Igneous Rocks : Characteristics, Types, Example & Significance

Rocks, the solid components of Earth’s crust, consist of various minerals. Though most rocks contain multiple minerals, their compositions can vary by region. Contrary to common belief, not all rocks are hard, as their texture can differ. They serve as a source of minerals and are categorized into three types based on their formation processes: Igneous, formed from cooled lava; Sedimentary, created by the accumulation of particles; and Metamorphic, produced under intense heat and pressure. This classification helps in understanding their distinct characteristics and origins.

Igneous Rocks

Igneous rocks, originating from the Latin word ‘ignis’ meaning “fire,” are primary rocks formed by the cooling and solidification of molten material known as magma.

Key features:

• Formation: They originate from magma, a hot liquid inside the Earth, or lava, which emerges at the surface during volcanic activity.

• Cooling and Crystallisation:
  • Below Earth’s Surface (Magma): Slow cooling leads to coarse-grained rocks.
  • Above Earth’s Surface (Lava): Rapid cooling results in fine-grained rocks, often with air cavities indicating escaped gas.
• Texture & Composition: Their texture varies based on crystal shape, size, cooling duration, and mineral composition within the rock. They’re identifiable by their density, colour, and proportion of minerals.

• Role in Rock Cycle: As the first to form from magma, they begin the rock cycle, earning the title of primary rocks.

In essence, igneous rocks are a testament to Earth’s fiery internal processes, visible in their diverse textures and compositions.

Characteristics of Igneous Rocks

Igneous rocks, formed deep inside the Earth, are unique for several reasons:

• Lack of Fossils: Unlike other rock types, igneous rocks don’t contain fossils. The intense heat destroys any potential fossils during the rock’s formation.

• Mineral Rich: These rocks are often composed of multiple minerals, making them diverse in composition.

• Texture Variety: Their appearance can vary greatly; some are glassy, while others are coarser.

• Acid Resistance: Igneous rocks typically don’t react to acids.

• Mineral Patches: The minerals in these rocks often appear in patches of varying sizes, adding to their distinct look.

Types of Igneous Rocks

Igneous rocks are fascinating natural formations, broadly categorized into two types based on the process of their formation. These two major types are:

• Intrusive Igneous Rocks

• Extrusive Igneous Rocks

Intrusive Igneous Rocks

• Formation: These rocks form from magma deep within the Earth, known as intrusive or plutonic rocks.

• Cooling Process: They cool slowly in magma chambers or inside surrounding rocks.

• Texture: This slow cooling allows large crystals to form, giving these rocks a coarse-grained appearance.

• Examples: Some well-known examples include granite, granodiorite, gabbro, and diorite.

Extrusive Igneous Rocks

• Formation: Also known as extrusive or volcanic rocks, these rocks are formed from lava that erupts onto the Earth’s surface.

• Eruption and Cooling: When molten rock, or lava, erupts onto the surface, it cools rapidly in contact with air or water.

• Texture: The quick cooling results in a fine-grained or glassy texture.

• Examples: Basalt, rhyolite, and trachyte are typical extrusive rocks.

In summary, igneous rocks vary based on where and how they cool. Each type offers a unique glimpse into Earth’s dynamic processes, from deep below the surface to dramatic volcanic eruptions. Intrusive rocks tell a story of slow, hidden changes, while extrusive rocks capture the swift drama of volcanic activity.

Types of igneous rocks- Mode of occurrence

Three types of igneous are identified based on their mode of occurrence :

Plutonic Rocks

• Named after Pluto, the Roman god of the underworld.

• Formed deep beneath the Earth’s surface or within magma chambers.

• Examples: Granite, Gabbro.

• Characteristics: Cool and solidify slowly, leading to large crystals.

• Occurrence: As intrusive bodies, like batholiths (e.g., Mount Abu, Ladakh batholith).

Volcanic Rocks

• Cool and solidify at the Earth’s surface, in contact with air or water.

• Examples: Basalt, Rhyolite.

• Characteristics: Rapid cooling results in fine-grained textures.

• Occurrence: As extrusive bodies, like lava flows (e.g., Deccan basalts in Maharashtra, Malani rhyolite in Jodhpur).

Hypabyssal Rocks

• Form at shallow depths near the Earth’s surface.

• Example: Dolerite

• Characteristics: Medium-grained texture

• Occurrence: Often found as dykes or sills.

From the depths of the underworld to the surface we walk on, igneous rocks are natural witnesses to Earth’s dynamic history.

Examples of Igneous Rocks

Rock Name	Colour	Texture	Characteristic Features
1. Rhyolite	Light coloured	Fine-grained Often has trapped gas bubbles.	Forms from rapidly cooling magma or lava. Rich in silica, like quartz.
2. Granite	Smoky grey or white	Coarse-grained to very coarse-grained	Forms deep in the earth. Made from magma, cooling slowly. Contains lots of silica, like quartz. Slow cooling leads to large crystals.
3. Diorite	Medium to dark grey	Coarse-grained	Diorite and granite form from magma deep in the earth. Unlike light-coloured granite, diorite lacks quartz and light minerals. It’s composed mainly of dark minerals.
4. Basalt	Dark grey to black	Fine-grained	Basalt is a volcanic rock. Formed from iron and magnesium-rich, silica-poor magma. Erupts as lava from volcanoes or fissures. Cools quickly, leading to a fine-grained texture. Sometimes, basalt contains gas bubbles, then it is known as vesicular basalt.
5. Diabase	Dark green to black	Medium grain size	Formed from iron and magnesium-rich magma. Poor in silica. Emerges in cracks or layers near the Earth’s surface. Similar to basalt but cools slower. Has slightly larger crystals. Develops larger crystals due to slow cooling.
6. Gabbro	Dark green to black	Large grain size	Forms deep underground. Comes from iron and magnesium-rich, silica-poor magma. Slow cooling creates large crystals. Related to basalt and diabase, but with slower formation.
7. Pumice	Light to medium grey	Full of gas bubbles, giving a sponge-like appearance. Very lightweight, it often floats in water. May have flow lines or bands.	Pumice is formed from explosive volcanic eruptions. It’s made of highly silicic magma, which is thick and sticky. Trapped gases cause the bubbly texture. Similar in composition to granite. Used for exfoliating skin and in abrasive cleaners.
8. Scoria	9. Dark coloured	Vesicular Texture Characterized by holes in the rock.	Forms at the top of lava flows. Originates from sticky lava. Cools quickly, limiting crystal formation.
10. Obsidian	Black	Glassy	Obsidian is a volcanic rock. Forms from quickly cooled, silica-rich lava. Rapid cooling prevents crystal formation.

Significance of Igneous Rocks

• Resistant to Weathering and Erosion: Igneous rocks like basalt and granite are resistant to weathering and erosion as compared to sedimentary rocks. Therefore sculptures, statues, buildings, and decorations will last longer if they were made from sedimentary rocks.

• Cost-effective: Granite is cost-effective as well as stunning in appearance. Moreover, as a natural stone, it is heat and scratch-resistant, it is one of the ideal materials for kitchen countertop options.

• Jewellery: Peridot, a kind of olivine used in jewellery, is sometimes mined from peridotite.

• Skin and Dental Aids: Pumice is a stone that can be used to polish skin or scrape away grime around the house. To act as an abrasive material to scrape teeth, ground-up pumice stone is occasionally added to toothpaste.

• Concrete Products: Pumice stone can be used in the production of lightweight concrete blocks and other lightweight concrete products.

• Landscaping and Horticulture: The pumice is used as a decorative ground cover in landscaping and planters.

• Used as a Drainage Rock and Soil Conditioner in plantings. Pumice and scoria are also popular rocks for use as substrates in hydroponic gardening.

• Making Roads: Basalt and dolerite are used for road making.

• Cash Crops: Deccan trap in India made up of basaltic igneous rocks is extensively used for the cultivation of cash crops like cotton, etc.

Conclusion

Igneous rocks, also called primary rocks which are formed when magma cools and solidifies. About 95% part of the earth’s crust is made up of igneous rocks. Magma (molten rock) cools and crystallises at volcanoes on the Earth’s surface or when the melted rock is still inside the crust, igneous rocks are formed. In essence, igneous rocks are a testament to Earth’s fiery internal processes, visible in their diverse textures and compositions. Igneous rocks vary based on where and how they cool. Each type offers a unique glimpse into Earth’s dynamic processes, from deep below the surface to dramatic volcanic eruptions. Intrusive rocks tell a story of slow, hidden changes, while extrusive rocks capture the swift drama of volcanic activity. From the depths of the underworld to the surface we walk on, igneous rocks are natural witnesses to Earth’s dynamic history.

FAQs- Igneous Rocks

1. Define Igneous Rocks.

Igneous rocks, originating from the Latin word ‘ignis’ meaning “fire,” are primary rocks formed by the cooling and solidification of molten material known as magma.

2. What are the two types of igneous rocks, based on their formation?

Igneous rocks are fascinating natural formations, broadly categorized into two types based on the process of their formation. These two major types are:

• Intrusive Igneous Rocks

• Extrusive Igneous Rocks

3. What are the characteristics of igneous rocks?

Igneous rocks, formed deep inside the Earth, are unique for several reasons:

• Lack of Fossils: Unlike other rock types, igneous rocks don’t contain fossils. The intense heat destroys any potential fossils during the rock’s formation.

• Mineral Rich: These rocks are often composed of multiple minerals, making them diverse in composition.

• Texture Variety: Their appearance can vary greatly; some are glassy, while others are coarser.

• Acid Resistance: Igneous rocks typically don’t react to acids.

• Mineral Patches: The minerals in these rocks often appear in patches of varying sizes, adding to their distinct look.

4. Name some igneous rocks.

Some igneous rocks are: Scoria, Pumice, Basalt, Diorite, Granite etc.

5. What are the features of igneous rocks.

The key features of igneous rocks are:

• Formation: They originate from magma, a hot liquid inside the Earth, or lava, which emerges at the surface during volcanic activity.

• Cooling and Crystallisation:
  • Below Earth’s Surface (Magma): Slow cooling leads to coarse-grained rocks.
  • Above Earth’s Surface (Lava): Rapid cooling results in fine-grained rocks, often with air cavities indicating escaped gas.
• Texture & Composition: Their texture varies based on crystal shape, size, cooling duration, and mineral composition within the rock. They’re identifiable by their density, colour, and proportion of minerals.

• Role in Rock Cycle: As the first to form from magma, they begin the rock cycle, earning the title of primary rocks.

In essence, igneous rocks are a testament to Earth’s fiery internal processes, visible in their diverse textures and compositions.