Overview
This lecture explains how magma viscosity and gas content control whether volcanic eruptions are explosive or mild, focusing on differences between composite and shield volcanoes.
Viscosity: Definition and Examples
- Viscosity is the resistance of a material to flow.
- Low viscosity liquids (e.g., water, milk) flow easily.
- Medium viscosity liquids (e.g., oils) have moderate resistance to flow.
- High viscosity substances (e.g., syrup, honey) flow slowly and resist movement.
Gas Content and Eruption Style
- Magma contains dissolved gases such as sulfur dioxide and carbon dioxide under high pressure.
- As magma rises, pressure decreases and gases come out of solution, increasing volume.
- Escaping gases propel magma upward and drive volcanic eruptions.
Influence of Viscosity on Eruptions
- Low viscosity magmas (mafic composition) let gases escape easily, causing mild, flowing eruptions.
- High viscosity magmas (intermediate or felsic composition) trap gases, leading to pressure buildup and violent eruptions.
- Demonstration: Blowing air into water (low viscosity) vs. a milkshake (high viscosity) shows how trapped gas can cause splattering.
Types of Volcanoes & Eruption Styles
- Shield volcanoes have low viscosity magma, resulting in gentle lava flows and occasional fountains.
- Composite volcanoes have high viscosity magma, causing trapped gas and explosive eruptions that produce tephra clouds.
Key Terms & Definitions
- Viscosity — resistance of a material to flow.
- Mafic composition — magma rich in magnesium and iron; low viscosity.
- Felsic composition — magma rich in silica; high viscosity.
- Tephra — fragments of volcanic rock and ash ejected during an eruption.
Action Items / Next Steps
- Define viscosity and give examples of materials with various viscosities.
- Explain how gas content and magma viscosity determine eruption style.
- Review differences between shield and composite volcano eruptions.