Igneous Rocks and Volcanoes

Igneous Rocks & the Rock Cycle

  • Igneous Rocks: Formed by cooling and crystallization of molten rock (magma/lava).

Types of Molten Rock

  • Lava: Molten rock that reaches Earth's surface.
  • Magma: Molten material beneath Earth's surface.

Textures of Igneous Rocks

  • Cooling Rate: Influences the type of igneous rocks formed.
    • Extrusive Rocks: Form from lava cooling on surface (e.g., volcanic rocks).
    • Intrusive Rocks: Form from magma cooling beneath the surface (e.g., plutonic rocks).

Classifying Igneous Rocks

  1. Felsic Rocks:

    • High in silica, low in iron/magnesium.
    • Low melting temperature, high viscosity.
    • Examples: Granite (coarse) and Rhyolite (fine).

  2. Intermediate Rocks:

    • Intermediate in silica and iron/magnesium.
    • Intermediate melting temperature and viscosity.
    • Examples: Diorite (coarse) and Andesite (fine).

  3. Mafic Rocks:

    • Low in silica but high in iron/magnesium.
    • High melting temperature, low viscosity.
    • Examples: Gabbro (coarse) and Basalt (fine).

  4. Ultramafic Rocks:

    • Very low in silica, very high in iron/magnesium.
    • Very high melting temperature, low viscosity.
    • Examples: Peridotite (coarse) and Komatiite (fine).

Evolution of Igneous Rocks

  • Bowen's Reaction Series: Outlines the order of mineral crystallization based on temperature.
    • High temperatures favor the formation of olivine and pyroxene, while lower temperatures favor quartz and muscovite.

Volcanic Compositions

  • Volcano: Structure that forms when magma erupts as lava.
  • Composition impacts:
    • Type of eruptions
    • Structure of volcanoes based on proportions of silica, iron, magnesium.

Volcanic Gas Content

  • 1-9% of magma may consist of gases; mainly water vapor and carbon dioxide.
  • Felsic magmas contain more gas, leading to more explosive eruptions.
  • Gas Expulsion: Gas bubbles form vesicles when magma rises and pressure decreases.

Lava Types

  1. Mafic Lava: 1000° - 1200°C; low viscosity; produces quiet eruptions.
  2. Intermediate Lava: 900° - 1200°C; intermediate viscosity; can produce explosive eruptions.
  3. Felsic Lava: 800° - 1200°C; high viscosity; tends to produce violent eruptions.

Mafic Lava Eruptions

  • Features:
    • Pahoehoe: Ropy texture from cooling mafic lava.
    • Aa: Jagged texture from broken surface; molten interior flows beneath.
    • Lava Fountain: Jets of incandescent lava.
    • Pillow Lava: Forms as lava erupts into seawater.

Intermediate/Felsic Lava Eruptions

  • Aerosols: Gas and particles ejected into the atmosphere.
  • Pyroclastic Material: Particles ejected by explosive eruptions, classified as:
    • Bombs/Blocks: largest fragments.
    • Lapilli: gravel-sized.
    • Ash: fine particles.
  • Pyroclastic Flow: Fast-moving ground-hugging avalanches of ash.
  • Lahar: Volcanic mud flow caused by melting snow or heavy rainfall mixing with ash.

Volcanic Architecture

  • Key features: magma chamber, fissures, vents, craters, and calderas.
    • Magma Chamber: A reservoir of magma, may contain intrusive rock.
    • Craters: Bowl-shaped depressions from eruptions.
    • Calderas: Large depressions resulting from a collapse of a volcano after a massive eruption.

Types of Volcanoes

  1. Shield Volcanoes: Gently sloping, low viscosity mafic lava, non-explosive.
  2. Cinder Cones: Steep sides, built from pyroclastic material, explosive eruptions.
  3. Composite Volcanoes: Alternating layers of lava and pyroclastic material, often explosive.

Eruptive Styles

  • Effusive Eruptions: Produce lava flows.
  • Explosive Eruptions: Release pressure catastrophically, creating pyroclastic flows and tephra rain.

Societal Impact of Volcanoes

  • Volcanic eruptions pose natural hazards. Past fatality causes include lava, gases, and pyroclastic flows.
  • Protection Strategies:
    • Danger assessment maps.
    • Evacuation plans for high-risk areas.
    • Community awareness programs to improve response efforts.

Climate Impact of Volcanoes

  • Eruptions can lead to short-term atmospheric cooling due to ash and aerosols blocking sunlight (e.g., Mt. Tambora, 1815).
  • Long-term effects include greenhouse gas emissions impacting climate.

Example Cases of Volcanic Eruptions

  • Mount Vesuvius: Buried Pompeii in 79 AD.
  • Mount St. Helens: Erupted in 1980 after 350 years of dormancy, showcasing the power of explosive eruptions.
  • Yellowstone Supervolcano: Last eruption 640,000 years ago, has potential for future activity.

Conclusion/Review Points

  • Understand the link between rock types and volcanic eruptions.
  • Study the composition's influence on eruption types.
  • Analyze the societal implications of volcanic activity and the importance of preparedness.