Rocks : Igneous, Sedimentary and Metamorphic Rocks

• Rocks are naturally occurring solid aggregates of one or more minerals that constitute the Earth’s crust.
• They form the fundamental building blocks of the Earth’s lithosphere.
• Most rocks are composed of minerals, although a few, such as Coal and Obsidian, are composed predominantly of non-mineral substances or volcanic glass.

Relationship between Rocks and Minerals

• A Mineral is a naturally occurring inorganic substance with a definite chemical composition and crystalline structure.
• A Rock is an aggregate of one or more minerals.
• The same type of rock may exhibit variations in mineral composition depending on its place of formation.
• Minerals are extracted from rocks through mining and are valuable sources of metals and industrial raw materials.

Characteristics of Rocks

• Rocks may be hard (Granite) or soft (Shale).
• They may be porous or non-porous.
• They can be layered or massive.
• Rocks are continuously transformed from one type to another through the Rock Cycle.

Classification of Rocks

Based on their mode of origin, rocks are classified into three major types:

• • Igneous Rocks
  • Sedimentary Rocks
  • Metamorphic Rocks

Igneous Rocks

• Igneous Rocks are formed by the cooling and solidification of molten magma or lava.
• They are also known as Primary Rocks because they were the first rocks to form during the cooling of the early Earth.

Formation

Igneous rocks originate in two ways:

• • Cooling of Magma beneath the Earth’s surface.
  • Cooling of Lava after volcanic eruptions on the Earth’s surface.

Characteristics

• Generally hard, compact, and crystalline.
• Non-layered or massive in structure.
• Do not contain fossils.
• Highly resistant to weathering.
• Rich in silicate minerals.
• Form many of the Earth’s mountain ranges and plateaus.

Types of Igneous Rocks

Intrusive (Plutonic) Igneous Rocks

• Form when magma cools slowly beneath the Earth’s surface.
• Slow cooling produces large crystals.
• Usually coarse-grained.
• Examples
• • Granite
  • Gabbro
  • Diorite

Extrusive (Volcanic) Igneous Rocks

• Form when lava cools rapidly on the Earth’s surface.
• Rapid cooling results in fine-grained or glassy texture.
• Examples
  • Basalt
  • Obsidian
  • Pumice
  • Andesite
  • Rhyolite

Composition of Igneous Rocks

Depending on silica content:

Distribution of Igneous rocks in India

• The Deccan Plateau is largely composed of Basalt, formed by extensive volcanic eruptions known as the Deccan Traps.

Economic Importance of Igneous rocks

• Source of building stones.
• Rich in metallic minerals.
• Used in road construction, monuments, and engineering works.

Sedimentary Rocks

• Sedimentary Rocks are formed through the deposition, compaction, and cementation of sediments over millions of years.
• This process is collectively known as Lithification.

Formation Process of Sedimentary Rocks

The formation involves:

• • Weathering
  • Erosion
  • Transportation
  • Deposition
  • Compaction
  • Cementation

Characteristics of Sedimentary Rocks

• Usually layered (Stratified).
• Contain fossils of plants and animals.
• Comparatively softer than igneous rocks.
• Highly susceptible to weathering and erosion.
• Occupy nearly 75% of the Earth’s land surface, although they account for only about 5% of the crust’s volume.

Types of Sedimentary Rocks

   Clastic Sedimentary Rocks

• • Formed from fragments of pre-existing rocks.
  • Examples:
    • Sandstone
    • Conglomerate
    • Shale

   Chemical Sedimentary Rocks

• • Formed through precipitation of dissolved minerals.
  • Examples:
    • Rock Salt
    • Gypsum
    • Limestone (Chemical Origin)

   Organic Sedimentary Rocks

• • Formed from remains of plants and animals.
  • Examples:
    • Coal
    • Chalk
    • Coral Limestone

Common Sedimentary Rocks

Economic Importance of Sedimentary Rocks

• Major source of Coal, Petroleum, and Natural Gas.
• Preserve Fossils used in geological history.
• Important for Groundwater Storage.
• Extensively used in the Cement Industry.

Metamorphic Rocks

• Metamorphic Rocks are formed when pre-existing Igneous or Sedimentary Rocks undergo transformation due to high temperature, high pressure, and chemically active fluids, without melting.
• The transformation process is known as Metamorphism.

Causes of Metamorphism

• High Temperature
• High Pressure
• Tectonic Compression
• Magma Intrusion
• Chemically Active Fluids

Characteristics of Metamorphic Rocks

• Usually hard, dense, and compact.
• May exhibit foliation (layered appearance).
• Original minerals are recrystallized.
• Fossils, if present originally, are generally destroyed.
• Possess excellent decorative and constructional value.

Types of Metamorphism

   Contact Metamorphism

• • Caused mainly by heat from nearby magma.

   Regional Metamorphism

• • Occurs over large regions due to tectonic pressure and temperature.

   Dynamic Metamorphism

• • Caused primarily by intense tectonic stress along fault zones.

Common Metamorphic Rocks

Economic Importance of Metamorphic Rocks

• Marble is widely used in architecture and sculpture.
• Slate is used for roofing and blackboards.
• Quartzite serves as an important construction material.
• Gneiss is commonly used as a building stone.

Rock Cycle

• The Rock Cycle is the continuous natural process through which Igneous, Sedimentary, and Metamorphic Rocks are transformed into one another over geological time.

Stages of the Rock Cycle

• Magma cools to form Igneous Rocks.
• Igneous rocks undergo Weathering, Erosion, and Deposition to form Sediments.
• Sediments are Compacted and Cemented into Sedimentary Rocks.
• Sedimentary and Igneous rocks undergo Metamorphism to become Metamorphic Rocks.
• Metamorphic rocks melt into Magma, completing the cycle.

Comparative Study of Rocks