What is the Difference Between Plutonic and Volcanic Rocks

The main difference between plutonic and volcanic rocks is that plutonic rocks form under the surface of Earth, whereas volcanic rocks form upon exposure to air.

There three major types of rocks on Earth, which are categorized depending on the source of rock formation. These are the sedimentary rocks, igneous rocks, and metamorphic rocks. When considering igneous rocks, we can classify them into two groups as plutonic and volcanic rocks, based on their location and source.

Key Areas Covered

1. What are Plutonic Rocks
     – Definition, Characteristics, Formation
2. What are Volcanic Rocks
     – Definition, Characteristics, Formation
3. What is the Difference Between Plutonic and Volcanic Rocks
     – Comparison of Key Differences

Key Terms

Plutonic Rocks, Volcanic Rocks

What are Plutonic Rocks

Plutonic rocks are one of two types of igneous rocks that form under the surface of Earth crust. They are also known as intrusive igneous rocks. These rocks form from magma that solidifies by slowly cooling below the surface of Earth. In general, we can call them “pluton”. Furthermore, we can categorize plutonic rocks into two groups by observing if they are tabular or not. We can also classify them further depending on the shape of the rock.

Tabular plutonic rocks have magma filled in a fracture or another plane that is weak. Non-tabular plutonic rocks are larger than tabular rocks. The formation of a plutonic rock underneath the Earth’s surface requires a very long time (ten to thousands of years or more). This is because magma cools very slowly. It allows individual magma crystals to grow large via coalescing. Therefore, we can observe that plutonic rocks are coarse-grained rocks.

The main way to identify plutonic rocks is observing tightly packed mineral grains that have a medium to large grain size, typically ranging from 1 to 5 mm. These rocks have a phaneritic structure. Moreover, these grains are usually of the same size. We call it an equigranular mixture. In construction fields, these rocks are named as commercial granite. In addition, plutonic rocks are among the most common type of rocks on Earth’s crust.

What are Volcanic Rocks

Volcanic rocks are a type of igneous rocks that form upon exposure to air. These rocks form from lava erupted from volcanoes. Therefore, it has a volcanic origin. Volcanic rocks are among the most common rock types on Earth’s crust. Particularly, these rocks occur in oceans. On the ground, these rocks are common in plate boundaries. According to different estimations, there are about 8% volcanic rocks covering the Earth’s land surface.

Settling of lava, tephra, volcanic bomb, lapilli, and volcanic ash cause the formation of volcanic rocks. Generally, volcanic rocks are fine-grained structures. Often, these rocks contain clasts or other rocks. Moreover, these rocks have a vesicular texture caused by voids left when volatile components trapped in molten lava leaves at the step of solidification.

Most volcanic rock types contain different minerals, such as silica (SiO2). In fact, more evolved volcanic rocks contain a high mineral content. We can identify a volcanic rock depending on the chemical composition and texture. For example, basalt is a common volcanic rock type with a low silica content, while rhyolite is a volcanic rock with high silica content. Another type of volcanic rocks is pyroclastic rocks, which forms from explosive volcanism.

Difference Between Plutonic and Volcanic Rocks

Definition

Plutonic rocks are a type of igneous rocks that form under the surface of Earth crust, while volcanic rocks are a type of igneous rocks that form upon exposure to air.

Formation

Plutonic rocks form underneath the Earth crust while volcanic rocks form above ground. 

Mineral Content

While plutonic rocks have a low mineral content, volcanic rocks have a high mineral content. 

Grain Structure

Moreover, plutonic rocks have medium to large grains while volcanic rocks have fine grains. 

Solidification

Plutonic rocks solidify due to slow cooling underneath the Earth while volcanic rocks solidify due to exposure to air.

Conclusion

We can classify igneous rocks into two groups as plutonic and volcanic rocks, based on their source and location. The main difference between plutonic and volcanic rocks is that plutonic rocks form under the surface of Earth, whereas volcanic rocks form upon exposure to air.

Reference:

1. Alden, Andrew. “All About Plutonic Rocks.” ThoughtCo, Feb. 11, 2020, Available here.
2. “Plutonic Rock.” Plutonic Rock – an Overview | ScienceDirect Topics, Available here.
3. “Volcanic Rock.” Wikipedia, Wikimedia Foundation, 18 Mar. 2020, Available here.

Image Courtesy:

1. “NotchPeak” By Qfl247 at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia
2. “Aa next to pahoehoe lava at Craters of the Moon NM-750px” (CC BY-SA 3.0) via Commons Wikimedia