Igneous Rocks
Structure of the Earth
The illustration below is an attempt to model the large scale internal structure of the Earth based on data
from Lutgens & Tarbuck. The habitable part of the Earth is a very thin layer. Though one might quibble
with the precision, the following captures the perspective "all life is confined to the space between the snow
of the mountain tops and the heat of the Earth's interior. This narrow stratum as compared with the diameter
of the Earth is but one half the thickness of one leaf of a thousand-page book." (Morrison)
The uppermost layer of the
mantle and the crust tend to act
together as a rigid shell.
Together they are called the
lithosphere, the "sphere of
rock". The lower level of the
mantle is called the
asthenosphere and it is softer
and weaker, particularly in its
upper portion where a small
amount of melting can occur. It
is at this level where the model
of plate tectonics suggests that
horizontal movement can occur
as a result of convection of heat
upward from the Earth's core.
The continental crust is made up of lighter granitic rock, while deep-sea drilling reveals that the oceanic
crust is basaltic in composition. Basalt is significantly more dense (about 3 gm/cm3) than granite (about 2.6
gm/cm3). The asthenosphere is thought to be a more dense rock like peridotite. This judgement comes from
the fact that lava reaching the surface in volcanic activity comes from the melting of the upper
asthenosphere. Lava of similar composition can be obtained by melting peridotite.
Modeling the core of the Earth must rest upon even more indirect evidence. We observe that the metallic
meteorites have cores of iron and nickel, and this correlates with other evidence that suggests that the
Earth's core is similarly composed of iron and nickel. Modeling the density of the center of the Earth yields
densities of about 14 times that of water, which could be obtainable by compressing iron and nickel, but not
surface type rocks. An iron core also gives us a circulating electrical conductor, which could provide the
necessary mechanism for creating the Earth's magnetic field.
The following table of density and depth data was taken from a USGS publication by Eugene C. Roberson
entitled The Interior of the Earth. (http://pubs.usgs.gov/gip/interior/)
Data on the Earth's Interior
Density
Thickness
(g/cm3)
Types of rock found
(km)
Top Bottom
Crust
30
Upper
mantle
720
Lower
mantle
2,171
Outer core
2,259
Inner core
1,221
Total
thickness
6,401
2.2
Silicic rocks
2.9
Andesite, basalt at base
Peridotite, eclogite, olivine, spinel,
garnet, pyroxene
3.4
4.4
4.4
5.6
9.9
12.2
12.8
13.1
Perovskite, oxides
Magnesium and silicon oxides
Iron + oxygen, sulfur, nickel alloy
Iron + oxygen, sulfur, nickel alloy
Robertson gives credit for most of the data to Anderson, Don L., Theory of the Earth:
Boston, Blackwell Publications, 1989.
Igneous Rock
Molten material within the Earth is called magma. In simple terms magma can be thought of as molten
rock. When magma cools, it solidifies to form rock which is called "igneous rock". That is deceptively
simple, since the solidification process can be very complex. There is a considerable range of melting
temperatures for different compositions of magma. Upon cooling from the completely molten state, it is
typical for silicon tetrahedra to form first, and they in turn join with each other and other ions to form the
nuclei for crystal growth. The minerals with the highest melting points will crystalize first, and their crystal
growth may continue unimpeded as long as the surrounding material remains molten. Depending upon the
surroundings and the rate of cooling, a great variety of textures and compositions of igneous rock can be
formed. When crystallization is complete, the result is a solid mass of interlocking crystals of different
sizes.
Igneous Rock Composition
Igneous rocks are commonly classified by their composition and texture. Most are composed of the eight
most abundant elements in the Earth's crust. Because of the dominance of oxygen and silicon in the crust,
igneous rocks are mostly made up of silicate minerals. These silicates can be generally divided into light
and dark silicates. The dark silicates are also called ferromagnesian because of the presence of iron and
magnesium in them. They include olivine, pyroxene, amphibole and biotite. The light-colored silicates
include quartz, muscovite and feldspar.
Solidification from magma produces great diversity in the mineral compositions which make up the rocks.
There are general catagories which are keyed to the amounts of light and dark silicates in the rocks. At the
light-colored extreme are rocks made up mainly of quartz and the feldspars, with about 70% silica. Such
rocks are called granitic rock. Rocks which contain large amounts of the ferromagnesian dark matter and
about 50% silica are said to have basaltic composition. Some organization was brought to the continuous
variation between these extremes by the Bowen reactions. This model of the process of solidification from
magma pictures the processes which causes the composition of the magma and the subsequent rocks to
change.
Elements Abundant in Igneous Rock
Eight elements make up about 98% by weight of most magmas from which igneous rocks are made. Click
on any element for further details.
The dominance of oxygen and silicon in the Earth's crust gaurantees that most igneous rocks are made up
of silicate minerals. The main differences in the composition of igneous rocks are the variations in the other
six elements.
Beyond the "big 8", manganese and titanium are present in small concentrations in magma and therefore
appear in a number of minerals.
Silicates
The most abundant elements in the Earth's crust are oxygen (46.6%) and silicon (27.7%). Minerals which
combine these two elements are called silicates, and combined they are the most abundant minerals on the
Earth. The silicates can be organized in terms of their chemical compositions and their crystal structures
(indicated by the existance of cleavage planes). The table below is an example of such organization from
Lutgens and Tarbuck.
Mineral
Idealized Formula
Cleavage
Olivine
(Mg,Fe)2SiO4
None
Pyroxene group
(Augite)
(Mg,Fe)SiO3
Two planes at
right angles
Amphibole group
(Hornblende)
Ca2(Fe,Mg)5Si8O22(OH)2
Two planes at
60° and 120°
Biotite
K(Mg,Fe)3AlSi3O10(OH)2
Muscovite
KAl2(AlSi3O10)(OH)2
Orthoclase
KAlSi3O8
Plagioclase
(Ca,Na)AlSi3O8
Micas
Feldspars
Quartz
SiO2
One plane
Two planes at 90°
None
Shipman, et al. comment that about 95% of the continental crust rocks are composed of the two types of
feldspar or quartz.
Abundances of the Elements in the Earth's Crust
Element
Approximate
% by weight
Oxygen
46.6
Silicon
27.7
Aluminum
8.1
Iron
5.0
Calcium
3.6
Sodium
2.8
Potassium
2.6
Magnesium
2.1
All others
1.5
Given the abundance of oxygen and silicon in the crust, it should not
be surprising that the most abundant minerals in the earth's crust are
the silicates. Although the Earth's material must have had the same
composition as the Sun originally, the present composition of the Sun
is quite different. The elemental composition of the human body and
life in general is quite different.
These general element abundances are reflected in the composition of
igneous rocks.
Magma
Magma is the term used to describe molten material within the Earth; in simple terms: molten rock. But the
molten rock usually contains some suspended crystals or dissolved gases. Igneous rocks form through the
crystallization of magma. There is a considerable range of melting temperatures for different compositions
of magma.
Melting Points of Rocks
Igneous rocks form through the crystallization of magma. There is a considerable range of melting
temperatures for different compositions of magma. All the silicates are molten at about 1200°C and all are
solid when cooled to about 600°C. Often the silicates are grouped as high, medium and low-melting point
solids.
ApproximateTemperature (°C) Minerals which are molten
1200
All molten
1000
Olivine, pyroxene, Ca-rich plagioclase
800
Amphibole, Ca/Na- plagioclase
600
Quartz, K-feldspar, Na-plagioclase, micas.
The pattern shown above where different kinds of minerals crystallize at different temperatures is further
developed in the Bowen reaction series. The crystallization temperatures play a large role in the
development of the different kinds of igneous rocks upon the cooling of magma.
Intrusive Rocks
Igneous rocks which form by the crystallization of magma at a depth within the Earth are called intrusive
rocks. Intrusive rocks are characterized by large crystal sizes, i.e., their visual appearance shows individual
crystals interlocked together to form the rock mass. The cooling of magma deep in the Earth is typically
much slower than the cooling process at the surface, so larger crystals can grow. Rocks with visible crystals
of roughly the same size are said to have a phaneritic texture.
Extrusive Rocks
Igneous rocks which form by the crystallization of magma at the surface of the Earth are called extrusive
rocks. They are characterized by fine-grained textures because their rapid cooling at or near the surface did
not provide enough time for large crystals to grow. Rocks with this fine-grained texture are called aphanitic
rocks. The most common extrusive rock is basalt.
Bowen's Reaction Series
In the early part of the 20th century, N. L. Bowen carried out experiments to characterize the process of
crystallization of igneous rocks from magma. The illustration below is patterned after Lutgens and
Tarbuck's perspective of that reaction series.
The difference in crystallization temperature for the different kinds of minerals plays a major role in the
differentiation of rock composition as the magma cools.
What do the terms mafic and felsic mean?
These are both made up words used to indicate the chemical composition of silicate minerals, magmas, and
igneous rocks.
Mafic is used for silicate minerals, magmas, and rocks which are relatively high in the heavier elements.
The term is derived from using the MA from magnesium and the FIC from the Latin word for iron, but
mafic magmas also are relatively enriched in calcium and sodium. Mafic minerals are usually dark in color
and have relatively high specific gravities (greater than 3.0). Common rock-forming mafic minerals include
olivine, pyroxene, amphibole, biotite mica, and the plagioclase feldspars. Mafic magmas are usually
produced at spreading centers, and represent material which is newly differentiated from the upper mantle.
Common mafic rocks include basalt and gabbro. (Please note that some geologists with questionable
motives switch the order of the magnesium and iron and come up with the term "femag." This term is not to
be confused with Femag, the dull-witted henchman of the Diabolical Dr. Saprolite.)
Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage
of the heavier elements, and are correspondingly enriched in the lighter elements, such as silicon and
oxygen, aluminum, and potassium. The term comes from FEL for feldspar (in this case the potassium-rich
variety) and SIC, which indicates the higher percentage of silica. Felsic minerals are usually light in color
and have specific gravities less than 3.0. Common felsic minerals include quartz, muscovite mica, and the
orthoclase feldspars. The most common felsic rock is granite, which represents the purified end product of
the earth's internal differentiation process.
It is important to note that there are many intermediate steps in the purification process, and many
intermediate magmas which are produced during the conversion from mafic to felsic. We call the magmas
associated with these intermediate stages "intermediate." Clever, huh?
Ultramafic Rock
The class of rock which crystallizes from silicate minerals at the highest temperatures is sometimes referred
to as "ultramafic" rock. It includes peridotite and komatiite. It is in the highest temperature range of the
Bowen reaction series.
Mafic or Basaltic Rock
The class of rock which crystallizes from silicate minerals at relatively high temperatures is sometimes
referred to as "mafic" rock. It is also sometimes called basaltic since the class includes basalt and gabbro. It
is in a high temperature range of the Bowen reaction series.
The term "mafic" is a short form indicator of the presence of a relatively large concentration of iron and
magnesium. The term "ferromagnesian" is also used.
Felsic Rock
The class of rock which crystallizes from silicate minerals at relatively low temperatures and with relatively
high percentage of silica is generally referred to as "felsic" rock. This class includes granite and rhyolite
and is at the low temperature extreme of the Bowen reaction series.
Andesitic Rock
The class of rock which crystallizes from silicate minerals at intermediate temperatures is sometimes
referred to as "andesitic" rock. This class includes andesite and diorite and is in an intermediate range of the
Bowen reaction series.
Igneous Rock Texture
Igneous rocks are commonly classified by their composition and texture. Texture is the term applied to the
overall appearance of a rock based on the size, shape, and arrangement of the interlocking mineral crystals
which form it. The table below summarizes the common classifications.
Aphanitic
Fine grained
Phaneritic
Coarse grained
Porphyritic Larger crystals with small crystal background
Glassy
Non-ordered solid from rapid quenching
Pyroclastic Composites of ejected fragments
Aphanitic Rock
Aphanitic rock is igneous rock in which the grain or crystalline structure is too fine to be seen by the
unaided eye. Such rock is formed when the material solidifies at or near the surface so that the cooling is
rather rapid. Such rocks are termed "extrusive" rocks. Under these conditions, there is not enough time for
the growth of large crystals. Basalt from surface lava flow often exhibits an aphanitic texture.
Since the crystals of individual minerals cannot be easily resolved for classification, aphanitic rocks are
classified in general terms like light, intermediate or dark in color.
The presence of voids called vesicles is common in aphanitic rock since the condition of cooling rapidly
may be associated with the upper portion of lava flows. These vesicles caused by gases escaping from these
lava flows will be most numerous in the upper portions of the flows.
Phaneritic Rock
Phaneritic rock is igneous rock with large, identifiable crystals of roughly equal size. Such crystals are
characteristic of rocks which solidified far below the surface so that the cooling was slow enough to enable
the large crystals to grow. Such rocks are termed "intrusive" rocks. When such rocks are found on the
surface, this can be taken to imply that the overlying material has been removed by erosion.
Porphyritic Rock
Porphyritic rock is igneous rock which is characterized by large crystals surrounded by a background of
material with very small crystals. The scenario for the production of such rocks involves the formation of
certain types of mineral crystals over a long period deep in the earth. Because of differences in melting
temperatures and growth rates, the surrounding material may not have appreciably crystallized. If this
material is suddenly ejected from the surface, as in a volcano, then the surrounding material will solidify
rapidly to form small crystals in the spaces between the large ones.
In such rocks the large crystals are called phenocrysts while the surrounding material is called groundmass.
The entire collection of material is called a porphyry.
Glassy Rock
When molten rock is suddenly ejected from a volcano, it may be cooled so rapidly that organized crystal
formation cannot occur. This results in igneous rock which has no internal structure. It has a glassy
appearance and produces no planes or crystal symmetry when broken.
Obsidian is a common natural glass occuring in lava flows.
Pyroclastic Rock
Sometimes material is violently ejected from volcanoes and then reassembled into igneous rocks from this
material. The material may range from fine dust or fine hair-like strands to large molten blobs. The
consolidation of such material into rocks produces what is called the pyroclastic texture.