Difference Between Intrusive and Extrusive Igneous Rocks
Igneous rocks are one of the three principal classes of rocks, alongside sedimentary and metamorphic rocks. They form when hot molten material known as magma cools and solidifies into a crystalline or sometimes glassy rock. This process can occur either beneath the Earth's surface or at the surface following a volcanic eruption. Understanding the chemical and physical properties of igneous rocks is essential in geophysics and earth science, as it provides insight into the planet's internal processes and material composition.
Formation and Composition of Igneous Rocks
Most igneous rocks are composed of silicate minerals, built mainly from silicon and oxygen atoms. Magma may also rarely contain carbonate-rich melts, but these are very limited in occurrence. The primary chemical factor affecting igneous rock types is the silica (SiO2) content of the magma. Rocks with high silica content are called felsic or silicic, while those with low silica are known as mafic or ultramafic, terms that reflect their magnesium and iron contents.
Silica content influences how minerals crystallize as magma cools:
- Magmas rich in silica develop light-colored minerals like quartz and feldspar.
- Magmas with low silica are dominated by dark, iron- and magnesium-rich minerals, such as pyroxene and olivine.
Classification by Silica Content
| Silica Category | Silica Content (%) | Typical Minerals | Rock Example |
|---|---|---|---|
| Felsic (Silicic) | > 66 | Quartz, Feldspar | Granite |
| Intermediate | 55 – 66 | Plagioclase, Amphibole | Diorite |
| Mafic | 45 – 55 | Pyroxene, Olivine | Basalt |
| Ultramafic | < 45 | Magnesium-rich Olivine | Peridotite |
As magma cools, silica combines with other oxides to form different minerals. For example, magnesium oxide (MgO) and silica can create pyroxene (MgSiO3), and excess magnesium will form olivine (Mg2SiO4). If there is surplus silica after most minerals form, pure quartz crystals may appear, resulting in rocks called supersaturated. If there is less silica, minerals such as magnesium-olivine, nepheline, or leucite may form instead, making the rock undersaturated regarding silica.
Key Chemical Characteristics
The oxides present in igneous rocks show a strong relationship with silica percentage:
- Low-silica rocks have higher magnesium oxide (MgO) and iron oxides, but lower soda (Na2O) and potash (K2O) content.
- High-silica rocks are enriched in soda and potash but depleted in magnesium and iron oxides.
- Calcium oxide (CaO) and alumina (Al2O3) content varies, being lower below 45% silica but increasing with more silica.
Crystallization Process and Mineral Formation
As cooling proceeds, each mineral crystallizes at its specific temperature. The relative abundance or shortage of silica, magnesium, sodium, calcium, and other elements decides which minerals dominate in each rock type. For instance, excess magnesium with less silica prompts olivine rather than only pyroxene to form.
Excess silica, after all essential minerals are formed, leads to free quartz in the final igneous rock. Conversely, if there is not enough silica to form all the common feldspars and pyroxenes, other minerals like nepheline and leucite may substitute.
Important Formulas and Physics Concepts
| Concept | Formula | Purpose/Application |
|---|---|---|
| Density | ρ = mass / volume | Compares rock types based on composition |
| Heat Change | Q = m × c × ΔT | Calculates heat exchanged during solidification (Q: heat, m: mass, c: specific heat, ΔT: temperature change) |
Example Physics Problem: Solidification of Basalt
Suppose a basalt sample (1000 g) cools from 1200 °C to 700 °C. If its specific heat capacity is 0.84 J/g °C, the heat lost is:
Q = m × c × ΔT
Q = 1000 × 0.84 × (1200 - 700)
Q = 1000 × 0.84 × 500 = 420,000 J
So, the basalt loses 420,000 Joules during solidification.
Stepwise Approach: Classifying Igneous Rocks by Chemistry
- Determine the silica (SiO2) content of the rock.
- Classify as felsic (>66%), intermediate (55–66%), mafic (45–55%), or ultramafic (<45%).
- Identify dominant minerals—light-colored silicates in felsic, dark iron-magnesium silicates in mafic.
- Check for special minerals (e.g., olivine, nepheline) indicating undersaturation in silica.
- Use density/formula-based calculation to compare physical properties.
Summary Table: Chemistry and Examples
| Category | Main Minerals | Color | Example |
|---|---|---|---|
| Felsic | Quartz, K-feldspar | Light (white, pink, grey) | Granite |
| Mafic | Pyroxene, Olivine, Ca-plagioclase | Dark (black, green) | Basalt |
| Ultramafic | Olivine, Pyroxene | Very dark/greenish | Peridotite |
Connect with More Earth Science Resources
- Rock Cycle – Overview
- Metamorphic Rocks – Formation and Changes
- Sedimentary Rocks – Types and Processes
- Volcanoes – Structure and Eruptions
Practice Tasks for Deeper Understanding
- Classify given igneous rock samples by silica content and mineral composition.
- Calculate the heat lost during the formation of various igneous rocks using the Q = m × c × ΔT formula.
- Compare physical properties such as density between felsic and mafic rocks.
Mastering igneous rocks and their chemistry not only strengthens your understanding of Earth materials but also helps you apply Physics problem-solving skills effectively. Continue practicing with Vedantu interactive resources and quizzes for thorough conceptual clarity.
FAQs on Igneous Rocks: Definition, Types, and Formation
1. What are igneous rocks?
Igneous rocks are rocks formed by the cooling and solidification of molten magma or lava. They are classified as one of the three main types of rocks, along with sedimentary and metamorphic rocks. Common features include:
• Formation from magma (underground) or lava (surface)
• Crystalline or glassy texture
• Examples: granite, basalt, obsidian
2. How are igneous rocks formed?
Igneous rocks form through the cooling and solidification of molten material from Earth's interior. The process involves:
1. Magma (molten rock beneath the surface) or lava (molten rock on the surface) begins to cool.
2. As the temperature drops, minerals crystallize, forming solid rocks.
3. The texture and crystal size depend on the cooling speed and location.
• Intrusive forms below the surface (slow cooling, large crystals).
• Extrusive forms at the surface (rapid cooling, small crystals).
3. What are the types of igneous rocks?
There are mainly two types of igneous rocks:
• Intrusive igneous rocks (Plutonic): Form below Earth's surface; cool slowly; have large crystals (e.g., granite, gabbro).
• Extrusive igneous rocks (Volcanic): Form at the surface; cool quickly; have fine crystals or glassy texture (e.g., basalt, obsidian, pumice).
4. What are examples of igneous rocks?
Common examples of igneous rocks include:
• Granite (intrusive, coarse-grained)
• Basalt (extrusive, fine-grained)
• Obsidian (extrusive, glassy)
• Pumice (extrusive, porous and light)
• Gabbro (intrusive, dark and coarse)
5. What is the difference between intrusive and extrusive igneous rocks?
Intrusive igneous rocks form below Earth's surface and cool slowly, resulting in large, visible crystals (e.g., granite). Extrusive igneous rocks form at or near the surface and cool rapidly, resulting in fine-grained or glassy texture (e.g., basalt, obsidian).
• Intrusive: Slow cooling, coarse-grained
• Extrusive: Fast cooling, fine-grained or glassy
6. What are the main characteristics of igneous rocks?
Igneous rocks are identified by these key characteristics:
• Hardness and crystalline structure
• Composed mainly of silicate minerals
• Generally non-porous and dense
• Texture varies from coarse-grained to glassy
• Colour depends on mineral composition (light-coloured = felsic, dark-coloured = mafic)
7. How can igneous rocks be classified based on composition?
Igneous rocks are classified by their silica content:
• Felsic (silicic): High silica, light-coloured (e.g., granite)
• Intermediate: Medium silica (e.g., diorite)
• Mafic: Low silica, rich in magnesium and iron, dark-coloured (e.g., basalt, gabbro)
• Ultramafic: Very low silica, very dark and dense (e.g., peridotite)
8. What physical properties help identify igneous rocks?
Key physical properties for identifying igneous rocks include:
• Grain size: Coarse (intrusive) or fine/glassy (extrusive)
• Colour: Light (felsic) or dark (mafic)
• Hardness: Generally hard and durable
• Density: Mafic rocks are denser than felsic
• Crystal structure and mineral composition
9. How does the cooling speed of magma affect the texture of igneous rocks?
The cooling speed of magma directly impacts the texture of the resulting igneous rock:
• Slow cooling (underground): Large, well-formed crystals → coarse-grained texture.
• Rapid cooling (surface): Small or no crystals → fine-grained or glassy texture.
This is why granite has large crystals and basalt is fine-grained.
10. What are the uses of igneous rocks?
Igneous rocks have many practical uses, such as:
• Granite: Building material, countertops, monuments
• Basalt: Road construction, railway ballast
• Pumice: Abrasive in cleaning and polishing; lightweight construction
• Obsidian: Surgical instruments (historically), decorative items
11. Which formula is used to calculate the heat lost during solidification of igneous rocks?
The formula for calculating heat lost or gained is:
Q = m × c × ΔT
Where:
• Q = heat lost or gained (Joules)
• m = mass (g)
• c = specific heat capacity (J/g°C)
• ΔT = change in temperature (°C)
12. Why are igneous rocks called 'primary rocks'?
Igneous rocks are termed 'primary rocks' because they were the first to form on the Earth's surface, originating directly from the cooling of magma or lava. All other rock types (sedimentary and metamorphic) are derived from the weathering, alteration, or transformation of these primary rocks.
