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An introduction to igneous rocks requires exploration of three core ideas. Each of these is introduced simplistically below, and then further explored in other pages of the site.
Formation, Classification, and Identification of Igneous Rocks (next).
Igneous rock evolution
Plate tectonics and igneous rock distribution
FORMATION, CLASSIFICTION AND IDENTIFICATION The inside of the earth is very hot - hot enough to melt rocks. And the deeper you go the hotter it gets. Below the surface the molten rock is called magma; at the earth's surface it becomes lava, although nothing has changed except the name.
The fresh magma is white hot, brillant enough that you would have trouble looking at it. But as it cools it turns yellow, and then various shades of red. Eventually it cools enough to solidify completely and form an igneous rock, such as the granite and basalt below. Granite and basalt are the two most abundant igneous rocks at the earth's surface.
Magma/lava is a mixture of elements such as silica, iron, sodium, potassium, etc. As the magma/lava cools these elements chemically combine, or crystallize, in geometric patterns to form the eight rock forming minerals. For example, in the granite above the pink is orthoclase, the black biotite, and clear to gray mostly quartz (see enlargement).
These eight minerals form the bulk of igneous rocks. They are arranged in Bowen's Reaction Series (BRS) by temperature of formation, high temperature ones at the top and low temperature ones at the bottom. Although it is useful to know these minerals they are not essential for a basic understanding of igneous rocks.
Constituent Percent of
Total Volume H2O
.20 Cooling is progressive in a magma/lava, some minerals becoming solid at high temperatures (top of BRS) and others at lower temperature (bottom BRS), so that part way through the cooling the magma/lava is a mixture of minerals and still molten rock.
Magma/lava also contains lots of gasses such as water, sulfur dioxide, carbon dioxide, etc., and these are driven off into the atmosphere during cooling. (See Table to right of gasses from the Hawaiian volcano.)
If cooling is "slow" (thousands to millions of years) below ground the minerals grow large enough to see with the eye, as with the granite to the left.
These are "coarse grained" (or phaneritic). Any rock in which the grains can be seen by eye are coarse grained.
If cooling is "quick" (days to weeks) as at the earth's surface, the minerals do not have enough time to grow, and so are microscopic in size. These are fine grained (or aphanitic). For example, the rhyolite to the left.
If cooling is "very quick" (hours to days) the elements and compounds are frozen in place, no minerals form, and the result is a glass. For example the scoria to above left and the obsidian above right.
Basalt and granite are two of the most common igneous rocks found at the earth's surface. They illustrate the diversity of properties igneous rocks have. Basalt
a mafic rock Granite
a felsic rock 1. Dark color
2. High specific gravity
3. Olivine/pyroxene/Calcic plagioclase rich
4. Fine grained - crystals only seen under high power
5. Forms at the surface, principally in the ocean basins, but also in isolated "hot spots" on the continents.
6. Forms high in Bowen's Reaction series 1. Light color
2. Low specific gravity
3. Quartz and orthoclase and sodium plagioclase rich
4. Coarse grained - crystals large enough to see by eye
5. Forms on the continents deep underground
6. Forms low in Bowen's Reaction series
Igneous rocks are classified in several different ways (link), but all rock classifications are a combination of texture and color/composition of the rock. The variety of igneous textures is in the table below Texture Cooling History Example Glassy
grain sizes) Very fast cooling; non-crystalline.
Very fast cooling with rapid gas
escape forming bubbles in the
Slow cooling; microscopic crystal
Very slow cooling; crystals grow
to visible size.
Two stage cooling; one slow
underground creating visible
phenocrysts, the second fast at
the earth's surface producing a
fine grained groundmass. Obsidian
Any aphanitic rock with
the adjective porphyry
The color/composition of the rock is at its simplest divided into dark colored rocks (mafic), intermediate colored rocks (intermediate), and light colored rocks (felsic). If we combine texture/cooling history and color/composition in a grid we get the classification in the table below.
A pdf version of this classification table will print as good as the original.
OBSERVE There are a few aphanitic and glassy/cellular rocks for which color will just not work for classification, and if you try to use color the identification will be wrong. There is no way to know this ahead of time. You just have to memorize these specimens.
Obsidian: Glassy, and black or red. It belongs in the light-colored felsic category because its chemistry is like those rocks. Obsidian is dark because it is a glass with many impurities which absorb the light and make it dark.
Scoria: Ranges from dark red to black. Composition ranges from intermediate to mafic.
OBSERVE Although mafic, intermediate and felsic are the main categories, many other kinds of igneous rocks exist. One additional category that does not easily fit onto the above classification table is ULTRAMAFIC. These, like mafic rocks are olivine or pyroxene rich, but lack plagioclase feldspar. Examples are dunite (mostly olivine) and peridotite (olivine and pyroxene).
That is, in Bowen's Reaction Series the composition of these rocks is high on the left.
If classification and identification was all there was to igneous rocks there would not be much use studying them. We classify rocks to learn what they can tell us about the earth.
There are two ideas about igneous rocks that are geologically important. The first idea is that igneous rocks evolve - they change from one kind of rock into another.
The second idea is that rocks are not randomly distributed across the earth. Specific kinds of rocks are always found in specific places for specific reasons, all tied into plate tectonic processes.
These ideas of igneous rock evolution and distribution are related, and eventually we will explore their relationships. For now, we just introduce the basic ideas.
IGNEOUS ROCK EVOLUTION One of the most important ideas geology has discovered is that igneous rocks evolve. That the earth began with a composition similar to that of the moon, that is, composed mostly of an mafic/ultramafic parent rock, and from that simple beginning all the other rocks have evolved through a sequence of fractionation processes. It is a core concept essential to understanding the Earth's evolution that is explored in the Wilson Cycle and a Plate Tectonic Rock Cycle.
The core idea is that an original rock, the parent rock, present when the earth formed gave rise not only to all other igneous rocks, but all rocks including sedimentary and metamorphic. The process occurs when the parent rock is fractionated, that is split into two fractions each with a composition different from the parent. Fractionation may occur during crystallization of a magma, or melting of a preexisting rock.
During fractional melting, for example the mafic parent rock selectively melts producing two fractions. The first fraction is a melt whose composition is closer to the bottom of Bowen's Reaction Series than the original rock. This melt is intermediate in composition. The second fraction is the unmelted crystal residue with a composition more mafic than the original rock. That is, its composition is higher in BRS than the original rock.
If time and conditions allow, the fractionation process can continue and the intermediate rock produced during the first fractionation can fractionate into a felsic magma (granite), leaving behind a crystal residue more mafic than the intermediate rock. The fractionation process continues until everything that can be fractionated out of the original composition has been removed. The process is then at an end.
PLATE TECTONICS, AND IGNEOUS ROCK DISTRIBUTION
Click image for larger version. Igneous rock evolution does not just happen anywhere. It takes special plate tectonic conditions. A basic understanding of plate tectonics is necessary here, and if you are unfamiliar with the ideas go to this link for a quick, simple introduction.
In terms of earth processes, fractionation occurs at two main locations, divergent plate boundaries, and convergent plate boundaries. Divergent plate boundaries (drawing or definition) are mostly under water so we do not easily see them working. But here magma rises up from deep in the earth and oozes out onto the ocean floor to form new oceanic lithosphere (simple drawing; or a more detailed drawing). In the process the parent rock of the earth's interior fractionates to form mafic igneous rocks, e.g. basalt and, at depth, gabbro.
At convergent plate boundaries (drawing or definition) part of the ocean lithosphere (created at divergent plate boundaries) descends into the earth again, where it heats up and fractionally melts (drawing). This generates intermediate rocks at first, such as diorite, but may eventually create felsic rocks such as granite.
On an earth time scale, igneous fractionation is responsible for the formation of all the world's volcanic arcs and continents, the implication being, the earth began without continents, and the total size of the continents has grown with geologic time. Contemplate the earth with no continents. It is easy to appreciate the importance of igneous fractionation to just about everything about the earth.
A final outcome of all this is that different igneous rocks are found in different places on the earth, and all these different distributions are related to plate tectonic processes, and to the history of the earth. At its simplest, continents are made of felsic igneous rocks (such as granite), ocean basins made of mafic igneous rocks (such as basalt and gabbro), and volcanic arcs of intermediate igneous rocks (such as diorite and andesite.)
Go On To:
Bowen's Reaction Series and Igneous Rock Forming Minerals
Kinds of Igneous Rock Classifications
Igneous Classification Chart with links to rock images
Keys to Igneous Rock Identification
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