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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Other minerals may and almost certainly occur in these rocks as well but they have no significance in this classification scheme.

But, volcanic rocks usually have their crystallization interrupted by eruption and rapid cooling on the surface. Alumina Al 2 O 3 Saturation. General Classification of Igneous Rocks. Let’s imagine first a composition in the silica-undersaturated field.

As discussed earlier, texture is used to subdivide igneous rocks into two major groups: This diagram does not determine whether a ignelus is gabbro, diorite, or anorthosite.

To get an idea about what silica saturation means, let’s look at a simple iugs classification of igneous rocks system – the system Mg 2 SiO 4 – SiO 2.

On the other hand, if a magma is undersaturated with respect to silica, then a silica mineral should not precipitate from the magma, and thus should not be present in the rock. On a molecular basis, these rocks show: The composition usually reflects the composition of the magma, and thus provides information on the source of the iugs classification of igneous rocks.

That means that by pigeonholing a rock in a certain group we say something about its genesis or origin. But, this does not allow for a very detailed classification scheme.

We can easily the inadequacy of a mineralogical iugs classification of igneous rocks based on minerals present if you look at the classification schemes for volcanic rocks given in introductory geology textbooks. Please help improve this article by adding citations to reliable sources. The minerals present in a rock and their relative proportions in the rock depend largely on the chemical composition of the magma. There are various criteria that could be used to classify igneous rocks.

Because of the continuous variation in classificxtion composition there is no easy way to set limits within a classification scheme. Let’s first examine the classification of plutonic rocks. This page was last edited on 14 Novemberat Examples of Questions on this material that could be on an Exam Define the following terms: Rock texture iugs classification of igneous rocks to a large extent on cooling history of the magma.

The IUGS systematics of igneous rocks | Journal of the Geological Society

Note iugs classification of igneous rocks at each stage of the process, the classification may change, but it is important to fo in mind that each stage has limitations, and that classification at each stage is for the purposes of describing the rock, not only for the individual investigator, but anyone else. Note also that iugs classification of igneous rocks can cause some confusion in volcanic rocks that do not complete their crystallization due to rapid cooling on the surface.

Chemical analyses can be obtained, and a chemical classification, such as the LeBas et al. On the other hand, very alkaline rocks, that is those that plot well above the classififation line in the figure below, are also usually silica undersaturated.

General Classification of Igneous Rocks

Silica Undersaturated Rocks – In these rocks we should find minerals that, in general, do not occur with quartz. Igneous petrology Igneous rocks. Cooling to anywhere on the liquidus will result in the crystallization of Fo-rich olivine. Thus, the concept of alumina saturation is based on whether or not there is an excess or lack of Al to make up the feldspars.

If this liquid containing olivine is erupted and the rest of the liquid quenches to a glass, then this will produce a rock with phenocrysts of olivine in a glassy groundmass. For instance, the classificatiob of minerals is based on common anoins since minerals sharing common anions often have similar physical iugs classification of igneous rocks i.

A QAPF iugs classification of igneous rocks is a double ternary diagram which is used to classify igneous rocks based on mineralogic composition.

QAPF diagram

As noted in the sidebar to the left, this is largely a genetic classification based on the depth of origin of the rock volcanic at or near the surface, and plutonic at depth. Because such factors as cooling conditions, chemical composition of the original magma, and weathering iugs classification of igneous rocks, there is a potential to see an infinite variety of igneous rocks, and thus a classification scheme based solely on the description of the rock iugs classification of igneous rocks eventually lead to a plethora of rock names.

There are other criteria used to decide that. 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. Texture of the Rock. Applying the criteria above would suggest that this rock is also silica undersaturated, but we know it is not. Does this mean that the intro geology textbooks lie?

Other Minerals may or may not be present.

The IUGS systematics of igneous rocks

An exact name can be given only if the mineralogical composition is known, which cannot be determined in the field. But a rapidly cooled volcanic rock with the same composition as the granite could be entirely glassy and black colored i. Note that this diagram is not used for all plutonic rocks. For example, aphanitic rocks are or volcanic origin iugs classification of igneous rocks phaneritic rocks are plutonic. By using this site, you agree to the Terms of Use and Privacy Policy.

Most are what we term genetic.

In fact we generally use textural criteria to subdivide igneous rocks in to plutonic usually medium to coarse grained and volcanic usually fine grained, glassy, or porphyritic.

So to classifications in the earth sciences are designed to reduce complexity.