Chapter – Classification of Igneous rocks
Classification of Igneous Rocks
1. Introduction to Igneous Rocks
Rocks are broadly divided into three major families based on their mode of formation: Igneous rocks, Sedimentary rocks, and Metamorphic rocks. Among these, igneous rocks are considered the "primary rocks" or "parent rocks" because they are the first rocks to have formed on Earth, and all other rock types are ultimately derived from them through weathering, erosion, deposition, and metamorphism.
Igneous rocks (from the Latin word "ignis" meaning fire) are rocks that are formed by the cooling and solidification of molten magma or lava, either below the Earth's surface or on it. They are also known as "primary rocks" since they form directly from molten material without involving any pre-existing rock.
1.1 How Igneous Rocks are Formed
Deep inside the Earth's crust and upper mantle, extremely high temperature and pressure keep rock material in a semi-liquid/molten state known as magma. When this magma finds a path of weakness in the crust, it moves upward. Depending on where it finally cools and solidifies, igneous rocks are formed in one of two broad settings:
- Below the surface — magma cools slowly inside the crust and forms intrusive igneous rocks.
- On the surface — magma erupts through volcanoes as lava, cools quickly in contact with air/water, and forms extrusive igneous rocks.
Magma is molten rock material found below the Earth's surface. Once it is erupted onto the surface through a volcano, it is called Lava. This single difference in terminology becomes the basis for one of the major classification systems discussed later in this chapter.
2. Formation Zones of Igneous Rocks (Diagram)
The diagram below shows a cross-section of the Earth's crust, indicating exactly where each category of igneous rock is formed — deep inside (Plutonic), somewhat shallow (Hypabyssal), and on the surface (Volcanic).
3. Bases of Classification of Igneous Rocks
Igneous rocks are extremely varied, so geologists classify them using four major criteria. It is important to understand that the same rock can be described using all four systems simultaneously (e.g., Granite is Plutonic + Acidic + Coarse-grained + Felsic).
A. Classification on the Basis of Mode of Occurrence (Place of Solidification)
This is the most common classification, based on where the magma cools and solidifies.
(i) Plutonic (Intrusive / Abyssal) Rocks
Rocks formed when magma cools and solidifies very slowly, deep inside the Earth's crust (several kilometres below the surface), without reaching the surface.
- Cooling is extremely slow → crystals get sufficient time to grow → rocks are coarse-grained.
- These rocks are exposed on the surface only after millions of years, once the overlying rock layers are removed by erosion (this is called denudation).
- Examples: Granite, Gabbro, Diorite, Syenite, Peridotite.
(ii) Hypabyssal (Sub-volcanic) Rocks
Rocks formed at a shallow to intermediate depth (between the surface and the deep plutonic zone), commonly as dikes, sills, or laccoliths.
- Cooling rate is moderate (faster than plutonic, slower than volcanic) → produces medium-grained rocks.
- Occur as intrusive bodies like Dikes (vertical, cut across rock layers), Sills (horizontal, parallel to rock layers), and Laccoliths (dome-shaped intrusions).
- Examples: Dolerite, Porphyry.
(iii) Volcanic (Extrusive) Rocks
Rocks formed when magma (now called lava) erupts onto the Earth's surface through volcanoes and cools very rapidly on contact with air or water.
- Cooling is very fast → little to no time for crystal growth → rocks are fine-grained or glassy.
- Often contain small holes/cavities called vesicles, formed by trapped gas bubbles.
- Examples: Basalt, Rhyolite, Obsidian, Pumice, Andesite.
B. Classification on the Basis of Silica (SiO₂) Content
This chemical classification is based on the percentage of silica (SiO₂) present in the rock. Silica content directly controls the rock's colour, density, and the type of minerals it contains.
| Category | Silica % | Colour | Density | Examples |
|---|---|---|---|---|
| Acidic (Felsic) | > 65% | Light (pink/white/grey) | Low (~2.7 g/cm³) | Granite, Rhyolite |
| Intermediate | 55% – 65% | Medium grey | Medium | Diorite, Andesite, Syenite |
| Basic (Mafic) | 45% – 55% | Dark grey / black | High (~3.0 g/cm³) | Gabbro, Basalt, Dolerite |
| Ultrabasic (Ultramafic) | < 45% | Dark green / black | Very high (~3.3 g/cm³) | Peridotite, Dunite, Pyroxenite |
As silica content decreases (Acidic → Ultrabasic): colour becomes darker, density/weight increases, melting point increases, and the rock becomes richer in iron & magnesium (Fe-Mg minerals) instead of quartz and feldspar.
C. Classification on the Basis of Texture (Grain Size)
Texture refers to the size, shape, and arrangement of mineral grains/crystals in a rock. It is directly controlled by the rate of cooling of magma/lava.
| Texture Type | Rate of Cooling | Crystal Size | Example |
|---|---|---|---|
| Coarse-grained (Phaneritic) | Very slow (deep underground) | Large, visible to naked eye | Granite, Gabbro |
| Medium-grained | Moderate (shallow depth) | Small but visible | Dolerite |
| Fine-grained (Aphanitic) | Fast (surface) | Very small, not visible to naked eye | Basalt, Rhyolite |
| Glassy (Vitreous) | Extremely fast (instant chilling) | No crystals form at all | Obsidian |
| Porphyritic | Two-stage cooling (slow, then fast) | Large crystals set in a fine-grained mass | Porphyry |
| Vesicular | Fast, with trapped gas | Full of small holes/cavities | Pumice, Scoria |
Fast cooling → Tiny/no crystals → Fine-grained or Glassy rock.
D. Classification on the Basis of Mineral Composition (Colour Index)
This classification is based on the proportion of light-coloured (felsic) minerals versus dark-coloured (mafic) minerals present in the rock. This proportion is called the Colour Index.
(i) Felsic Rocks
Rich in feldspar and silica (quartz) → light coloured → low colour index (0–25% dark minerals). Example: Granite, Rhyolite.
(ii) Intermediate Rocks
Balanced mix of light and dark minerals (25–50% dark minerals). Example: Diorite, Andesite.
(iii) Mafic Rocks
Rich in magnesium and ferric (iron) minerals → dark coloured → high colour index (50–90% dark minerals). Example: Gabbro, Basalt.
(iv) Ultramafic Rocks
Almost entirely composed of dark, iron-magnesium minerals with little to no feldspar/quartz (90–100% dark minerals). Example: Peridotite, Dunite.
4. Complete Classification at a Glance (Flowchart)
5. Master Comparison Table (Common Igneous Rocks)
| Rock | Mode of Occurrence | Silica Type | Texture | Colour | Common Use / Location |
|---|---|---|---|---|---|
| Granite | Plutonic | Acidic | Coarse-grained | Pink / Grey | Building/flooring stone; found in Aravalli hills |
| Gabbro | Plutonic | Basic | Coarse-grained | Dark grey/black | Construction aggregate |
| Diorite | Plutonic | Intermediate | Coarse-grained | Salt-and-pepper grey | Decorative stone, sculpture |
| Peridotite | Plutonic | Ultrabasic | Coarse-grained | Dark green | Source rock of upper mantle; diamonds occur in related kimberlite pipes |
| Dolerite | Hypabyssal | Basic | Medium-grained | Dark grey | Forms dikes; road metal |
| Basalt | Volcanic | Basic | Fine-grained | Black | Deccan Traps, India; ocean floor rock |
| Rhyolite | Volcanic | Acidic | Fine-grained | Light grey/pink | Volcanic lava flows |
| Andesite | Volcanic | Intermediate | Fine-grained | Grey | Common in the "Pacific Ring of Fire" volcanoes |
| Obsidian | Volcanic | Acidic | Glassy | Black, glassy | Used historically for tools/weapons |
| Pumice | Volcanic | Acidic | Vesicular (full of holes) | Light grey, very light weight | Floats on water; used for skin scrubbing |
6. Key Points & Quick Revision
- Igneous rocks are called primary rocks because all other rocks are ultimately derived from them.
- Molten rock below the surface = Magma; molten rock on the surface = Lava.
- Rate of cooling is inversely related to crystal/grain size — slower cooling gives bigger crystals.
- Silica content decreases from Granite → Diorite → Gabbro → Peridotite, while colour becomes progressively darker.
- Igneous rocks are generally non-fossiliferous (they do not contain fossils) since they form from molten material.
- Igneous rocks are usually hard, compact, crystalline, and resistant to weathering (except where jointed/fractured).
7. Practice / Revision Questions
- Define igneous rocks and explain why they are called "primary rocks."
- Differentiate between Plutonic, Hypabyssal, and Volcanic rocks with one example each.
- What is the difference between magma and lava?
- Classify igneous rocks on the basis of silica content. Why does colour change with silica percentage?
- Explain the terms "Dike," "Sill," and "Laccolith" with a labelled diagram.
- Why are igneous rocks generally non-fossiliferous?
- Compare Granite and Basalt on the basis of mode of occurrence, texture, and silica content.
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