Igneous Rocks

GROUP 4 IGNEOUS ROCKS

1. LEARNING OBJECTIVES

  • Identification Skills: Students should be able to identify igneous rocks.

  • Classification: Understand how igneous rocks are classified.

  • Characterization: Be able to characterize the properties of igneous rocks.

  • Composition: Learn the composition of igneous rocks.

2. WHAT ARE IGNEOUS ROCKS?

  • Definition: Also known as magmatic rocks, they form from the cooling of magma.

  • Etymology: Derived from the Latin word "ignis" meaning fire.

  • Types of Rocks: One of the three major rock types along with sedimentary and metamorphic.

  • Intrusive and Extrusive: Includes both intrusive (plutonic) and extrusive (volcanic) rocks.

3. FORMATION OF IGNEOUS ROCKS

  • Igneous rocks are formed when molten rock crystallizes and solidifies

  • Conditions: Formed at very high temperatures either below the Earth's crust or on the surface.

4. COMPOSITION OF IGNEOUS ROCKS

  • Magma and Lava: Formed from both magma (underground) and lava (above ground).

  • Characteristics: May include crystalline, solid and gaseous fractions.

  • Divisions: Can be categorized based on chemical composition.

5. MAGMA VS LAVA

  • Magma:

    • Molten rock found underground (in the mantle)

    • Hypothetical melt, not yet exposed

  • Lava:

    • Exposed molten rock expelled from volcanoes

    • Known components when it erupts

6. CHARACTERISTICS OF MAGMA

  • Types of Magma:

    • Basaltic Magma:

    • Andesitic Magma:

    • Rhyolitic Magma:

7. CLASSIFICATION OF IGNEOUS ROCKS

7.1 Based on Silica Content

  • Acidic Rocks:

    • Silica Content: More than 66%

    • Composition: High in silica, poor in Fe and Mg.

    • Example: Granites

  • Basic Rocks:

    • Silica Content: Less than 50%

    • Composition: Rich in Fe and Mg, poor in silica.

    • Example: Basalt

7.2 Based on Solidification Depth

  • Volcanic (Extrusive) Rocks:

    • Formed when magma erupts at the surface.

    • Cool rapidly, resulting in fine-grained textures.

    • Examples: Basalt, Rhyolite.

  • Plutonic (Intrusive) Rocks:

    • Form at depths of 7 to 10 km.

    • Slow cooling leads to coarse-grained textures.

    • Examples: Granite, Gabbro.

  • Hypabyssal Rocks:

    • Form at intermediate depths.

    • Textures between volcanic and plutonic.

    • Example: Porphyry.

8. TEXTURE OF IGNEOUS ROCKS

  • Definition: Refers to the size, shape, and arrangement of minerals within the rock.

  • Factors Affecting Texture: Include cooling rate, degree of crystallization, and overall fabric.

8.1 Types of Texture

  • Equigranular Texture: Similar sized crystals, exemplified by granite.

  • Felsitic Texture: Micro granular sails seen in rhyolite.

  • Porphyritic Texture: Large phenocrysts in a finer groundmass in rocks like basalt.

  • Intergranular Texture: Spaces between crystals, can be glassy.

9. IMPORTANT IGNEOUS ROCKS

  • Pluton: Deep-seated intrusive rock formation.

  • Batholiths: Large mass formed deep in the crust, predominantly granite.

  • Dikes: Solidified magma that cuts across other rock layers.

  • Sills: Tabular intrusions between older sedimentary layers.

  • Laccoliths: Mushroom-shaped intrusive bodies.

  • Volcanic Necks: Hardened lava plugs within a volcano vent, can lead to eruptions.

10. IGNEOUS ROCK PROPERTIES

10.1 Shapes

  • Euhedral: Well-formed crystals.

  • Subhedral: Partially formed crystals.

  • Anhedral: Irregularly shaped crystals.

10.2 Granularity**

  • Coarse-Grain (Phaneritic): Crystals visible to the naked eye, formed from slow cooling.

  • Medium-Grain (Porphyritic): Mix of large and small crystals, forms from multiples cooling stages.

  • Fine-Grain (Aphanitic): Microscopic crystals, formed from rapid cooling.