope,
then true identification becomes a problem in the field. Generalized
terms used in this classification technique include felsic, intermediate,
mafic and ultramafic. Felsic rocks, similar to acidic rocks, contain
a lot of silica, sodium and calcium and form
quartz and
feldspar minerals; felsic is short for feldspar/silica.
Mafic, similar to basic rocks, have
a lot of magnesium and iron (ferrous) and is short for magnesium/ferrous.
Generally the important aspect of this classification is the ratio of three
minerals and tertiary diagrams are used to classify the rocks. In rocks
that contain feldspars and quartz then the ratio of quartz to plagioclase
feldspars to calcium or alkali feldspars is used. For example, an igneous
rock that has around 50% quartz to 25% plagioclase and 25% alkali feldspar
is classified as granite. The boundaries between rock names are arbitrary
and a classification diagram is necessary. Ultramafic rocks which generally
do not have many quartz or feldspar minerals are classified based on their
percentages of olivine,
pyroxene and
hornblende.
Feldspathoid rocks which
do not have quartz are classified based on their plagioclase feldspar/alkali
feldspar/feldspathoid percentages.
Calcite can be the predominant mineral
in a special case of igneous rock and then the rock is classified as a
carbonatite.
A third general type of classification uses a variety of characteristics
and is therefore more subjective, but sometimes easier to use for the
average rock hound. It involves the use of the rocks texture, color
and mineralogy. Although not an official classification technique, it
nevertheless produces a field-identifiable way to classify igneous
rocks. Rocks are divided by their texture from glassy to fine grained
(aphanitic) to course grained (phaneritic). The terms such as
fragmented,
vesicular (having a lot of holes), porphoritic (large crystals in a glassy
or fine grained matrix) and pegmatitic (all crystals are large) are used
to further describe the texture. After texture is applied, color comes
into play. Generally the rock is described from light to dark in color.
Finally the mineralogy is applied if known. This is generally used for
rocks that contain a large amount of one particular mineral such as
calcite
for carbonatite or
anorthite for
anorthosite. Using this technique is
easier, but leads to misidentifications and is not useful for scientific
analysis. But at least we generally know what igneous rock we are looking
at. And isn't that the purpose of classification anyways?
Below are the main types of igneous rocks and their general attributes
that place them into the various classification schemes:
PLUTONIC - Intrusive
igneous rocks:
|
NAME: |
CHEMISTRY
|
MINERALS
|
COLOR
|
TEXTURE
|
ORIGINS
|
|
ANORTHOSITE
|
Acidic
|
Plagioclase Feldspars
|
White to Black
|
Phaneritic
|
ancient plutons,
mountains of the moon
|
|
CARBONATITE
|
Acidic
|
Calcite,
Trona and other rare
carbonates
|
shades of gray
|
Aphanitic
|
igneous intrusive structures,
very rarely as a volcanic rock
|
|
DIORITE
|
Intermediate
|
Mostly
plagioclase feldspars and
little or no quartz
|
white with dark specks
|
Phaneritic
|
Plutons associated with volcanic
arcs
|
|
DUNITE
|
Ultrabasic
|
Mostly
olivine with some
pyroxenes
and chromite
|
dark green to black
|
Phaneritic to aphanitic
|
Deep Oceanic crust
|
|
GABBRO
|
Basic
|
pyroxenes,
amphiboles,
olivine
and
plagioclase feldspars
|
Dark green to black
|
Phaneritic
|
Oceanic crust and magmatic segregation
|
|
GRANITE |
Acidic
|
quartz,
plagioclase feldspars,
alkali feldspars
and micas
|
generally light colors from white,
pink to gray
|
Phaneritic to pegmatitic
|
Continental crusts and mountain
belts
|
|
KIMBERLITE
|
Ultrabasic
|
olivine,
phlogopite,
pyroxenes,
ilmenite,
diopside,
chromite,
spinel,
pyrope and rare
diamonds
|
Dark green to black
|
Phaneritic to porphyritic
|
Deep mantle to crust intrusions
|
|
LAMPROPHYRES
|
Ultrbasic
|
Biotite,
amphiboles,
pyroxenes,
orthoclase, and
plagioclase feldspars |
Dark gray to black
|
Porphyritic
|
Dikes and some pluton extremeties
|
|
MONZONITE
|
Intermediate
|
A variety of
feldspars
and some
hornblende,
Biotite
and rarely
quartz
if any
|
generally light colors of white,
pink, green and gray
|
Phaneritic
|
Crustal intrusions - plutons
|
|
PEGMATITE
|
Acidic
|
quartz,
feldspars,
micas,
tourmalines
and others
|
generally light colors of white,
pink, green and gray
|
Pegmatitic
|
Granitic intrusions
|
|
PERIDOTITE
|
Ultrabasic
|
olivine
and
pyroxenes
with some
plagioclase feldspars,
amphiboles
and
chromite
|
dark green to black
|
Phaneritic to aphanitic
|
Deep oceanic crust
|
|
PYROXENITE
|
Ultrabasic
|
Predominantly
pyroxenes
|
black to dark gray
|
Aphanitic
|
Dikes
|
|
SYENITE |
Acidic
|
orthoclase,
plagioclase feldspars,
hornblende,
micas
and little or no
quartz
|
Light colors from white to pink
to orange
|
Phaneritic
|
Plutons
|
VOLCANIC - Extrusive
igneous rocks:
NAME:
|
CHEMISTRY
|
COLOR
|
TEXTURE
|
| ANDESITE |
Intermediate
|
Variable from white or
gray to nearly black
|
Aphanitic to porphyritic
|
| BASALT |
Basic
|
Generally black
|
Glassy, vesicular and
porphyritic
|
| DACITE |
Acidic
|
light colors
|
Aphanitic to porphyritic
|
KOMATIITE
|
Ultrabasic
|
Various shades of gray
|
Unique spinifex texture
|
OBSIDIAN
|
Acidic (generally)
|
Black
|
Glassy some with phenocryst
of cristobalite
|
| PUMICE |
Generally acidic to intermediate
|
light to dark gray
|
Vesicular to glassy chards
|
| RHYOLITE |
Acidic
|
white to light gray colors
|
Aphanitic to porphyritic.
|
| SCORIA |
Intermediate to basic
(generally)
|
black to dark red
|
Vesicular and ropy glassy
fragments
|
|
