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:
Plagioclase Feldspars
White to Black
ancient plutons,
mountains of the moon
Calcite, Trona and other rare carbonates
shades of gray
igneous intrusive structures, very rarely as a volcanic rock
DIORITE Intermediate
Mostly plagioclase feldspars and little or no quartz
white with dark specks
Plutons associated with volcanic arcs
DUNITE Ultrabasic
Mostly olivine with some pyroxenes and chromite
dark green to black
Phaneritic to aphanitic
Deep Oceanic crust
pyroxenes, amphiboles, olivine and plagioclase feldspars
Dark green to black
Oceanic crust and magmatic segregation
quartz, plagioclase feldspars, alkali feldspars and micas
generally light colors from white, pink to gray
Phaneritic to pegmatitic
Continental crusts and mountain belts
olivine, phlogopite, pyroxenes, ilmenite, diopside, chromite, spinel, pyrope and rare diamonds
Dark green to black
Phaneritic to porphyritic
Deep mantle to crust intrusions
Biotite, amphiboles, pyroxenes, orthoclase, and plagioclase feldspars Dark gray to black
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
Crustal intrusions - plutons
quartz, feldspars, micas, tourmalines and others
generally light colors of white, pink, green and gray
Granitic intrusions
olivine and pyroxenes with some plagioclase feldspars, amphiboles and chromite
dark green to black
Phaneritic to aphanitic
Deep oceanic crust
Predominantly pyroxenes
black to dark gray
orthoclase, plagioclase feldspars, hornblende, micas and little or no quartz
Light colors from white to pink to orange

VOLCANIC - Extrusive igneous rocks:
ANDESITE Intermediate
Variable from white or gray to nearly black
Aphanitic to porphyritic
Generally black
Glassy, vesicular and porphyritic
light colors
Aphanitic to porphyritic
Various shades of gray
Unique spinifex texture
Acidic (generally)
Glassy some with phenocryst of cristobalite
PUMICE Generally acidic to intermediate
light to dark gray
Vesicular to glassy chards
white to light gray colors
Aphanitic to porphyritic.
SCORIA Intermediate to basic (generally)
black to dark red
Vesicular and ropy glassy fragments

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