Volcanic Rocks vs Plutonic Rocks: What’s the difference?

Earth has two types of rocks in terms of how they were formed. They are either sedimentary rocks or igneous rocks.

Sedimentary rocks are made from the deposits of eroded materials that have been cemented together over time. On the other hand, igneous rocks are made from the solidification of molten lava or magma.

Igneous rocks are further classified into two types: plutonic and volcanic. Plutonic rocks are formed when the magma cools slowly inside the Earth.

Volcanic rocks, on the other hand, are formed when the magma cools quickly on the Earth’s surface.

Magma from a Volcano

The main difference between volcanic and plutonic rocks is where they are formed. Volcanic rocks are formed on the Earth’s surface while plutonic rocks are formed inside the Earth.

But it isn’t just the location of where they are formed that separates these two types of igneous rocks. The process of how they are formed is also different.

In this post, we will compare and contrast these two types of rocks in more detail so that you can have a better understanding of their key differences.

Volcanic Rocks

Volcanic rocks are formed when the magma cools quickly on the Earth’s surface. This can happen in a number of ways.

The most common way is through an explosive volcanic eruption. When the magma erupts from the volcano, it finds itself in contact with the much cooler air. This causes the magma to cool quickly and solidify into rock.

volcanic rocks
Volcanic rocks

Since the magma cools quickly, it doesn’t have time to crystallize. This results in volcanic rocks having a lower density and a more porous structure than plutonic rocks that we will mention further.

Volcanic rocks can also be formed by the lava flows from a less explosive eruption. When the magma is not explosive, it will slowly ooze out of the volcano. As it flows down the side of the volcano, it will cool and solidify into rock.

The last way that volcanic rocks can be formed is through the process of hydrothermal alteration.

This happens when the magma comes into contact with the groundwater. The heat from the magma will cause the water to evaporate and leave behind minerals that will eventually solidify into rock.

Basalt and gabbros are two examples of common volcanic rocks.

Basalt is a dark-colored, fine-grained igneous rock that is typically composed of plagioclase and pyroxene minerals.

Gabbros are also dark-colored, fine-grained igneous rocks but they are typically composed of plagioclase, pyroxene, and olivine minerals.

Plutonic Rocks

Plutonic rocks are formed when the magma cools slowly inside the Earth. The magma will slowly make its way to the surface but will never reach it.

plutonic rock close up
Close up to a plutonic rock

As the magma cools, it will begin to crystallize. This process can take millions of years. During this time, the magma will slowly solidify into rock.

Since the magma cools slowly, it has time to crystallize. This results in plutonic rocks having a higher density and a less porous structure than volcanic rocks.

Granite and diorite are two examples of common plutonic rocks.

Granite is a light-colored, coarse-grained igneous rock that is typically composed of quartz, feldspar, and mica minerals.

Diorite is also a light-colored, coarse-grained igneous rock but it is typically composed of plagioclase, hornblende, and augite minerals.

What are the differences between volcanic rocks and plutonic rocks?

Although the difference between these two types of rocks may seem small, they have a big impact on the Earth. The most significant difference between them is where they are formed.

Here is a summary of the key differences between these two types of rocks:

Formation

Volcanic rocks are formed when the magma cools quickly on the Earth’s surface.

While plutonic rocks are formed when the magma cools slowly inside the Earth. The difference in how these rocks are formed results in some key physical differences that we will explore next.

Location

Volcanic rocks are found on the Earth’s surface while plutonic rocks are found inside the Earth.

Since plutonic rocks are formed deep inside the Earth, they are not exposed to the elements and are not weathered as much as volcanic rocks.

Composition

Volcanic rocks are made up of minerals that have not had time to crystallize while plutonic rocks are made up of minerals that have had time to crystallize.

Crystallization is a process where the minerals in the magma form distinct crystals. The rocks that result from this process have a higher density and are less porous than those that do not go through this process.

Grain Structure

The grain size is the average size of the crystals in the rock. Volcanic rocks have a smaller grain size than plutonic rocks due to faster cooling rates.

Much slower cooling rates allow for the growth of larger crystals, resulting in a coarser-grained texture as seen in plutonic rocks.

Porosity

Porosity is the percentage of empty space in a rock. Volcanic rocks have a higher porosity than plutonic rocks because they cool quickly and do not have time to crystallize.

volcanic rock with void space
Volcanic rock with a large void space

The lack of time for crystallization results in rock with more empty space.

Mineral Content

Volcanic rocks typically have a lower mineral content than plutonic rocks. This is because the magma that forms volcanic rocks cools quickly and doesn’t have time to collect all of the minerals from the surrounding rocks.

On the other hand, plutonic rocks have a higher mineral content because the magma that forms them cools slowly and has time to collect minerals from the surrounding rocks.

Color

Both volcanic and plutonic rocks can be found in a variety of colors. However, volcanic rocks are typically dark-colored. On the other hand, plutonic rocks are usually lighter colored than volcanic rocks.

Although the color is not a reliable way to distinguish between these two types of rocks, it can be helpful in some cases.

Structure

Volcanic rocks have a lower density and are more porous than plutonic rocks. This is because the magma that forms volcanic rocks cools quickly, which does not allow for the formation of large crystals.

The quick cooling also results in a more glassy texture and a lower overall density. In contrast, plutonic rocks have a higher density because the magma that forms them cools slowly, allowing for the formation of large crystals.

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