Comprehensive Guide to Igneous Rocks

What are Igneous Rocks?

Igneous rocks are formed from magma, a molten rock material located beneath the Earth’s surface. When magma cools and solidifies, either deep within the Earth or after being expelled to the surface as lava, it transforms into igneous rocks. A significant portion of the Earth’s crust is composed of these rocks.

How Does Magma Form Igneous Rocks?

Magma solidifies into igneous rocks as it cools. The environment where the cooling occurs plays a crucial role in determining the characteristics of the rock. For instance, slow cooling beneath the surface allows larger crystals to form, whereas rapid cooling on the surface results in fine-grained textures.

Types of Igneous Rocks

There are two primary types of igneous rocks:

• Intrusive (Plutonic) Rocks: These rocks form when magma cools and solidifies beneath the Earth’s surface. The slow cooling process facilitates the formation of visible crystals, resulting in a coarse texture. Examples include granite and diorite.
• Extrusive (Volcanic) Rocks: These rocks are created when magma erupts as lava and cools rapidly on the Earth’s surface. This rapid cooling process produces rocks with a fine-grained texture or even glassy surfaces. Examples include basalt and pumice.

Influences on Texture and Composition

The texture and composition of igneous rocks are influenced by several factors:

• Cooling Rate: Slow cooling leads to the formation of large crystals, while rapid cooling results in smaller crystals or none at all.
• Mineral Composition: The specific minerals present in magma dictate the color and density of the resulting rocks.

Significance of Igneous Rocks in India

In India, igneous rocks play a vital role in shaping its geological features. The Deccan Traps, one of the largest volcanic provinces in the world, are formed from extensive basaltic lava flows. Granite, another igneous rock, is widely utilized in construction and sculpture, while various igneous rocks serve as essential sources of minerals like mica and feldspar.

Frequently Asked Questions (FAQs)

Q1. What are the primary characteristics of igneous rocks?
Answer: Igneous rocks are characterized by their crystalline texture, which results from the cooling and solidification of magma. They can be coarse or fine-grained depending on the cooling rate and location of formation.

Q2. How do intrusive and extrusive igneous rocks differ?
Answer: Intrusive igneous rocks form from magma cooling beneath the Earth's surface, leading to larger crystals, while extrusive igneous rocks form from lava cooling quickly on the surface, resulting in smaller crystals or a glassy texture.

Q3. Why are igneous rocks important in construction?
Answer: Igneous rocks like granite are durable and aesthetically pleasing, making them ideal for construction and decorative purposes, including buildings, monuments, and countertops.

Q4. What role do igneous rocks play in India's geology?
Answer: Igneous rocks, particularly those forming the Deccan Traps, are crucial for understanding India's geological history and contribute to the diversity of mineral resources in the country.

Q5. What is the significance of the Deccan Traps?
Answer: The Deccan Traps are significant due to their vast basaltic lava flows that have shaped the landscape of India and are important for studying volcanic activity and its effects on the environment.

UPSC Practice MCQs

Question 1: What type of igneous rock is formed when magma cools slowly underground?
A) Extrusive igneous rock
B) Intrusive igneous rock
C) Sedimentary rock
D) Metamorphic rock
Correct Answer: B

Question 2: Which of the following is a characteristic of extrusive igneous rocks?
A) Coarse texture
B) Formed beneath the Earth's surface
C) Fine-grained texture
D) High mineral density
Correct Answer: C

Question 3: The Deccan Traps are primarily composed of which type of rock?
A) Sedimentary rock
B) Metamorphic rock
C) Basalt
D) Limestone
Correct Answer: C

Question 4: Which mineral is commonly derived from igneous rocks?
A) Coal
B) Mica
C) Sandstone
D) Clay
Correct Answer: B

Question 5: What process leads to the formation of crystals in igneous rocks?
A) Erosion
B) Cooling of magma
C) Sedimentation
D) Compaction
Correct Answer: B