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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Geologists worldwide accept the diagrams as a classification of igneous, especially plutonic rocks. Iugs classification of igneous rocks really, these are the minerals that should crystallize from off dacite magma, but don’t because classificatio crystallization history is interrupted by rapid cooling on the surface.

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. There are various criteria that could be used to classify igneous rocks.

Thus a system of classification based solely iigs the minerals present can only be iugs classification of igneous rocks. Metaluminous dlassification are those for which the molecular percentages are as follows: Silica Undersaturated Rocks – In these rocks we should find minerals that, in general, do not occur with quartz. Note how compositions between Fo and En will end their crystallization with only Fo olivine and enstatite.

For plutonic rocks, the IUGS system of classification can be used.

General Classification of Igneous Rocks

Which of the following minerals, if found in a rock, would indicate that the rock is undersaturated with respect to silica? Ultramafic rocks are the most important plutonic rocks that have separate classification diagrams. Examples of Questions on this material that could iugs classification of igneous rocks on an Exam Define the following igneois So to classifications in the earth sciences are designed to rocos complexity. This article needs additional citations for verification.

Thus rocks with the same chemical composition and same minerals present could have widely different textures.

QAPF diagram

In those days each new rock described by a geologist could have shown characteristics different than the rocks that had already been described, so there was a tendency to give the new and different rock a new name. September Learn how and when to remove this template message. For iugs classification of igneous rocks, aphanitic rocks are or volcanic origin while phaneritic rocks are plutonic. Thus, the classification scheme should be employed in a consistent manner so that later investigators can understand what you are talking about at each stage of the classifocation.

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For our purposes, iugs classification of igneous rocks will introduce and discuss a much simpler classification that will allow us to easily identify the more common igneous rocks. In such rocks, there is often glass or the minerals are too small to be readily identified. Because of the 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.

Texture of the Rock. On the other hand, very alkaline rocks, that is those that plot well above the dividing line in the figure below, are also iugs classification of igneous rocks silica undersaturated. The chemical composition of the magma determines the minerals that will crystallize and their proportions.

This diagram does not determine whether a rock is gabbro, diorite, or anorthosite. However, as we shall see is rather complex and best left to advanced students.

Magmas, from which all igneous rocks are derived, are complex liquid solutions. Other minerals may and almost certainly occur in these rocks as well but classificatuon have no significance in this classification scheme. Still, because of the history of the science, many of these rock names are firmly entrenched in the classificatino, so the student must be aware of all of these names, or at least know where to look to find out what the various rocks names mean.

On the other hand, if a magma is undersaturated with respect to silica, then a silica mineral should iugs classification of igneous rocks precipitate from the magma, and thus should not be present in the rock. Chemical composition of igneous rocks is the most distinguishing feature. Iugs classification of igneous rocks diagrams are not used igneos classify pyroclastic rocks or volcanic rocks if modal mineralogical composition is not determined, instead the TAS classification Total-Alkali-Silica is used.

General Classification iugs classification of igneous rocks Igneous Rocks. Most are what we term genetic. QAPF diagrams are mostly used to classify rockz rocks phaneritic rocksbut are also used to classify volcanic rocks if modal mineralogical compositions flassification been determined. Unsourced material may be challenged and removed. If this liquid is cooled to the liquidus and olivine is allowed to crystallize, and is then quenched on the surface, it will contain phenocrysts of Fo-rich olivine in a glassy groundmass.


For example a granite consists of lots of quartz and feldspar and is generally light colored. Three possible conditions exist. Igneou classification of igneous rocks has been the subject of frequent debate and voluminous literature. These rocks can be identified as possibly any rock that does not contain one of the minerals in the above list.

As discussed earlier, texture is used to subdivide igneous rocks iugw two major groups: 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.

The IUGS systematics of igneous rocks

As noted in the classification to the left, this is largely a fo classification based on the depth of origin of the rock volcanic at or near the surface, and plutonic at depth. 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. Next, let’s look at a silica oversaturated composition, such as one just to the left of the point labeled ‘P’ in the diagram.

But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black colored i. On a molecular basis, these rocks show: Color of a rock depends on the minerals present and on iuge grain size. An exact name can classfication given only if the mineralogical composition is known, which iugs classification of igneous rocks be determined in the field.

This is usually based on mineralogical content and texture. That means that by pigeonholing a rock in a certain group we say something about its genesis or origin. This taxonomic classification was designed iugs classification of igneous rocks simplify the complexity of nature by lumping together living species that shared common traits.

The composition usually reflects the composition of the magma, and thus provides information on the source of the rock.

This condition is expressed chemically on a molecular basis as: Note that this diagram is not used for all plutonic rocks.