Decoding the Earth’s Magmatic Mysteries: Unraveling the Distinction Between Subvolcanic and Plutonic Rocks

Getting Started

Magmatism is a fundamental process in Earth science that plays a critical role in the shaping of the Earth’s crust and the formation of igneous rocks. Igneous rocks are classified based on their texture, mineral composition, and mode of formation. Two major classifications of igneous rocks are subvolcanic rocks and plutonic rocks. While they share some similarities, there are distinct differences between these two rock types. In this article, we will explore the characteristics and processes of formation of subvolcanic and plutonic rocks, shedding light on their unique features.

Subvolcanic rocks

Subvolcanic rocks, also known as hypabyssal rocks, are formed when molten magma solidifies below the Earth’s surface but relatively close to it. These rocks are considered intermediate between volcanic and plutonic rocks in terms of depth of formation. Subvolcanic rocks typically have a fine-grained texture due to the relatively rapid cooling of the magma at shallow depths. This rapid cooling results in the formation of small mineral crystals that are not easily visible to the naked eye.

One of the most common subvolcanic rocks is diabase, which consists primarily of plagioclase feldspar and pyroxene minerals. Diabase often forms dikes or sills, which are tabular intrusions that cut across existing rock layers. These intrusions are formed when magma is injected into fractures in the surrounding rock and solidifies. Subvolcanic rocks such as diabase may also contain other minerals such as quartz, olivine, or amphibole, depending on the specific composition of the magma.

Plutonic rocks

Plutonic rocks, also known as intrusive rocks, are formed from magma that cools and solidifies deep within the Earth’s crust. Unlike subvolcanic rocks, plutonic rocks form at great depths and are characterized by a coarse-grained texture. The slow cooling of magma deep underground allows the growth of larger mineral crystals that are visible to the naked eye. This results in the typical granular appearance of plutonic rocks.

Granite is one of the most common plutonic rocks. It is composed primarily of quartz, feldspar, and mica minerals. Granite forms massive bodies called plutons that can range in size from a few meters to several kilometers in diameter. These plutons are often exposed at the Earth’s surface by erosion and weathering of the overlying rocks. Other examples of plutonic rocks include gabbro, syenite, and peridotite, each with its own unique mineral composition.

Differences between subvolcanic and plutonic rocks

Although subvolcanic and plutonic rocks are both classified as igneous rocks, there are several important differences between them. The first and most important difference is the depth of their formation. Subvolcanic rocks form at relatively shallow depths, closer to the Earth’s surface, while plutonic rocks form at much greater depths within the Earth’s crust. This difference in depth results in differences in cooling rates and consequently in the texture and grain size of the rocks.

Another difference is cooling time. Subvolcanic rocks cool relatively quickly due to their proximity to the surface, resulting in fine-grained textures. On the other hand, plutonic rocks cool slowly over a longer period of time, allowing for the growth of larger mineral crystals and coarse-grained textures.

The final difference lies in their mode of occurrence. Subvolcanic rocks are often found as small intrusions, such as dikes or sills, that cut across existing rock formations. Plutonic rocks, on the other hand, form large, solid bodies called plutons that can be exposed at the surface by erosion and uplift.

Conclusion

In summary, subvolcanic rocks and plutonic rocks are two different types of igneous rocks that differ in their formation processes, depth of formation, texture, and mode of occurrence. Subvolcanic rocks form closer to the Earth’s surface and have fine-grained textures, while plutonic rocks form at greater depths and have coarse-grained textures. Understanding these differences is essential for the study of Earth’s igneous processes, as well as for geological exploration and interpretation of rock formations in different regions.

FAQs

What is the difference between subvolcanic rocks and plutonic rocks?

Subvolcanic rocks and plutonic rocks are both types of igneous rocks that form beneath the Earth’s surface, but they differ in their specific characteristics and formation processes.

How do subvolcanic rocks form?

Subvolcanic rocks form when magma rises towards the Earth’s surface but gets trapped in the crust before it reaches the surface. The magma cools and solidifies underground, resulting in the formation of subvolcanic rocks.

How do plutonic rocks form?

Plutonic rocks, also known as intrusive rocks, form when magma cools and solidifies deep within the Earth’s crust. The slow cooling process allows the minerals in the magma to crystallize over a long period of time, resulting in the formation of coarse-grained plutonic rocks.

What is the texture of subvolcanic rocks?

Subvolcanic rocks generally have a fine-grained or porphyritic texture. The fine-grained texture is a result of relatively rapid cooling near the Earth’s surface, while porphyritic texture indicates that the rock contains larger crystals (phenocrysts) embedded in a fine-grained matrix.

What is the texture of plutonic rocks?

Plutonic rocks typically have a coarse-grained texture due to their slow cooling process deep within the Earth’s crust. The slow cooling allows larger mineral grains to form, resulting in the coarse-grained appearance of plutonic rocks.

Can you provide examples of subvolcanic rocks?

Examples of subvolcanic rocks include diabase, basaltic andesite, and andesite. These rocks are commonly found in dikes, sills, and volcanic necks, which are formed by the solidification of magma beneath the Earth’s surface.

Can you provide examples of plutonic rocks?

Examples of plutonic rocks include granite, diorite, and gabbro. These rocks are often associated with large intrusive bodies such as batholiths and stocks, which are formed by the solidification of magma at considerable depths within the Earth’s crust.