Geology 101: Chapter 5 Lecture - Volcanoes and Volcanism

Recap of Chapter 4

• Igneous Processes: Formation of igneous rocks from magmas and lavas.
• Focus of Chapter 5: Processes of magma formation and its emergence as lava on Earth's surface.

Understanding Volcanoes

• Conventional Image: Steep mountainsides, a crater, gases, and lava.
• Variety of Forms: Different shapes and locations, not just a mountain with a vent.
• Definition: Broad definition including vents which are pathways for lava and gases.

Significance of Volcanoes

• Habitat Creation: Volcanic eruptions release gases, including water vapor, making Earth habitable.
• Oceanic Crust Formation: At divergent plate boundaries, magma forms new oceanic crust.
• Economic Impact: Eruptions like Iceland's 2011 eruption caused significant economic disruption.
• Historic Insight: Pompeii's preservation offers insight into life in 79 AD.

Volcanism

• Process: Movement of magma due to its lower density than surrounding materials.
• Eruption Types: Magma that reaches the surface releases gases and becomes lava.
• Volcano Categories:
  • Active: Documented eruptions in historic time, potentially predictable behavior.
  • Dormant: Potential to erupt, showing activity but no historic eruptions.
  • Extinct: No eruptions or activity, unlikely to erupt.

Global and Extraterrestrial Volcanism

• Mount Vesuvius: Active with historical eruptions; detailed eruption history.
• Extraterrestrial Volcanism: Volcanic activity on Jupiter’s moon Io.

Volcanic Eruptions - Components

1. Volcanic Gases: Majority is water vapor, essential for forming the hydrosphere.
2. Lava Flows:
   • Lava Tubes: Insulated flow within solidified crust.
   • Pahoehoe: Ropey, stringy texture.
   • Aa: Blocky, angular formations.
   • Pillow Lavas: Rounded shapes at divergent boundaries.
   • Columnar Joints: Polygonal shapes as lava cools.
3. Pyroclastic Materials: Solid ejecta like ash, lapilli, and volcanic bombs.

Types of Volcanoes

• Crater vs. Caldera:
  • Craters: Small, bowl-like features from explosive collapse.
  • Calderas: Large collapse features, e.g., Crater Lake.

Four Types of Volcanoes

1. Shield Volcanoes: Low, rounded; formed by mafic lava flows.
2. Cinder Cones: Small, steep-sided, often secondary.
3. Stratovolcanoes (Composite): Steep, explosive, layered; common at convergent boundaries.
4. Lava Domes: Formed from high viscosity magma, blocking vents.

Volcanic Eruption Zones

• Global Distribution:
  • Ring of Fire: High volcanic activity along the Pacific plate.
  • Mediterranean Belt: Lesser extent of volcanic activity.
• Plate Boundary Activity:
  • Divergent Boundaries: Mafic magmas, oceanic ridges.
  • Convergent Boundaries: Intermediate to felsic magmas, composite volcanoes.
• Hot Spots: Create islands like Hawaii through mantle plumes.

Volcanic Explosivity Index (VEI)

• Purpose: Measures volume of ejecta and plume height.
• Scale: Ranges from gentle to cataclysmic.
• Case Study: Mount Pinatubo's 1991 eruption (VEI 6).

Conclusion

• Homework: Chapter 5 MindTap assignment.
• Support: Discussion board and office hours for questions.