Introduction to Volcanism: Formation, Hazards, and Magma Characteristics

Introduction to Volcanism
  • Focus for the Week: Introduction to volcanism, covering fundamental basics, types of volcanoes, specific facts, and hazards.
  • Scale of Volcanic Eruptions:
    • An eruption can produce an estimated 101110^{11} kilograms of material, roughly the same order of magnitude as the estimated mass of the Great Wall of China.
  • Case Study: Eyjafjallajökull Eruption (Iceland):
    • Event: A volcanic eruption in Southern Iceland near a glacier.
    • Characteristics: Produced rivers of lava over five football fields long and sent smoke more than two miles high.
    • Impact on Air Travel (April 2010):
      • Caused the worst disruption in air travel since 9/119/11.
      • Initially, over 4,0004,000 flights were canceled, primarily in Europe (many from the U.S. to Europe).
      • By the fifth day, over 63,00063,000 flights had been canceled since Thursday.
      • Affected almost 7,000,0007,000,000 passengers.
      • Estimated cost to airlines was 200,000,000200,000,000 per day.
      • The ash cloud followed winds across the Atlantic, impacting a vast area.
      • The eruption strengthened, and a new ash cloud moved south and east toward the UK.
      • Largest shutdown of airspace since World War II.
      • The eruption lasted for about 3939 days, with the volcano eventually quieting months later.
    • Observations:
      • The eruption plume sometimes broke the sound barrier, creating a visible white cloud.
      • Scientists used snowmobiles to get a closer look.
      • Massive ash cloud disrupted travel across six continents.
      • After six days, flights like British Airways from Vancouver started landing at Heathrow Airport again, met with cheering and applause.
    • Precursors: Volcanic eruptions often provide good indicators well in advance, primarily based on seismicity (earthquakes).
    • Location and Risk (Iceland):
      • Eyjafjallajökull is near Katla, an even larger and more active volcano.
      • The melting of glacial ice caps due to eruptions (jökulhlaup or glacial floods) creates massive amounts of water channeled into valleys.
      • Southern Iceland has a main road ringing the island with many fishing villages that are vulnerable to these floodwaters.
      • During the Eyjafjallajökull event, Iceland's president warned that Katla's eruption was coming and urged European governments and airline authorities to prepare.
  • Volcanic Activity in Iceland: The entire island of Iceland is essentially a giant volcano because it is situated directly on the Mid-Atlantic Ridge, the longest volcanic feature on Earth.
  • Global Distribution of Volcanoes: Most volcanoes are found along plate boundaries, with approximately two-thirds located around the Pacific Plate Boundary, collectively known as the "Ring of Fire."
Magma vs. Lava
  • Magma: Molten rock underground.
  • Lava: Molten rock that has erupted to the surface.
Where Magma is Created
  • Earth's Interior: It's crucial to understand that the Earth's interior is not entirely liquid; while extremely hot (1000exts1000 ext{s} of degrees Celsius), immense pressure keeps most of the mantle solid.
  • "Magma Factories" (Primary Locations): Magma generation is mostly associated with plate tectonic boundaries and specific anomalies.
    • Spreading Centers (Divergent Plate Boundaries): Examples include the Mid-Atlantic Ridge and other Mid-Ocean Ridges.
    • Subduction Zones (Convergent Plate Boundaries): Where oceanic crust is forced back into the mantle.
    • Hotspots: Areas of volcanic activity not directly linked to plate boundaries.
How Magma is Created (Mechanisms)
  • 1. Decompression Melting:
    • Process: As solid rock in the mantle rises towards the surface at spreading centers, the overlying pressure decreases significantly (more so than a temperature change).
    • Effect: This reduction in pressure allows the solid rock to melt, forming magma.
    • Location: Primarily occurs at divergent plate boundaries like mid-ocean ridges.
  • 2. Flux Melting (Addition of Volatiles/Water):
    • Process: At subduction zones, oceanic crust carrying water (volatiles) is subducted into the hot mantle.
    • Effect: The presence of water lowers the melting temperature of the mantle rock, causing it to melt and form magma.
    • Location: Primarily occurs at convergent plate boundaries (subduction zones).
  • 3. Mantle Plumes / Hotspots (Hypothesis):
    • Concept: These are thought to be stationary columns of hot rock rising from the deep mantle, possibly originating at the outer core-mantle boundary.
    • Mechanism: A disturbance creates a "plume head" that slowly rises (over hundreds of millions of years), burning through the overlying crust.
    • Relationship to Plate Tectonics: Mantle plumes behave independently of the main plate tectonic convective network.
    • Volcano Formation: As tectonic plates move over these stationary plumes, a chain of volcanoes can form.
    • Example: The Hawaiian island chain is a classic example of volcanoes formed by a plate moving over a stationary mantle plume.
Magma Composition and Associated Rocks
  • Classification Basis: Magma compositions are primarily differentiated by their silica content.
  • Compositional Groups:
    • Felsic Magmas:
      • Silica Content: High.
      • Mineralogy: Rich in feldspar and silica.
      • Chemical Affinity: Compositionally similar to continental crust.
      • Associated Volcanic Rock: Rhyolite (e.g., Rhyolitic volcanoes).
    • Mafic Magmas:
      • Silica Content: Low.
      • Mineralogy: Rich in magnesium and iron.
      • Chemical Affinity: Compositionally close to the mantle.
      • Associated Volcanic Rock: Basalt (e.g., Basaltic volcanoes).
    • Intermediate Magmas:
      • Silica Content: Between felsic and mafic.
      • Associated Volcanic Rock: Andesite (e.g., Andesitic volcanoes).
  • Magma Evolution:
    • Magma originates in the mantle (mafic/ultramafic composition).
    • As magma rises through the crust, it can interact with and incorporate (melt and assimilate) the overlying continental rock.
    • This process allows mafic magmas to chemically evolve, becoming more intermediate or felsic in composition. This explains how volcanoes can exhibit a range of compositions.
Upcoming Topics
  • Future discussions will delve deeper into trapped gases, specific volcano types, and hazards, with a lab session focused on volcanic rocks.