Lecture 2.2 igneous rocks

Igneous Rocks Overview

• Formation: Igneous rock forms when liquid rock (magma) cools and crystallizes into a solid form.

• Magma vs. Lava:

  • Magma: Molten rock beneath the Earth's surface.

  • Lava: Molten rock that has erupted onto the surface through volcanic activity.

Classification of Igneous Rocks

• Two primary classification criteria:

  • Texture: The size and arrangement of mineral crystals.

  • Composition: The mineral and chemical makeup.

• Cooling Rate: Influences mineral crystal size; slower cooling allows larger crystals to form.

Types of Igneous Rock

Extrusive (Volcanic) Igneous Rocks

• Formed when magma exits and cools at or near Earth's surface.

• Characteristics:

  • Fine-grained or glassy texture due to rapid cooling.

  • Common textures include:

    • Aphanitic: Very fine mineral crystals, not visible to the naked eye.

    • Vesicular: Contains gas bubbles trapped in solidified lava.

    • Glassy: Hard, glassy texture resulting from extremely rapid cooling (e.g., obsidian).

• Examples: Rhyolite (fine-grained), Basalt (fine-grained), Obsidian (glassy).

Intrusive (Plutonic) Igneous Rocks

• Formed from magma that cools slowly beneath Earth's surface.

• Characteristics:

  • Coarse-grained texture due to longer cooling time, allowing crystals to grow larger.

  • Common textures:

    • Phaneritic: Large, easily visible crystals (e.g., granite, gabbro).

• Examples: Granite (felsic), Diorite (intermediate), Gabbro (mafic).

Volcanoes and Eruptions

• Volcanoes can be characterized by the nature of eruptions:

  • Gentle Eruptions: Produce fluid lava flows, typically less dangerous.

  • Explosive Eruptions: Can result in significant hazards, as seen in the 1980 eruption of Mount St. Helens.

• Volcanic Hazards: Include lava flows, ash fallout, pyroclastic flows, and lahars, which are mudflows of volcanic material.

Composition of Igneous Rocks

• Rock Types: Classified into four main chemical groups based on silica content:

  • Felsic: High silica (65-75%), light-colored minerals (e.g., quartz, feldspar).

  • Intermediate: Moderate silica (55-60%), mixed light/dark minerals.

  • Mafic: Low silica (45-50%), rich in dark minerals (e.g., pyroxene, olivine).

  • Ultramafic: Very low silica (<40%), primarily composed of olivine.

• Chemical Composition: Varies continuously, with overlapping characteristics between categories.

Bowen's Reaction Series

• Describes the sequence of mineral crystallization from cooling magma:

  • Temperature Scale: Minerals solidify at different temperatures; higher temperatures yield early-forming minerals (e.g., olivine) and lower temperatures yield later crystallizing minerals (e.g., quartz).

  • Magmatic Differentiation: Processes that change magma composition can lead to diverse igneous rocks.

    • Includes crystal settling, partial melting, mixing of magmas, and assimilation of surrounding rocks.

Textures in Igneous Rocks

• Distinct textures indicate different cooling histories:

  • Porphyritic: Contains both large and small crystals, indicating two cooling stages.

  • Pegmatitic: Extremely large crystals forming in water-rich magma environments.

• Continuum of Textures: Ranges from fine-grained aphanitic to coarse-grained phaneritic, depending on the environment of formation.

Summary

• Classification: Based primarily on texture (size of crystals) and composition (chemical makeup).

• Cooling Rate Impact: Rapid cooling leads to fine textures (extrusive), while slow cooling leads to coarse textures (intrusive).

• Understanding these classifications provides insights into volcanic activity and the geological processes that shape igneous rocks.