Igneous Rocks

Igneous Rocks

  • Formation: Crafted from the cooling and solidification of molten rock; occurs either on or below the Earth’s surface.

Igneous Rock Identification Scheme

  • Crystal Texture Size:

    • Obsidian: Non-vesicular, glassy (typically black).

    • Pumice: Vesicular, lightweight.

    • Scoria: Vesicular with gas pockets.

    • Rhyolite: Fine-grained, non-vesicular.

    • Basalt: Fine-grained, can be vesicular.

    • Andesite / Diorite: Intermediate textures.

    • Granite: Coarse-grained, non-vesicular.

    • Pegmatite: Very coarse, large crystals.

Magma vs. Lava

  • Magma: Hot, liquid rock beneath the Earth's surface.

  • Lava: Magma that reaches Earth’s surface.

  • Formation Locations: Magma forms in the mantle and crust.

Sources of Heat for Melting

  • Heat from below: Conduction and convection from Earth's core (high temperature).

  • Geothermal Gradient: Temperature increases with depth.

  • Radioactive Decay: Heat from radioactive elements in the crust.

  • Friction: Generated from tectonic movements, especially in mountain-building regions.

Composition of Magma

  • Approximately 99% of Igneous Rocks consist of eight key elements: Oxygen, Silicon, Aluminum, Iron, Calcium, Sodium, Potassium, Magnesium.

Types of Magma

  1. Felsic: High in silica and aluminum; lighter in color.

  2. Mafic: Rich in iron and magnesium; darker in color.

  3. Intermediate: Between felsic and mafic; medium colors.

  4. Ultramafic: Very high in iron and magnesium; rare volcanic compositions.

Melting Temperature Controls

  • Pressure: Increased pressure raises melting point; reduction may cause melting.

  • Water Under Pressure: Lowers melting point; enhances magma formation.

  • Mixed Minerals: Combinations can lower overall melting temperatures.

Types of Volcanic activity**

  • Spreading Centers, Subduction Zones, and Hot Spots: Major mechanisms driving volcanic activity, resulting in various types of igneous rocks based on composition and eruption types.

Textures of Igneous Rocks

  • Phaneritic: Coarse-grained, prominently visible crystals (intrusive).

  • Aphanitic: Fine-grained, crystals require a microscope to see (extrusive).

  • Glassy: Very rapid cooling results in a non-crystalline texture.

  • Vesicular: Characterized by gas bubbles forming holes or vesicles.