Classification of Plutonic Rocks

In geology, a pluton is a body of trespassing igneous rock (known as a plutonic rock) which is crystallized from magma steadily cooling beneath the Earth’s surface. Plutons include batholiths, dikes, stocks, sills, lopoliths, laccoliths, and other igneous creations. 

In practice, “pluton” generally implies a peculiar mass of igneous rock, essentially several kilometres in dimension, non-existing with a tabular, or flat, shape like those of dikes and sills.

Plutonic Rocks Examples

Examples of plutonic rocks include Cuillin in Skye, Cardinal Peak in Washington State, Denali in Alaska, Mount Kinabalu in Malaysia; and Stone Mountain in the United State of Georgia.

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The most common rock types in plutons include monzonite, granite, granodiorite, tonalite, and quartz diorite. Usually, coarse-grained, light coloured plutons of these compositions are called granitoid.

Classification of Igneous Rocks

Classification of igneous rocks is actually one of the confusing facts of geology. This is partly because of the historical reasons, partly because of the nature of magmas, and partly because of the different benchmark that could potentially be used to classify rocks. That being said, let’s find out what the names of the different rocks mean.

Criteria of Classification of Igneous Rocks

There are different benchmarks that could be used to classify igneous rocks. Among many of them, various are:

1. Minerals Present in the Rock: 

The minerals in a rock and their corresponding proportions in the rock depend highly on the chemical composition of the magma. This further works well as a classification scheme if all of the minerals that could presumably crystallize from the magma have done so - generally the case for steadily cooled plutonic igneous rocks.  But, volcanic rocks generally have their crystallization interfered with the explosion and quick cooling on the surface.  In such rocks, there are most commonly minerals or glass which are too small to be readily determined. Hence, a system of classification based entirely on the minerals present can only be used.

2. Texture of the Rock: 

to a large extent, rock texture depends on the cooling history of the magma. Hence, rocks with similar minerals present and chemical composition could have largely different textures. In fact, we usually use the textural criteria to subdivide igneous rocks into plutonic (generally moderate to coarse-grained) and volcanic (generally glassy, fine-grained, or porphyritic) varieties.

3. Colour: 

The colour of rocks typically depends on the minerals it contains in addition to their grain size. Usually, rocks that contain ample quartz and feldspar are light-coloured, and rocks that consist of abundant amphiboles (ferromagnesium minerals), olivines, and pyroxenes are dark-coloured. However, remember that colour can be misleading when applied to rocks of similar composition but different grain size.

For example, granite contains a large amount of quartz and feldspar and is usually light-coloured. But a rapidly cooled volcanic rock with a similar composition as the granite could be completely glassy and black coloured (i.e. obsidian). Still, we can divide rocks in general into felsic rocks and mafic rocks.

4. Chemical Composition: 

Chemical composition of igneous rocks is a very distinctive feature. The composition generally reflects the composition of the magma, and thus offers information on the source of the rock. The chemical composition of the magma identifies the minerals which will crystallize and their proportions.

A set of hypothetical minerals which can crystallize from a magma having similar chemical composition as the rock (known as the Norm), can expedite comparison between rocks. Still, since chemical composition can vary continuously, there are several natural breaks to expedite divisions between different rocks. Chemical composition cannot be easily identified in the field, making classification based on chemistry somewhat impractical.

Magmas, from which all igneous rocks are extracted, are compounded liquid solutions. Since they are solutions, their chemical composition can differ incessantly within a range of compositions. Due to an ongoing fluctuation in chemical composition, there is no simple way to set limits within a classification scheme.