Introduction to Igneous Rocks
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~
Below the surface of the earth, molten rock is called magma; at the earth's surface it is called lava.
Fresh magma is white hot 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
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 mineral is Orthoclase, the black is Biotite, and clear to gray is Quartz.
The eight rock-forming minerals constitute the bulk of all igneous rocks. They are arranged in Bowen's Reaction Series
(BRS) by temperature; minerals that format high temperatures are at the top and low temperature ones are at the bottom.
Cooling is progressive in a magma/lava; some minerals become 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 gasses such as water, sulfur dioxide, carbon dioxide, etc., and these are driven off into the atmosphere during cooling. The Table to right of includes gasses from a Hawaiian volcano.
If cooling is "slow" (thousands to millions of years) minerals forming below the surface can grow large enough to see with the naked 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 into visible crystals, and so are microscopic in size. These are fine grained (or aphanitic). An example is 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
(above left) and Obsidian
are two of the most common igneous rocks found at the earth's surface. They illustrate the diversity of properties igneous rocks have.
Igneous rocks are classified in several different ways (link
), but all rock classifications are a combination of texture
of the rock. The variety of igneous textures is in the table below
|Very fast cooling; non-crystalline.
Very fast cooling with rapid gas escape forming bubbles in the non-crystalline rock.
Slow cooling; microscopic crystal growth.
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.
Any aphanitic rock with
the adjective porphyry
The color/composition of a rock may be 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.
Several aphanitic and glassy/cellular rocks cannot be classified by color:
* Obsidian: Glassy, and black or red. It belongs in the light-colored felsic category because of similar chemistry. 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.
|Mafic, intermediate and felsic are the main categories for igneous rocks, but one additional category 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). In Bowen's Reaction Series the composition of these rocks is high on the left.
Rocks are classified with the main objective of learning 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.
~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 all the other rocks have evolved through a sequence of fractionation. 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
, which was 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,or 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.
Plate Tectonics and Igneous Rock Distribution
Igneous rock evolution requires specific conditions of plate tectonic processes (go to this link
for an introduction on plate tectonics). In terms of earth processes, fractionation occurs at two main locations; divergent plate boundaries, and convergent plate boundaries. Divergent plate boundaries (drawing) are typically in a subaqueous environment and here magma rises up from deep in the earth and oozes out onto the ocean floor to form new oceanic lithosphere. 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, part of the ocean lithosphere (created at divergent plate boundaries) descends into the earth again, where it heats and fractionally melts. This generates intermediate rocks at first, such as diorite
, but may eventually create felsic rocks such as granite
On earth's 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. In summary, 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.)
Contributed by Lynn Fichter