Differentiate Between Extrusive And Intrusive Rocks

Differentiating Extrusive and Intrusive Igneous Rocks: A Comprehensive Guide

Igneous rocks, formed from the cooling and solidification of molten rock (magma or lava), represent a fundamental building block of our planet's crust. Understanding their formation and characteristics is crucial for geologists and anyone interested in Earth's dynamic processes. A key distinction within the igneous rock family lies between extrusive and intrusion rocks, differentiated primarily by where they cool and solidify. This comprehensive guide will delve into the nuances of these two rock types, exploring their formation, textures, mineral compositions, and key distinguishing features.

Formation: The Defining Difference

The fundamental difference between extrusive and intrusive igneous rocks boils down to their cooling environment:

Extrusive Rocks: A Rapid Cooling Process

Extrusive rocks, also known as volcanic rocks, form when magma reaches the Earth's surface as lava, rapidly cools, and solidifies. This rapid cooling process, often occurring within days, weeks, or months, limits the time for mineral crystals to grow. As a result, extrusive rocks typically exhibit fine-grained or even glassy textures. The rapid cooling prevents the formation of large, easily visible crystals.

• Volcanic eruptions: The primary mechanism for extrusive rock formation. Eruptions can range from gentle lava flows to explosive pyroclastic events, each influencing the resulting rock's texture and composition.

• Lava flows: Extensive sheets of lava spreading across the landscape are a common feature of effusive eruptions. The rapid cooling of these flows generates characteristic textures.

• Pyroclastic flows: High-velocity, high-temperature currents of gas and volcanic debris can create a range of rock types, including welded tuffs and ash deposits. The rapid cooling in these flows often results in fine-grained rocks.

Intrusive Rocks: A Slow Cooling Symphony

Intrusive rocks, also called plutonic rocks, form when magma cools and solidifies slowly beneath the Earth's surface. This slow cooling process, which can take thousands or even millions of years, allows ample time for mineral crystals to grow large. Consequently, intrusive rocks typically display coarse-grained textures, with easily visible crystals.

• Magmatic intrusions: These represent the various ways magma intrudes into pre-existing rock formations. Examples include batholiths (massive, irregularly shaped intrusions), dikes (tabular intrusions that cut across pre-existing structures), sills (tabular intrusions that lie parallel to pre-existing structures), and laccoliths (lens-shaped intrusions that arch overlying strata).

• Slow cooling: The insulating effect of the surrounding rocks significantly slows the cooling rate, allowing ample time for crystal growth and the development of a coarse-grained texture.

• Deep subsurface environments: Intrusive rocks are typically found deep beneath the Earth's surface, often only exposed through uplift and erosion over geological timescales.

Texture: A Tale of Cooling Rates

The texture of an igneous rock is a crucial indicator of its formation environment. This directly relates to the cooling rate, influencing the size and arrangement of mineral crystals:

Extrusive Rock Textures:

• Aphanitic: A fine-grained texture, characterized by mineral crystals too small to be seen with the naked eye. This indicates rapid cooling. Examples include basalt and andesite.

• Glassy: An amorphous texture, lacking visible crystals. This rapid cooling is so fast that mineral crystals cannot form. Obsidian is a classic example.

• Porphyritic: A texture with larger crystals (phenocrysts) embedded in a finer-grained matrix (groundmass). This suggests a two-stage cooling process: initial slow cooling allowing large crystal growth, followed by rapid cooling resulting in the finer matrix. Porphyritic textures can occur in both extrusive and intrusive rocks.

• Vesicular: A texture characterized by holes or vesicles, formed by escaping gases during the cooling process. This is common in extrusive rocks like pumice and scoria.

Intrusive Rock Textures:

• Phaneritic: A coarse-grained texture, where individual mineral crystals are easily visible to the naked eye. This indicates slow cooling. Granite and gabbro are prime examples.

• Pegmatitic: An exceptionally coarse-grained texture, with crystals often exceeding several centimeters in size. These form under conditions of extremely slow cooling and often contain rare minerals.

• Porphyritic: As mentioned earlier, porphyritic textures can also be found in intrusive rocks, reflecting a change in cooling rates during their formation.

Mineral Composition: A Window into Magma Chemistry

The mineral composition of igneous rocks is largely determined by the chemical composition of the parent magma. While both extrusive and intrusive rocks can exhibit a wide range of compositions, certain minerals are more commonly associated with one type than the other:

Common Minerals in Extrusive Rocks:

• Plagioclase feldspar: A common mineral found in many extrusive rocks, particularly basalts and andesites.

• Pyroxene: A group of silicate minerals often present in mafic extrusive rocks.

• Olivine: A magnesium-iron silicate mineral abundant in basalts.

• Amphibole: Another group of silicate minerals found in intermediate and mafic extrusive rocks.

• Quartz: While less common than in felsic intrusive rocks, quartz can be found in some rhyolitic extrusive rocks.

Common Minerals in Intrusive Rocks:

• Quartz: A major constituent of felsic intrusive rocks like granite.

• Orthoclase feldspar: A potassium-rich feldspar commonly found in granite.

• Plagioclase feldspar: Also present in many intrusive rocks, but often in different proportions than in extrusive rocks.

• Biotite mica: A dark-colored mineral common in many granite varieties.

• Muscovite mica: A lighter-colored mica that may be found in some granites.

• Amphibole: Found in various intrusive rocks, particularly those of intermediate composition.

Distinguishing Features: A Summary Table

Beyond the Basics: Exploring Specific Rock Types

Let's delve into a few examples of specific extrusive and intrusive rock types to solidify our understanding:

Extrusive Examples:

• Basalt: A dark-colored, mafic extrusive rock, abundant in oceanic crust and volcanic regions worldwide. Its rapid cooling gives it a fine-grained texture.

• Andesite: An intermediate extrusive rock, often found in volcanic arcs associated with subduction zones. It's typically gray to dark gray in color.

• Rhyolite: A felsic extrusive rock, often light-colored and glassy or very fine-grained. It is the extrusive equivalent of granite.

• Obsidian: A volcanic glass formed by the rapid cooling of viscous lava. Its lack of crystalline structure gives it a smooth, glassy texture.

• Pumice: A highly vesicular extrusive rock, so light that it can float on water. The numerous vesicles are a result of escaping gases during cooling.

Intrusive Examples:

• Granite: A light-colored, felsic intrusive rock, known for its coarse-grained texture and abundance of quartz and feldspar. It's a common component of continental crust.

• Gabbro: A dark-colored, mafic intrusive rock, the intrusive equivalent of basalt. It has a coarse-grained texture and is often found in oceanic crust.

• Diorite: An intermediate intrusive rock, with a composition intermediate between granite and gabbro. Its mineral composition varies, reflecting its intermediate nature.

• Pegmatite: An exceptionally coarse-grained intrusive rock, known for its large crystals, some reaching exceptional sizes. They form under very specific conditions of slow cooling and often contain rare minerals.

Applications and Significance

The study of extrusive and intrusive rocks is fundamental to various geological disciplines:

• Understanding plate tectonics: The distribution of igneous rocks, particularly their type and location, provide critical insights into plate boundaries and tectonic processes.

• Volcanic hazard assessment: Understanding the formation of extrusive rocks helps in assessing volcanic hazards and predicting future eruptions.

• Resource exploration: Igneous rocks often host valuable mineral deposits, and understanding their formation aids in the exploration of these resources.

• Geochronology: The isotopic dating of igneous rocks is crucial for determining the age of various geological formations and events.

Conclusion

The distinction between extrusive and intrusive igneous rocks is a cornerstone of petrology and geology. By understanding their formation, textures, and mineral compositions, we gain invaluable insights into Earth's dynamic processes. Their contrasting characteristics offer a window into the diverse conditions under which magma cools and solidifies, shaping the landscapes and resource potential of our planet. This understanding is crucial not only for geological research but also for managing resources, assessing hazards, and appreciating the complexity of our Earth's history.