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.