TC

volcanoes

Volcanoes: Chapter 5 Study Notes

Introduction

  • Volcanic Eruptions: The study of volcanic eruptions is framed by various myths and interpretations. Roman mythology includes Vulcano, where Vulcan, the god of fire, is associated with volcanic activity.

What is a Volcano?

  • A volcano is defined as an erupting vent through which molten rock surfaces. It can also be described as a mountain built from magmatic eruptions.

Distribution of Volcanoes

  • Types of Volcanoes:

    • Island Arc (I)

    • Continental Arc (C)

    • Rift (R)

    • Hot Spot (H)

Volcanic Materials

  • The materials produced during a volcanic eruption can be categorized into three main forms:

    • Lava Flows: Molten rock that moves over the ground.

    • Pyroclastic Debris: Fragments that are blown out of a volcano.

    • Volcanic Gases: Expelled vapor and aerosols.

Lava Flows

  • The characteristics of lava flows depend on their viscosity, which is influenced by:

    • Composition (especially silica content, SiO2).

    • Temperature.

    • Gas content.

    • Crystal content.

    • Mass.

Viscosity Classification

  1. Mafic Lavas

    • Silica content: 45-52%

    • Characteristics: Low viscosity, high temperature (~1200 °C), capable of flowing great distances.

  2. Intermediate Lavas (Andesite)

    • Silica content: 52-66%

    • Characteristics: Moves slowly (1-5 m per day), influenced by heat and gas content, usually flows less than 10 km.

  3. Felsic Lavas (Rhyolitic)

    • Silica content: 66-76%

    • Characteristics: Highest viscosity, rarely flows, more explosive, usually forms lava domes instead.

Types of Mafic Lavas

  • Pahoehoe: Smooth, ropy lava

  • Aa: Rough, angular lava

  • Lava tubes: Flow channels that allow lava to continue moving over long distances underground

  • Pillow Basalts: Formed when mafic lava erupts underwater and cools rapidly, resulting in blob-like shapes.

Cooling Characteristics of Lava

  • As lava cools, it contracts and fractures into hexagonal columns, known as columnar jointing.

Volcaniclastic Deposits

  • Volcaniclastic deposits often consist of significant amounts of fragments, including:

    • Pyroclastic debris: Lava fragments that solidify in air.

    • Preexisting rock: Broken apart by eruption forces.

    • Landslide debris: Material that has rolled down the slopes from the eruption.

    • Lahars: Volcanic ash and debris mixed with water.

Pyroclastic Debris Characteristics

  • Produced mostly during basaltic, andesitic, or rhyolitic eruptions, pyroclastic debris can include:

    • Lapilli: Pea-sized fragments.

    • Pele’s tears: Frozen droplets of lava.

    • Pele’s hair: Thin strands of volcanic glass.

    • Blocks: Large solid fragments.

    • Bombs: Streamlined, molten rock ejected.

Pyroclastic Flows

  • Definition: Fast-moving avalanches of hot ash, ranging 200 °C to 450 °C, capable of reaching speeds up to 300 km/h. They can incinerate everything in their path and are associated with catastrophic eruptions like those at Mount Vesuvius, Mount Pelée, and Mount Augustine.

Volcanic Architecture

  • Volcanoes have distinctive structural features:

    • Magma Chamber: A reservoir of magma beneath the surface.

    • Fissures and Vents: Openings that allow magma to escape.

    • Craters: Bowl-shaped depressions formed around the vent.

    • Calderas: Large depressions formed after the collapse of a volcano.

    • Profiles:

    • Shield Volcanoes: Broad, slightly dome-shaped, made from low-viscosity lava flows.

    • Cinder Cones: Small, conical, built from tephra.

    • Stratovolcanoes: Large, cone-shaped, characterized by alternating layers of lava and tephra.

Eruptive Styles

  • Eruptive styles vary and can be categorized as:

    • Effusive Eruptions: Produce large amounts of lava flowing gently from a vent, often associated with basaltic magma.

    • Explosive Eruptions: Characterized by the release of gas pressures, leading to significant eruptions and pyroclastic flows.

Volcanic Hazards

  • Key volcanic hazards include:

    • Lava flows: Moving slowly but can destroy property.

    • Pyroclastic flows: Extremely deadly, moving at 100-300 kph and destroying everything in a significant range.

    • Lahars: Fast-moving mudflows that can occur without an eruption, posing traffic and property risks nearby.

Historical Eruptions

  • Significant Events:

    • Mount Vesuvius: 79 A.D. eruption destroyed Pompeii, killing approximately 20,000 people.

    • Yellowstone: Experiences caldera formation and eruptions, last noted 600,000 years ago with potential significant future risk.

Current Volcanic Dangers in the U.S.

  • Notable volcanoes include:

    • Mount Saint Helens: Active with a history of devastating eruptions.

    • Mount Rainier: Considered highly dangerous due to potential lahars that could affect populated areas nearby.

Forecasting Eruptions

  • Long-term forecasting helps identify volcanoes that may erupt in the next several decades or centuries, while short-term forecasting relies on monitoring physical changes, seismic activity, and gas emissions.

Conclusion

  • Understanding volcanoes, their behaviors, materials, hazards, and monitoring helps mitigate risks associated with volcanic activity.

References

  • Additional materials and studies may be reviewed to gain deeper insights into the specific mechanisms, examples, and implications of volcanic activity.