1 Sep In order to create a sustainable classification of igneous rocks which all geologists might use, an international body was set up by the IUGS: the. 18 Mar A classification of the phaneritic igneous rocks. b. Gabbroic rocks. c. Ultramafic rocks. After IUGS. 1/17/ Petrology-Spring , Goeke. basic system which classifies rocks on the basis of where they form. work by the IUGS Subcommission on the Classification of Igneous Rocks or Streckeissen .

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QAPF diagram

In the field, a simple field based classification must be used. Examples of Questions on this material that could be on an Exam Define the following terms: So to classifications in the earth sciences are designed to reduce complexity. Applying the criteria above for identifying silica undersaturated rocks would tell us that this is a silica-undersaturated rock, which we know to be correct. They are characterized by lack of an Al 2 O 3 -rich mineral and lack of sodic pyroxenes and amphiboles in the mode.

That means that by pigeonholing a rock in a certain group we say something about its genesis or origin. Please help improve this article by adding citations to reliable sources. The iugs classification of igneous rocks usually reflects the composition of the magma, and thus provides information on the source of the rock.

QAPF diagrams are mostly used to classify plutonic rocks phaneritic rocksbut are also used to classify volcanic rocks if modal mineralogical compositions have been determined. By using this site, you agree to the Terms of Use and Privacy Policy. Igneous rocks are classified on the basis of mineralogy, chemistry, and texture. Because they are solutions, their chemical composition can vary continuously within a range of iugs classification of igneous rocks. Unsourced material may be challenged and removed.

Because of iugs classification of igneous rocks limitations of the various criteria that can used to classify igneous rocks, geologists use an approach based on the information obtainable at various stages of examining the rocks. But, this does not allow for a very detailed classification scheme. Still we can divide rocks in general into felsic rocks those with lots of feldspar and quartz and mafic rocks those with lots of ferromagnesium minerals.


In such rocks, there is often glass or the minerals are too small to be readily identified. An iugs classification of igneous rocks name can be given only if the mineralogical composition is known, which cannot be determined in the field.

These are rocks that contain just enough silica that quartz does not appear, and just enough silica that one of the silica undersaturated minerals does not appear. Rock texture depends to a large extent on cooling history of the magma.

If this liquid containing olivine is erupted iugs classification of igneous rocks the rest of the liquid quenches to a clasaification, then this will produce a rock with phenocrysts of olivine in a glassy groundmass. This works well as a classification scheme if all of the minerals that could potentially crystallize from the magma have done so – usually the case for slowly cooled plutonic igneous rocks.

General Classification of Igneous Iugs classification of igneous rocks. The first and last of these terms are most easily seen. For our purposes, we will introduce and discuss a much simpler classification that will allow us to easily identify the more common igneous rocks.

Metaluminous rocks are those for which the molecular percentages are as follows: But, volcanic rocks usually have their crystallization interrupted by eruption and rapid cooling on the surface. A QAPF diagram is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition.

Examples iugs classification of igneous rocks Questions on this material that could be on an Exam. Silica Undersaturated Rocks – In these rocks we should find minerals that, in general, do not occur with quartz. There are various criteria that could be used to classify igneous rocks.

As discussed earlier, texture is used to subdivide clasaification rocks into two major groups: Once the rocks are brought back to the laboratory and thin sections can be made, these iugs classification of igneous rocks examined, mineralogical content can be more precisely determined, and refinements in the mineralogical and textural classification can be made. Igneous petrology Igneous rocks.

Minerals Present iugs classification of igneous rocks the Rock the mode. Next, let’s look at a igneos oversaturated composition, such as one just to the left of the point labeled ‘P’ in the diagram.


To get an idea about what silica saturation means, let’s look at a simple silicate system – the system Mg 2 SiO 4 – SiO 2 Note how compositions between Fo and En will end their crystallization with only Fo olivine and enstatite. Thus rocks with the same chemical composition and same minerals present could have widely different textures. Let’s first examine the classification of plutonic rocks. Magmas, from which all igneous rocks are derived, are complex liquid solutions.

QAPF diagram – Wikipedia

Thus, if we find any of these minerals in a rock, with an exception that we’ll see in a moment, then we can expect the rock to be silica undersaturated.

September Learn how and when to remove this template message. Iugs classification of igneous rocks classifications also seek to reduce complexity. Does this mean that the intro geology textbooks lie?

In fact we generally use textural criteria to subdivide igneous rocks in to plutonic usually iggneous to coarse grained and volcanic usually fine grained, glassy, or porphyritic. Texture of the Rock. Not really, these iugss the minerals that should igneouss from a dacite magma, but don’t because the crystallization history is interrupted by rapid cooling on the surface. Ultramafic rocks are the most important plutonic rocks that have separate classification diagrams.

Applying the criteria above would suggest that this rock is also silica undersaturated, but we know it is not. To get an idea about what silica iugs classification of igneous rocks means, let’s look at a simple silicate system – the system Mg 2 SiO 4 – SiO 2.

General Classification of Igneous Rocks

But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black colored i.

For example a granite consists of lots of quartz and feldspar and is generally light colored. Chemical composition of igneous rocks is the most distinguishing feature.