Which Of The Following Is Igneous Rock

Which of the Following is Igneous Rock? A Deep Dive into Igneous Rock Formation and Identification

Identifying igneous rocks requires understanding their unique formation process and characteristics. This comprehensive guide will explore the fascinating world of igneous rocks, helping you confidently distinguish them from other rock types. We'll delve into the processes that create them, their diverse textures and compositions, and provide clear examples to solidify your understanding.

Understanding Igneous Rock Formation: From Magma to Mountain

Igneous rocks are formed from the cooling and solidification of molten rock, known as magma or lava. This process, called igneous rock formation, is a fundamental aspect of the rock cycle, shaping Earth's landscapes for billions of years. Let's break down the key aspects:

Magma: The Molten Heart of Igneous Rocks

Magma is a complex mixture of molten silicate minerals, dissolved gases, and crystals. It originates deep within the Earth's mantle and crust, where temperatures are high enough to melt rock. The composition of magma is crucial in determining the type of igneous rock that will eventually form. Variations in chemical composition lead to the formation of diverse igneous rocks with varying mineral content and physical properties.

Intrusive vs. Extrusive Igneous Rocks: A Tale of Two Cooling Rates

The cooling rate of magma significantly influences the texture and mineralogy of the resulting igneous rock. This leads to the two primary categories:

Intrusive Igneous Rocks: Slow Cooling, Large Crystals

Intrusive igneous rocks, also known as plutonic rocks, form when magma cools and solidifies slowly beneath the Earth's surface. This slow cooling process allows ample time for large crystals to grow, resulting in a coarse-grained texture. Examples of intrusive igneous rocks include:

• Granite: A very common and well-known intrusive rock, often light-colored with visible quartz, feldspar, and mica crystals. Granite is used extensively in construction and countertops.
• Gabbro: A dark-colored intrusive rock, typically rich in plagioclase feldspar and pyroxene. It's often found in large intrusive bodies called batholiths.
• Diorite: An intermediate igneous rock in composition, possessing a mix of plagioclase feldspar and hornblende. Its color can vary, depending on the mineral proportions.
• Pegmatite: An extremely coarse-grained intrusive rock, characterized by exceptionally large crystals. These are often found in the margins of granite intrusions.

Extrusive Igneous Rocks: Rapid Cooling, Small Crystals

Extrusive igneous rocks, also called volcanic rocks, form when magma (now called lava once it reaches the surface) erupts onto the Earth's surface and cools rapidly. The rapid cooling process prevents the formation of large crystals, resulting in a fine-grained texture or even a glassy texture. Examples of extrusive igneous rocks include:

• Basalt: A very common dark-colored extrusive rock, often found in volcanic flows and ocean crust. Basalt is rich in plagioclase feldspar and pyroxene.
• Rhyolite: A light-colored extrusive rock, often with a fine-grained texture. It is the extrusive equivalent of granite.
• Andesite: An intermediate extrusive rock, similar in composition to diorite.
• Obsidian: A volcanic glass, formed by the extremely rapid cooling of lava. It has a glassy texture and conchoidal fracture.
• Pumice: A volcanic rock with a very porous texture, formed by the rapid cooling of gas-rich lava. It is so light that it often floats on water.
• Scoria: A dark-colored volcanic rock with a vesicular (containing many cavities) texture. It forms from the rapid cooling of lava containing abundant gas bubbles.

Identifying Igneous Rocks: A Practical Guide

Identifying igneous rocks requires careful observation of their physical characteristics. Key features to consider include:

1. Texture: A Window into Cooling History

The texture of an igneous rock, as discussed above, is a direct reflection of its cooling history. Key textural terms include:

• Coarse-grained: Large crystals are visible to the naked eye. This indicates slow cooling.
• Fine-grained: Crystals are too small to see without magnification. This indicates rapid cooling.
• Porphyritic: Contains both large and small crystals, suggesting a two-stage cooling process.
• Glassy: No crystals are visible, indicating extremely rapid cooling.
• Vesicular: Contains many small holes (vesicles) formed by escaping gases.

2. Mineral Composition: Unveiling the Chemical Fingerprint

The mineral composition of an igneous rock is determined by the chemical composition of the parent magma. Common minerals found in igneous rocks include:

• Feldspar: A group of silicate minerals, often light-colored, that are abundant in many igneous rocks.
• Quartz: A hard, crystalline mineral that is often found in felsic igneous rocks.
• Pyroxene: A group of dark-colored silicate minerals that are commonly found in mafic igneous rocks.
• Amphibole (Hornblende): A dark-colored silicate mineral found in intermediate igneous rocks.
• Mica (Biotite and Muscovite): Sheet silicate minerals, biotite is dark and muscovite is light.
• Olivine: A green, magnesium-iron silicate mineral often found in ultramafic rocks.

3. Color: A First Impression

While not definitive, color can provide a valuable initial clue. Generally:

• Light-colored (felsic) igneous rocks are rich in silica and aluminum.
• Dark-colored (mafic) igneous rocks are rich in iron and magnesium.
• Intermediate igneous rocks have a mix of light and dark minerals.

Examples of Igneous Rocks and Their Identification

Let's examine some specific examples to reinforce your understanding:

Scenario 1: You find a rock with large, easily visible crystals of quartz, feldspar, and mica. It's light-colored.

Identification: This is likely granite, a coarse-grained, felsic intrusive igneous rock.

Scenario 2: You discover a dark-colored rock with fine-grained texture. It's dense and heavy.

Identification: This could be basalt, a fine-grained, mafic extrusive igneous rock.

Scenario 3: You encounter a rock with a glassy texture, exhibiting a conchoidal fracture.

Identification: This is likely obsidian, a volcanic glass formed by rapid cooling of lava.

Scenario 4: You find a light-colored rock with a porphyritic texture, displaying large crystals embedded in a fine-grained matrix.

Identification: This could be a porphyritic rhyolite or other porphyritic igneous rock, indicative of a two-stage cooling history.

Beyond the Basics: Specialized Igneous Rock Classifications

The classification of igneous rocks can be further refined based on more detailed chemical and mineralogical analyses. These classifications often involve diagrams like the QAPF diagram (for classifying plutonic rocks) and TAS diagram (for volcanic rocks), which consider the relative proportions of quartz, alkali feldspar, plagioclase feldspar, and feldspathoids. However, the principles discussed above provide a solid foundation for identifying the most common types of igneous rocks.

Conclusion: Mastering the Art of Igneous Rock Identification

Identifying igneous rocks requires a combination of observation and understanding. By carefully examining texture, mineral composition, and color, and by considering the processes of magma formation and cooling, you can confidently distinguish between different types of igneous rocks. This knowledge is essential for geologists, earth science enthusiasts, and anyone fascinated by the dynamic processes that shape our planet. Remember, practice is key! The more rocks you examine, the sharper your identification skills will become. So, grab your hand lens and get exploring!