What are 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.
FAQs on Classification of Plutonic Rocks
1. What are plutonic rocks and how are they fundamentally different from volcanic rocks?
Plutonic rocks, also known as intrusive igneous rocks, are formed when magma cools and solidifies slowly deep beneath the Earth's surface. The primary difference from volcanic (extrusive) rocks lies in their formation environment. Plutonic rocks cool underground over thousands of years, while volcanic rocks form from lava that erupts and cools rapidly on the surface. This difference in cooling rate is the main reason for their distinct textures.
2. What are the main criteria used for the classification of plutonic rocks?
The classification of plutonic rocks is primarily based on two key criteria:
- Mineral Composition: This is the most common method, focusing on the percentage of silica (SiO₂) present. Rocks are categorised along a spectrum from felsic (high silica) to mafic and ultramafic (low silica).
- Texture: This refers to the size, shape, and arrangement of the mineral crystals. Plutonic rocks are typically characterised by their coarse-grained (phaneritic) texture.
3. How does silica content influence the types of plutonic rocks?
Silica (SiO₂) content is a crucial factor in classifying plutonic rocks as it determines their mineralogy and colour. The classification is generally as follows:
- Felsic Rocks: These have high silica content (>65%), are light in colour, and include rocks like Granite.
- Intermediate Rocks: With moderate silica content (55-65%), they have a mix of light and dark minerals. Diorite is a common example.
- Mafic Rocks: These are low in silica (45-55%), rich in magnesium and iron, and dark in colour. Gabbro is a prime example.
- Ultramafic Rocks: With very low silica content (<45%), these are the darkest plutonic rocks, such as Peridotite.
4. What are some common examples of plutonic rocks and their typical uses?
Some of the most common examples of plutonic rocks include:
- Granite: A light-coloured, coarse-grained rock rich in quartz and feldspar. Due to its durability and aesthetic appeal, it is widely used for kitchen countertops, flooring, and building facades.
- Gabbro: A dark, coarse-grained rock, chemically equivalent to basalt. It is often used as crushed stone for road construction and concrete aggregate.
- Diorite: An intermediate rock with a salt-and-pepper appearance. It is very hard and is used for paving blocks and as a dimension stone in large buildings.
5. Why do plutonic rocks develop a coarse-grained texture?
Plutonic rocks develop a coarse-grained (phaneritic) texture because the magma from which they form is insulated by the surrounding rock deep within the Earth's crust. This insulation causes the magma to cool extremely slowly over many thousands or even millions of years. This extended cooling period allows the mineral crystals ample time to grow large enough to be easily visible to the naked eye.
6. What is the importance of the IUGS (QAPF) classification for plutonic rocks?
The IUGS (International Union of Geological Sciences) classification provides a standardised, quantitative method for naming plutonic rocks. It uses the QAPF diagram, which plots the relative percentages of four key mineral groups: Quartz, Alkali feldspar, Plagioclase feldspar, and Feldspathoid. This system is crucial for geologists because it allows for precise and universally understood rock identification, moving beyond simple field observations to a classification based on specific mineralogical data.
7. Can two rocks, one plutonic and one volcanic, have the same chemical composition but different names?
Yes, absolutely. This is a key concept in igneous petrology. A plutonic rock and a volcanic rock can be chemical equivalents, meaning they originated from the same type of molten magma but cooled in different environments. For example:
- Granite is a plutonic rock that cools slowly underground.
- Rhyolite is its volcanic equivalent, which cools quickly on the surface.
They share a similar felsic composition but are given different names because their textures (and therefore their formation histories) are completely different.
8. Where do hypabyssal rocks fit in the classification of igneous rocks?
Hypabyssal rocks represent an intermediate case between plutonic and volcanic rocks. They are formed when magma cools at a medium depth below the surface, in structures like dikes or sills. Their cooling rate is faster than plutonic rocks but slower than volcanic rocks. This results in a texture that is typically finer than granite but coarser than basalt, often a medium-grained or porphyritic texture (large crystals in a finer groundmass). Dolerite (or diabase) is a classic example of a hypabyssal rock.
