Igneous Rocks
Geography › Rocks, Erosion and Landforms · Topic 5.1
Igneous rocks are primary rocks formed by the cooling and solidification of molten magma or lava. Intrusive or plutonic igneous rocks, such as granite and gabbro, cool slowly underground and have large crystals, while extrusive or volcanic rocks, such as basalt and rhyolite, cool quickly on the surface and are fine-grained or glassy. They are crystalline, unlayered and hold no fossils.
5 min read · UPSC Prelims · Geography · Rocks, Erosion and Landforms
At a Glance
- Origin Cooling of magma or lava
- Also called Primary or parent rocks
- Intrusive Granite, gabbro, diorite
- Extrusive Basalt, rhyolite, andesite, obsidian
- Features Crystalline, hard, no layers, no fossils
- Exam link UPSC Prelims, Geography: Rocks, Erosion and Landforms
Where Does It Fit?
This is the first topic of the category on rocks, erosion and landforms. The previous category dealt with volcanoes and earthquakes, which produce igneous rock. The next topic covers sedimentary rocks. Previous topic: The 2004 Indian Ocean Tsunami and Warning Systems. Next topic: Sedimentary Rocks.
What Are Igneous Rocks?
In short: igneous rocks form when molten rock cools and hardens.
A rock is a natural aggregate of one or more minerals, and the rocks of the Earth’s crust are of three major families: igneous, sedimentary and metamorphic. Igneous rocks are the oldest and are called primary or parent rocks, because the other two kinds are formed from them. The word igneous comes from the Latin for fire. They form from magma, molten material inside the Earth, or lava, the same material on the surface, as discussed in Volcanism – Causes and Lava Types. Igneous rocks make up the bulk of the crust by volume, along with metamorphic rocks, though sedimentary rocks cover most of the surface.
How Are Igneous Rocks Classified?
They are classified by where they cool and by their silica content.
| Group | Cooling | Texture | Examples |
|---|---|---|---|
| Intrusive (plutonic) | Slowly, deep underground | Coarse, large visible crystals | Granite, gabbro, diorite, peridotite |
| Hypabyssal | At moderate depth, in dykes and sills | Medium | Dolerite |
| Extrusive (volcanic) | Quickly, on the surface | Fine-grained or glassy | Basalt, andesite, rhyolite, obsidian, pumice |
| By silica | Silica content | Colour and density | Examples |
|---|---|---|---|
| Acidic (felsic) | More than about 65 per cent | Light-coloured, lighter | Granite, rhyolite |
| Intermediate | About 52 to 65 per cent | Medium | Diorite, andesite |
| Basic (mafic) | About 45 to 52 per cent | Dark, denser; rich in iron and magnesium | Gabbro, basalt |
| Ultrabasic | Less than 45 per cent | Very dark, very dense | Peridotite |
Intrusive rocks, also called plutonic rocks, are coarse-grained because of slow cooling, while extrusive rocks are fine-grained because of fast cooling. Obsidian is a glassy rock formed when lava cools so fast that crystals cannot grow, and pumice is a light, frothy volcanic rock full of gas bubbles that can float on water.
What Are the Features of Igneous Rocks?
- They are crystalline or glassy, with interlocking grains, and are generally hard and resistant to erosion.
- They lack layers or bedding and contain no fossils, because the high temperature of the molten material destroys any remains.
- They are mostly impermeable to water, though water can move along joints and cracks.
- They often have columnar jointing, a pattern of polygonal cracks formed on cooling, as in the basalt of the Deccan Traps.
- They are a source of many metallic minerals.
What Are the Uses and Indian Examples of Igneous Rocks?
Igneous rocks yield valuable minerals: ores of iron, copper, nickel, chromium, lead, zinc, gold and platinum are associated with them, and diamonds come from kimberlite, an igneous rock that rises from the mantle in pipes. Granite is a major building and ornamental stone and is used for monuments and road metal, and basalt is a common crushed stone for roads and railway ballast. In India, igneous rocks include the basalts of the Deccan Traps over much of Maharashtra, Gujarat and Madhya Pradesh, whose weathering gives the black soil, as described in Movement of the Indian Plate. The granites and gneisses of the Peninsular shield, such as those of Karnataka, Andhra Pradesh and Jharkhand, and the Archaean rocks, are of igneous origin, and are discussed in Archaean Rock System of India. As an analytical point, the Himalayan granites are products of the continental collision, whereas those of the Peninsula are far older, so the same rock name can represent very different geological ages.
How Do Igneous Rocks Link to the Rock Cycle?
Igneous rocks form the starting point of the rock cycle. When exposed at the surface they weather and erode, and the fragments are carried away and become sedimentary rocks. If they are buried and subjected to heat and pressure, they turn into metamorphic rocks. If they are melted again they return to magma. The next rock families are described in Sedimentary Rocks and Metamorphic Rocks and the Rock Cycle.
Why Is This Topic Important?
- Basics Foundation of rock questions
- Resources Minerals and soils
- India Deccan basalt and Peninsular granite
What Came Next?
The next topic describes sedimentary rocks. Next topic: Sedimentary Rocks.
Key People and Terms
- Magma Molten rock underground
- Plutonic Cooled deep underground
- Felsic Rich in silica; light
- Mafic Rich in iron and magnesium; dark
- Kimberlite Diamond-bearing igneous rock
- Columnar jointing Polygonal cooling cracks
Exam Corner
Points to Remember
- Igneous rocks are primary rocks.
- Granite is intrusive; basalt is extrusive.
- They have no fossils and no layers.
- Obsidian is volcanic glass.
- Kimberlite carries diamonds.
Do Not Confuse With
- Granite and basalt Slow cooling coarse rock versus fast cooling fine rock
- Acidic and basic rocks High silica light versus low silica dark
- Magma and lava Underground versus surface
Memory Hook
Fire-born, fossil-free; granite deep and slow, basalt fast at the top.
Mains Angle
Questions ask for classification and examples. These points add depth.
- Cooling Controls crystal size
- Silica Controls colour and density
- Resources Source of ores
A Question You May Face
An original practice question, not a past paper question.
“Classify igneous rocks and explain how the place of cooling affects their texture.”
How to Answer
- Define Primary rocks.
- Classify Intrusive and extrusive.
- Texture Cooling rate.
- Examples Granite and basalt.
GS Relevance
Frequently Asked Questions
What are igneous rocks?
Igneous rocks are rocks formed by the cooling and solidification of molten magma or lava. They are called primary or parent rocks because sedimentary and metamorphic rocks form from them. They are crystalline, have no layers and contain no fossils.
What is the difference between intrusive and extrusive igneous rocks?
Intrusive igneous rocks, such as granite and gabbro, cool slowly beneath the surface and have large crystals. Extrusive igneous rocks, such as basalt and rhyolite, cool quickly on the surface after eruption and are fine-grained or glassy.
Why do igneous rocks not contain fossils?
Igneous rocks form from molten material at very high temperatures, which destroys any remains of living things. Fossils are found mainly in sedimentary rocks, which are formed by the layered deposition of sediments in which dead organisms get buried.
Which are examples of igneous rocks?
Granite, gabbro, diorite and peridotite are intrusive igneous rocks, and basalt, andesite, rhyolite, obsidian and pumice are extrusive ones. The Deccan Traps of India are made mainly of basalt, and the Peninsular shield has large areas of granite.
What is the economic importance of igneous rocks?
Igneous rocks are a source of metallic ores such as iron, copper, nickel, chromium, gold and platinum, and diamonds come from kimberlite pipes. Granite and basalt are widely used as building stone, road metal and ornamental stone.
PYQ Practice — Statement Analysis
1 Igneous rocks are called primary rocks.
Other rocks form from them.
2 Fossils are commonly found in igneous rocks.
Heat destroys any remains.
3 Granite is an extrusive igneous rock.
Granite is intrusive.
4 Basalt is a fine-grained extrusive rock.
It cools quickly on the surface.
Related Topics
Browse the full Geography index