Which Of The Following Is An Igneous Rock

Which of the Following is an Igneous Rock? Understanding Igneous Rock Formation and Identification

Igneous rocks are one of the three main rock types, alongside sedimentary and metamorphic rocks. Understanding what constitutes an igneous rock is crucial for anyone studying geology, earth science, or simply curious about the fascinating processes shaping our planet. This comprehensive guide will delve into the formation of igneous rocks, their classification, and how to identify them, ultimately answering the question: which of the following is an igneous rock? We'll explore various examples and differentiate them from sedimentary and metamorphic rocks.

What are Igneous Rocks?

Igneous rocks are formed from the cooling and solidification of molten rock, known as magma (beneath the Earth's surface) or lava (on the Earth's surface). This process of cooling can occur slowly beneath the Earth's crust, resulting in large crystals, or rapidly on the surface, leading to smaller or even glassy textures. The rate of cooling, along with the chemical composition of the magma/lava, significantly influences the final characteristics of the igneous rock.

The Igneous Rock Formation Process: A Detailed Look

The formation of igneous rocks is a complex process that can be broadly divided into these stages:

1. Melting: The initial stage involves the melting of existing rocks within the Earth's mantle and crust. This melting is primarily driven by heat from the Earth's core, pressure changes, and the addition of water.

2. Magma Generation and Movement: The molten rock, now magma, is less dense than the surrounding solid rock and thus begins to rise. This ascent can be slow or rapid, depending on factors like the magma's viscosity and the surrounding rock's permeability.

3. Crystallization: As the magma rises and cools, it begins to crystallize. The rate of cooling significantly impacts the size of the crystals formed. Slow cooling allows for larger crystals to grow, while rapid cooling leads to smaller crystals or even a glassy texture.

4. Eruption (Extrusive Igneous Rocks): If the magma reaches the Earth's surface through volcanic eruptions, it is now called lava. Lava cools relatively quickly, forming extrusive igneous rocks like basalt, obsidian, and pumice. These rocks are often characterized by fine-grained textures or glassy appearances due to the rapid cooling process.

5. Intrusion (Intrusive Igneous Rocks): If the magma cools and solidifies beneath the Earth's surface, it forms intrusive igneous rocks. These rocks cool much more slowly, allowing for the formation of larger crystals. Examples include granite, gabbro, and diorite. These rocks are typically coarse-grained.

Classifying Igneous Rocks: Key Characteristics

Igneous rocks are classified based on two primary factors:

• Mineral Composition: This refers to the types and proportions of minerals present in the rock. The major minerals found in igneous rocks include feldspar, quartz, mica, pyroxene, amphibole, and olivine. The specific minerals present are directly related to the chemical composition of the parent magma.

• Texture: Texture refers to the size, shape, and arrangement of the mineral grains within the rock. Texture is primarily determined by the cooling rate of the magma or lava. Common textural descriptions include:

  • Phaneritic (Coarse-grained): Visible crystals (greater than 1mm). Indicates slow cooling. Examples found in intrusive rocks.
  • Aphanitic (Fine-grained): Crystals too small to see without magnification. Indicates rapid cooling. Examples found in extrusive rocks.
  • Porphyritic: A mixture of large and small crystals. Indicates a two-stage cooling process – initial slow cooling followed by rapid cooling.
  • Glassy: No visible crystals. Indicates extremely rapid cooling. Examples include obsidian.
  • Vesicular: Contains many holes (vesicles) formed by escaping gases during cooling. Common in extrusive rocks like pumice and scoria.

Identifying Igneous Rocks: A Practical Guide

Identifying an igneous rock requires careful observation of its texture and mineral composition. Here's a step-by-step guide:

1. Examine the Texture: Is the rock coarse-grained, fine-grained, glassy, porphyritic, or vesicular? This will give you initial clues about the cooling history of the rock.

2. Identify the Minerals: Use a hand lens or microscope to examine the minerals present. Can you identify feldspar, quartz, mica, etc.? Refer to mineral identification charts for assistance. Consider color, luster, hardness, and cleavage.

3. Assess the Overall Appearance: Consider the rock's color (light-colored or dark-colored) and overall appearance. This, combined with texture and mineral identification, will help you narrow down the possibilities.

4. Utilize Reference Materials: Use field guides, online resources, or textbooks to compare your observations with known igneous rocks.

Examples of Igneous Rocks: A Diverse Group

Let's examine some common igneous rocks to illustrate the diversity within this rock type:

Intrusive Igneous Rocks (Plutonic Rocks):

• Granite: A light-colored, coarse-grained rock composed mainly of quartz, feldspar, and mica. It is a very common igneous rock found in many mountain ranges.
• Gabbro: A dark-colored, coarse-grained rock rich in pyroxene and plagioclase feldspar. It is often found in large intrusive bodies called batholiths.
• Diorite: An intermediate-colored, coarse-grained rock with a mixture of plagioclase feldspar and amphibole.

Extrusive Igneous Rocks (Volcanic Rocks):

• Basalt: A dark-colored, fine-grained rock rich in pyroxene and plagioclase feldspar. It is the most abundant volcanic rock on Earth, forming extensive lava flows and volcanic plateaus.
• Rhyolite: A light-colored, fine-grained rock similar in composition to granite. It is often found in volcanic domes and lava flows.
• Andesite: An intermediate-colored, fine-grained rock with a composition between basalt and rhyolite.
• Obsidian: A dark-colored, glassy rock formed by the rapid cooling of lava. It has a smooth, conchoidal fracture.
• Pumice: A light-colored, vesicular rock formed from frothy lava. Its high porosity makes it lightweight and can float on water.
• Scoria: A dark-colored, vesicular rock similar to pumice but denser and darker in color.

Differentiating Igneous Rocks from Sedimentary and Metamorphic Rocks

It's essential to distinguish igneous rocks from sedimentary and metamorphic rocks. Here's a comparison:

Conclusion: Answering the Question

So, which of the following is an igneous rock? The answer depends on the options provided. However, by understanding the formation process, classification, and identification methods of igneous rocks discussed in this comprehensive guide, you can confidently identify igneous rocks from other rock types. Remember to observe the texture and mineral composition carefully and consult reference materials when needed. The world of igneous rocks is vast and fascinating, offering a window into the dynamic processes shaping our planet. Further exploration into specific types of igneous rocks and their geological context will deepen your understanding of this essential rock type.