6. Types of Rocks

1. Introduction to Rocks and Minerals

The Earth is primarily made up of rocks and minerals. The rocks forming the solid crust (lithosphere) of the Earth are in a solid state. A rock can be described as an aggregate or mixture of minerals in different proportions, meaning it has no definite chemical composition. A naturally occurring material in the crust from which a mineral or minerals of economic value can be extracted profitably is called an ore.

Concept

Difference Between Rocks and Minerals:

Rocks: They are naturally occurring aggregates of minerals. They do not have a definite chemical composition. Examples include Basalt, Sandstone, Granite, and Slate.
Minerals: They are solid, inorganic substances occurring naturally. They have a definite chemical composition and physical properties. Elements are organized to form compounds known as minerals. The four chief mineral groups are silicates, carbonates, sulphides, and oxides. Examples include Feldspar, Mica, Magnetite, and Pyrite.

Based on their mode of formation, rocks are broadly classified into three main types: Igneous, Sedimentary, and Metamorphic.

2. Igneous Rocks

The word "Igneous" derives from the Latin word 'Ignis', which means fire. Igneous rocks are formed by the cooling and solidification of hot, molten matter (magma) from the interior of the Earth.

A. Characteristics of Igneous Rocks

They are the first rocks to be formed, so they are also called primary rocks.
They are typically very hard and compact.
They do not have layers (unstratified) and generally do not contain fossils.
They are less affected by chemical weathering because they are non-porous.

B. Types Based on Mode of Formation

Igneous rocks are divided into two main categories based on where the magma solidifies:

Intrusive Igneous Rocks: Formed when molten magma finds its way into clefts, cracks, or spaces but does not reach the surface, solidifying below the Earth's crust.
- Because cooling happens slowly deep inside the Earth, large-sized crystals are formed. Deep-seated rocks with large crystals are called Plutonic Rocks (e.g., Granite, Dolerite, Gabbro).
- If magma solidifies at a higher place in the crust (shallow depths), medium-grained intrusive rocks are formed, known as Hypabyssal Rocks.
Extrusive Igneous Rocks (Volcanic Rocks): Formed when hot magma (now called lava) erupts onto the Earth's surface and solidifies.
- Because cooling on the surface is rapid, the grains formed are small and fine.
- Basalt is the best example. Lava flows can form vast lava plateaus, such as the Deccan Trap in Peninsular India (creating basaltic regur/black soil) and the Columbia/Snake Plateau in the USA.

C. Acid vs. Basic Igneous Rocks

Acid Igneous Rocks: Contain a high percentage of silica. They are less dense and lighter in colour. Example: Granite (contains quartz, feldspar, and mica).
Basic Igneous Rocks: Contain a lower percentage of silica but a high percentage of basic oxides (like iron and magnesium). They have small, dense grains and are darker in colour. Example: Basalt.

3. Sedimentary Rocks

Sedimentary rocks are formed by the accumulation, compaction, and cementation of sediments (particles, grains, or fragments) derived from previously existing parent rocks. They are the most widespread rocks on the surface of the Earth.

A. Characteristics of Sedimentary Rocks

They are distinguished by the presence of distinct layers or strata. Hence, they are called stratified rocks.
They are formed from materials recovered from other rocks and laid down over a long course of time.
They often contain the remains of plants and animals preserved between the layers.

Fact

Fossils: Sedimentary rocks are unique because they are the only rocks that contain fossils—the preserved remains or impressions of ancient sea plants and animals that were buried in the sediments millions of years ago.

B. Types Based on Mode of Formation

1. Mechanically Formed Sedimentary Rocks:

Formed when loose particles (sand, gravel, clay) of parent rocks are compacted and cemented together by pressure or cementing materials.

Shale: Formed from very fine-grained particles on sea beds; can be easily separated into flakes.
Mudstone: Fine-grained rock formed on the floor of water bodies, similar to shale but lacks fissility (doesn't split easily).
Sandstone: Formed by the consolidation of coarse loose sand particles.
Siltstone: Formed from silt deposits (particles between clay and sand size).
Note: Based on deposition agents, these can be Riverine/Alluvial (rivers), Lacustrine (lakes), Glacial (moraines/conglomerates), or Aeolian (wind-borne, like Loess).

2. Organically Formed Sedimentary Rocks:

Formed by the accumulation of shells, skeletons, and organic remains.

Calcareous Rocks: Formed from the shells and skeletons of organisms (like clams, oysters, corals) that extract calcium carbonate from sea water. Examples: Limestone and Chalk.
Carbonaceous Rocks: Formed when ferns, plants, and trees are buried in swamps. Under heat and pressure, they decay, leaving mostly carbon. Examples: Peat, Lignite, and Coal.

3. Chemically Formed Sedimentary Rocks:

Formed by the evaporation of water containing dissolved minerals, leaving the precipitated salts behind. Example: Gypsum, nitrates, and potash.

4. Metamorphic Rocks

The term comes from "Meta" (change) and "Morph" (form). Sometimes, due to tremendous pressure and extremely high temperatures deep within the Earth, igneous and sedimentary rocks undergo a complete alteration in their appearance and chemical composition, transforming into Metamorphic Rocks.

Important

Common Rock Transformations:
• Limestone (Sedimentary) $\rightarrow$ Marble
• Clay / Shale (Sedimentary) $\rightarrow$ Slate / Schist
• Sandstone (Sedimentary) $\rightarrow$ Quartzite
• Granite (Igneous) $\rightarrow$ Gneiss
• Coal (Sedimentary) $\rightarrow$ Graphite

Types of Metamorphism

Thermal Metamorphism: Transformation driven primarily by high temperature (heat from magma intrusion or friction of moving rock layers).
Dynamic Metamorphism: Transformation caused mainly by immense mechanical deformation and pressure over long periods.
Regional Metamorphism: Metamorphism taking place over a very large area, usually caused by large-scale tectonic movements of the Earth's crust.
Contact Metamorphism: Occurs when rocks are directly heated by coming into contact with an intrusion of hot magma.

5. The Rock Cycle

The continuous process by which rocks forming the crust of the Earth change from one type to another is called the Rock Cycle.

Magma from the interior solidifies to form primary Igneous rocks.
Agents of denudation (wind, water, ice) weather and erode these rocks, depositing sediments that compact to form Sedimentary rocks.
Under intense heat and pressure, both igneous and sedimentary rocks transform into Metamorphic rocks.
Eventually, these rocks may be buried deep inside the Earth due to crustal movements, melt back into magma, and the cycle repeats.

6. Economic Importance of Rocks

Rocks are of immense economic value to mankind in several ways:

Soil Formation: Disintegration of rocks creates valuable agricultural soils (e.g., black regur soil from basaltic rocks).
Minerals: Rocks are the storehouses of a huge variety of metallic and non-metallic minerals critical for manufacturing industries.
Fuel: Carbonaceous sedimentary rocks provide essential fuel minerals like coal.
Construction: Rocks provide stones (granite, marble, sandstone), lime, and cement used heavily in building houses, roads, and dams.
Precious Metals/Stones: Igneous and metamorphic rocks are sources of precious metals like gold, silver, platinum, and various gemstones used in jewelry.

7. Landforms of Intrusion

When magma solidifies inside the Earth's crust (intrusive igneous activity), it creates distinct landforms that may later be exposed by erosion:

Sills: Magma that solidifies horizontally between beds of sedimentary rock.
Dykes: Magma that solidifies vertically, cutting across rock beds through cracks or fissures.
Laccoliths: Large, blister-like or dome-shaped intrusions of acid lava that push the overlying strata upwards.
Phacoliths: Lens-shaped mass of igneous rock situated in the crest of an anticline or the trough of a syncline.
Lopoliths: Saucer-shaped intrusions formed in shallow basins.
Batholiths: Massive, deep-seated coarse-grained igneous intrusions that often form the core/base of major mountain ranges.