Volcanism and Volcanoes

Chapter 5: Volcanism and Volcanoes

  • Example of Rock:

    • Gabbro: A type of rock that is black and characterized by visible mineral crystals, classified as plutonic (formed beneath the surface).

    • Other commonly found plutonic rocks include felsic rocks like granite.

Processes of Volcanism

  • Definition of Volcanism:

    • Describes the processes leading to the extrusion of magma, gases, and pyroclastic materials onto the Earth’s surface and into the atmosphere, via explosive eruptions and lava flows.

  • Volcano Types Based on Activity:

    • Active Volcanoes: Currently erupting or nearing eruption. Examples include Hawaii's volcanoes, Mt. St. Helens, Mt. Fuji, Mt. Vesuvius, and Mt. Pinatubo.

    • Dormant Volcanoes: Not erupted in recent history but may erupt in the future (e.g., San Salvador Volcano).

    • Extinct Volcanoes: No evidence of future eruptions.

Volcanic Gases

  • Composition and Impact:

    • 50-80% of volcanic gases consist of water vapor; others include carbon dioxide (CO2), nitrogen, sulfur dioxide (SO2), hydrogen sulfide, and carbon monoxide (CO).

    • Gases expand in rising magma, leading to potential violent explosions

  • Hazards: Exposure to toxic gases can lead to fatalities due to suffocation or toxic exposure.

Lava Flows

  • Characteristics:

    • Lava flows, which typically follow low known paths, move slow, rarely a risk to humans

    • Types of Lava Flows:

    • Pahoehoe: smooth, ropy surface, similar to taffy, and is low in viscosity.

    • Aa: Contains rough, jagged blocks and fragments, exhibiting higher viscosity.

  • Cooling Process:

    • Columnar joints form in cooling lavas, creating fractures that extend downward into the flow, commonly exhibiting a hexagonal shape.

Pillow Lava Formation

  • Underwater Eruptions:

    • Pillow lava is formed during underwater volcanic activity when lava is rapidly chilled by water, resulting in bulbous shapes.

Pyroclastic Materials

  • These solid fragments are ejected during explosive volcanism and include:

    • Ash: Diameter < 2 mm, can travel long distances from the eruption site.

    • Lapilli: Diameter 2-64 mm.

    • Bombs/Blocks: Diameter > 64 mm.

Nature of Volcanic Eruptions

  • Several factors influence whether a volcano erupts violently or gently:

    • Composition of Magma

    • Temperature of Magma

    • Dissolved Gases present in Magma

Summary of Factors Affecting Eruptions

  • Gas Expansion: Gases within magma expand near the surface due to decreased pressure, influencing the explosion's violence.

  • Eruption Characteristics:

    • Basaltic lavas generally produce quiet eruptions, allowing gases to escape easily.

    • Highly viscous lavas, such as rhyolite, produce more explosive eruptions.

What are Volcanoes?

  • The term volcano originates from the Roman god of fire, Vulcan.

  • Defined as a conical mountain composed of erupted lava, gases, and pyroclastic materials.

  • Most volcanoes feature:

    • A circular depression at the summit known as a crater, usually with a diameter of < 1 km.

    • Larger depressions called calderas, which are > 1 km in diameter, result from volcanic collapse.

Five Major Types of Volcanoes

  1. Caldera: Formed by the collapse of a volcano after a magma chamber empties.

    • Example: Crater Lake, Oregon.

  2. Shield Volcano: Broad, gently sloping volcano formed mainly from fluid lava flows (e.g., Hawaiian volcanoes like Mauna Loa).

  3. Cinder Cone: Simple volcano-shaped structure composed of ejected blobs of lava around a single vent. Typically <400 m high with steep slopes (up to 33°).

    • Examples: Eldfell, Iceland; Paricutin, Mexico.

  4. Stratovolcano (Composite Volcano): Built from alternating layers of lava and pyroclastic materials, exhibiting steep slopes near the summit and a more gradual slope at the base. Examples include Mt. Fuji, Mt. St. Helens, Mt. Vesuvius.

  5. Lava Dome: Formed when highly viscous lava is forced up and accumulates around the vent, resulting in steep-sided formations.

Fissure Eruptions and Basalt Plateaus

  • In some regions, volcanic activity does not lead to the development of traditional volcanoes.

  • Fissure Eruptions: Lava extruded along fractures in the crust leads to basalt plateaus, distinct from central-vent eruptions.

    • Historical example: The Laki eruption in Iceland (1783) produced massive flows covering 588 km², leading to a significant population decline due to famine.

Super-Eruptions of Flood Basalts

  • These eruptions have dramatic effects on both climate (leading to global darkness) and environment (CO2 enrichment). Examples include the Deccan Traps in India, which involved vast outpourings of lava.

Volcanic Explosivity Index (VEI)

  • VEI Definition: A scale that ranks eruptions from 0 (unexplosive) to 8 (megacolossal) based on the volume of material ejected and the height of the eruption plume. Duration of eruptions may range from days to years, with most historic eruptions lasting 1-6 months.

Predicting Eruptions

  • Monitoring of volcanoes involves tracking physical and chemical changes, such as:

    • Tiltmeter changes, seismic activity, magnetic/electrical field variations, gas emissions, and groundwater temperature fluctuations.

  • Despite advancements, volcanoes remain inherently unpredictable.

Distribution of Volcanoes

  • Volcanoes are predominantly located near plate boundaries, arranged in defined belts:

    • Circum-Pacific Ring of Fire: Over 60% of the world's volcanoes.

    • Mediterranean Belt: About 20%.

    • Linear mid-ocean trends also recognized.

Plate Tectonics and Volcanic Activity

  1. Divergent Plate Boundaries: Mostly associated with igneous activity, where ocean crust is predominantly made of basalt and gabbro (e.g., Mid-Atlantic Ridge).

  2. Convergent Plate Boundaries: Associated with composite volcanoes formed during partial melting processes, resulting in intermediate/felsic magmas (e.g., Mt. Pinatubo and Mt. Vesuvius).

  3. Intraplate Volcanism: Occurs as tectonic plates move over stationary hot spots, with Hawaiian Islands as a key example formed by mantle plume activity.