An Introduction To Igneous Petrology
Rocks, whether sedimentary, metamorphic or igneous, tell a story. But before you can read the story you must first learn the syntax and meaning of the words that make up the language of rocks. In this article I will focus on the language of igneous rocks, and towards the end, I will talk a little about certain metamorphic rocks that are relevant to the discussion of igneous petrology.
You don’t need a background in geology to be able to understand this article. It is intended for the layperson. The first thing that you need to know about igneous rocks is that there are essentially two types of igneous rock: extrusive (volcanic) and intrusive (plutonic), which are characterized by both texture and composition. We will start off with a discussion of texture.
Phaneritic Texture: Intrusive igneous rocks produced from magma that has cooled and crystallized underground (e.g. granite) have a phaneritic texture, meaning a coarse-grained texture with crystals that are readily seen by the unaided eye. This is because intrusive igneous rocks cool and crystallize slowly underground, allowing them time to form larger crystals.
Extrusive igneous rocks that are produced by volcanic eruptions (e.g. rhyolite) can have one of four types of textures.
Aphanitic Texture: An aphanitic texture is the result of lava having been extruded out of a volcano cooling and crystallizing much faster than magma that has cooled and crystallized underground. The resulting crystals are extremely small–too small, in fact, to be seen with the unaided eye.
Porphyritic Texture: A porphyritic texture, results when some of the minerals have cooled and crystallized while the magma was still underground, thus producing larger crystals, while other minerals cooled and crystallized above ground after having been extruded from a volcano in the form of lava. The rock that crystallizes from lava above ground quickly cools into a surrounding matrix of rock with an aphanitic texture around the larger crystals, thus creating a texture of large crystals called phenocryst embedded in a matrix of tiny crystals, too small to be seen with the unaided eye. Porphyritic andesite or porphyritic diorite are examples of rocks with this kind of a texture. Also rhyolites and granites sometimes have a porphyritic texture.
Courtesy of Lynn S. Fichter, James Madison University, Harrisonburg, Virginia
Glassy Texture: If an igneous rock solidifies so quickly that it does not have time to form crystals, then it is said to have a glassy texture. Obsidian is an example of an igneous rock with this type of texture.
- Photo courtesy yananine of Flickr.com under Creative Commons license
Vesicular Texture: If a rock has a felsic mineral composition with a lower melting point, it tends to be very viscous. As a result of the high viscosity of these types of rocks (e.g. rhyolite), gas bubbles can get trapped in them as they cool and crystallize. When this happens these rocks end up having what is called a vesicular texture. Pumice is such a rock and is the vesicular form of rhyolite. Scoria, the vesicular form of basalt or andesite or sometimes other rocks, is another example of this type of texture.
Pumice (Vesicular Rhyolite)
- From my own collection
Basaltic magma has a low viscosity, but it has a higher melting point than felsic magma such as rhyolite, which means it cools much faster than the more viscous rhyolite once it comes in contact with air. As a result, the gas escaping from the basalt gets trap as the lava cools producing the vesicular texture characteristic of scoria as seen below.
Scoria (Vesicular Basalt)
These are the four textures that you will see in volcanic igneous rock.
Bowen’s Reaction Series
So why do some minerals in the magma in a volcano crystallize underground while others don’t crystallize until they are released from the volcano in the form of lava? Well, the answer to that question lies in Bowen’s reaction series (above). Early in the 20th century the petrologist, Norman L. Bowen wondered how it was possible to get so many different kinds of rocks from essentially the same magma. Well, there were several things he realized.
One of the things that Bowen realized was that as magma cools, crystals that form in the magma react with the surrounding magma and change in composition as they react. He also realized that some of these reactions were continuous, while others were discontinuous. Those in the discontinuous series describe the formation of the mafic minerals olivine, pyroxene, amphibole, and biotite mica. The strange thing about the discontinuous series is that at higher temperatures the magma crystallizes into the mineral olivine, but below a certain temperature the olivine would react with the remaining magma and change into pyroxene. As the temperature of the magma lowered even further, at specific temperature cut-off points, pyroxene reacting with the remaining magma changes into amphibole, and at a still lower temperature, amphibole into biotite mica. These discontinuous minerals are what is known as the mafic minerals because of their high iron/magnesium content.
In the continuous side of the series you have the calcium-rich plagioclase feldspars (e.g. anorthite) slowly changing into the sodium-rich feldspars (e.g. albite) as it reacts with the cooling magma surrounding it.
At a certain temperature, as can be seen in the diagram, the two series converge into one. At this point what we know as the felsic minerals high in silicates, sodium and aluminum are formed, i.e., orthoclase feldspar, muscovite mica, and quartz.
Now you will notice from the diagram that various minerals form different kinds of rock. For example, the mafic minerals pyroxene and calcium-rich plagioclase feldspar make up the mafic igneous rocks, basalt and gabbro. The only difference between basalt and gabbro is that basalt is an extrusive (volcanic) igneous rock, whereas gabbro is an intrusive (plutonic) igneous rock. Mineralogically they are identical. In fact, all of the intrusive igneous rocks have an extrusive igneous rock counterpart, as illustrated in the table that I’ve created below (Fig. 1).
(Fig. 1) Extrusive Igneous Rocks and Their Intrusive Counterparts
My own illustration.
Flow Characteristics of Volcanic Rocks
When a rock type has a flow characteristic that is low in viscosity (e.g basalt from crystallized pahoehoe lava) then it is said to be ‘thin’ and ‘runny’. When a rock type has crystallized from a felsic lava which is very viscous (e.g. rhyolite), then it is said to be ‘thick and ‘sticky’. As a rule, the higher that the silica content of a particular lava is, the more viscous or “sticky” it is. The chart below (Fig.2) gives you an idea of the silica content and the flow characteristics (i.e. viscosity) of two of the igneous rock type (i.e. felsic and mafic rock types.)
(Fig.2) Classification&Flow Characteristics of Volcanic Rocks
Now that I have explained the difference between mafic and felsic igneous rocks, I would like to turn your attention to the process known as differentiation. Mafic igneous rocks are much denser than felsic igneous rocks. This is why oceanic plates, made primarily of the mafic igneous rocks basalt and gabbro, are subducted below the much lighter continental plates, made primarily of the the less dense felsic igneous rocks, granite and rhyolite. Now intrusive magma can lie underground in a variety of different ways. When magma intrudes into already existing rock, we call these intrusions plutons. Below is an illustration of each type of pluton–dikes, sills, lacoliths (lenticular in shape) and the largest of all, batholiths.
Various Types of Plutons
Courtesy of the USGS
Now, say we have magma contained in a pluton like, for example, a sill. Our magma in this example contains both mafic and felsic constituents, pretty much uniformly distributed throughout the magma. Since mafic magmas crystallize at higher temperatures, they will be the first to crystallize. And because mafic minerals are denser than felsic minerals, the crystallized mafic minerals are going to tend to sink to the bottom of the sill. For example, pyroxene, which is a mafic mineral, will do this. So what happens is that you get symmetrically shaped crystals floating down to the bottom and piling up, leaving the less dense and lighter felsic magma up at the top of the sill. As a result of this process, when all of the magma in the sill has cooled and solidified, you end up with a rock that has mostly mafic minerals at the bottom and felsic minerals with a high silica content at the top. This process is what is known as differentiation, and it is a significant part of Bowen’s reaction series.
Stoping and Xenoliths
Many times when magma intrudes into existing rock, it works its way into fractures and crevices in the surrounding rock. As a result, chunks–sometimes large and sometimes small–of the solid host or source rock break away. These chunks are what is referred to as Xenoliths–the name derived from the Greek, meaning ‘strange rock’. The process by which these Xenoliths are separated from the host or source rock is known as stoping, which is illustrated below.
Similar to the process of differentiation, the stoped blocks tend to sink to the bottom of the magma chamber, as seen in the illustration above. When all of the magma has solidified and is expose by erosion or mining, the rocks look like the image below.
Courtesy of Ane Engvik, Geological Survey of Norway.
The image above is of granitoid xenoliths of gneissic and magmatic origin in coarse-grained syenite. The dimension of the xenoliths is evident from the size of the person dressed in red in the lower part of the image.
A wonderful paper put out by The Smithsonian/NASA Astrophysics Data System (ADS) entitled Xenolith incorporation, distribution, and dissemination in a mid-crustal granodiorite, Vega pluton, central Norway can be found by going to this site: The Smithsonian/NASA Astrophysics Data System (ADS) This is only the abstract of the paper, but the paper, in it’s entirety, is available to the public or at least I think it is.
Eclogite, Kimberlite and Lamproite
Even though this essay is about igneous rocks, I would be remiss, especially after talking about xenoliths, if I did not discuss the metamorphic rock, eclogite. Eclogite is a metamorphic rock very similar in composition to basalt. But the group of eclogites also includes igneous members that rise up through the crust in the form of plumes of intrusive igneous magma from the upper mantle. The metamorphic rock is created at great depths under extremely high temperatures and pressures. Because eclogite is so very dense, it tends to sink into the less dense upper mantle at convergent plate boundaries where oceanic plates are being subducted. It is believed by many petrologist that when basaltic oceanic plates return to the upper mantle by means of subduction, that the basalt by means of high-presure/high-temperature metamorphism is converted into eclogite. But this is a point of some contention.
The basic composition of eclogite consists primarily of garnet and sodium rich pyroxene with quartz, kyanite, and rutile sometimes present. The chemistry is rather complicated, but it is from the eclogites groups that we get pyrope, almandine, and grossular garnets. From the pyroxene in the eclogite we get the inosilicate minerals, jadeite and diopside.
If you’re into the technical stuff, here’s a wonderful paper written by Don L. Anderson of Caltech on the subject of eclogite : Eclogite.
Below are images of eclogite as well as a chart showing where eclogite fits into the scheme of the various types of metamorphism–all obtained from this wonderful site, Metamorphic Rocks Home Page.
The Various Kinds of Metamorphism
Kimberlite and Lamproite
Even though the magma that solidifies into kimberlite and lamproite are not the source of diamonds, diamonds are perhaps the most well-known mineral associated with these igneous rocks. Diamonds, which are formed at depths greater than 150 km (93 mi) in the Earth’s upper mantle merely hitch a ride, along with other minerals, with the magma that forms kimberlite and lamproite as it makes its way up towards the surface. At one time it was thought that only kimberlite became embedded with and carried diamonds to the surface. But since 1979 diamonds have been found in olivine lamproite. Most diamonds found in lamproite are of industrial grade, but about 5% are of high quality gemstone grade.
Because the magmas that form kimberlite and lamproite have such large amounts of water and carbon dioxide dissolved in them, the volcanoes that they produce are consequently highly explosive. The magma, which solidifies into kimberlite, makes its way to the surface through cracks and dikes. Because of the way kimberlite exploits cracks and dikes in existing rock, it is one of the magmas responsible for the process of stoping, mentioned above, which creates xenoliths from the existing rock. When it cools, kimberlite is one of several types of igneous rock that can become the groundmass in which the xenoliths are embedded, as illustrated in the photo above.
Only when the kimberlite producing magma gets close to the Earth’s surface does it begin to carve out a carrot-shaped pipe. This is because the gases of water vapor and carbon dioxide begin to expand in a region of the carrot-shaped pipe known as the root (i.e. as in the root of a carrot) located towards the bottom of the pipe. This creates a build up in pressure which eventually exceeds the strength of the roof rock overhead and causes an explosive eruption–much the same way soda water shoots out of a bottle when it is shaken and the pressure is released–that creates the carrot-shape of the pipe. Lamproite, on the other hand, carves out a bowl-shaped pipe during an eruption. That is why diamond mines in South Africa have that same morphology as these two pipe shapes of the respective kimberlite and lamproite volcanic eruptions.
Below is a photograph of a specimen of kimberlite. Unfortunately, I was not able to find a photograph of lamproite.
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Copyright © 2010 Eric F. Diaz