Volcanoes

Magma

  • Mix of molten rock, mineral grains, and dissolved gases.

  • In the last 10,000 years, over 1500 volcanoes have erupted.

How Magma Forms

  • Forms deep beneath Earth's surface at 800°C to 1200°C.

  • Found beneath the lithosphere in the asthenosphere.

  • Factors affecting magma formation include pressure, water, and temperature.

Pressure

  • Pressure increases with depth, requiring higher temperatures to melt rocks.

  • Higher pressure keeps the lower mantle solid.

Water

  • Increased water content lowers the melting point of rocks.

Temperature

  • Temperature increases with depth, and rocks have specific melting points.

Types of Magma

  • Three types: basaltic, andesitic, rhyolitic.

    • Basaltic: low silica, not very explosive (e.g., Hawaiian Islands).

    • Andesitic: very explosive (e.g., Mount St. Helens).

    • Rhyolitic: high silica, intensely explosive (e.g., Yellowstone).

Magma Composition

  • Viscosity affects magma composition.

    • Hotter magma = lower viscosity.

    • Basaltic magma: 1000 to 1250°C.

    • Rhyolitic magma: 700-900°C.

    • Rhyolitic magma has greater viscosity due to lower temperature, resulting in thicker magma.

Types of Lava

Lava Type

Silica Content

Viscosity

Eruption Temperature

Violence of Eruption

Basaltic

Low

Low (runny)

>950°C

Least violent

Andesitic

Medium

Medium

750-950°C

Medium

Rhyolitic

High

High (thick)

<750°C

Most violent

Intrusive Activity

  • Plutons: Intrusive igneous rock bodies classified by size and shape.

    • Batholith: Largest plutons, mostly granite, formed over millions of years.

Volcano Characteristics

  • Vent: Opening in the crust where lava emerges.

  • Crater: Bowl-shaped depression at the volcano's top (<1km diameter).

  • Calderas: Large depressions (up to 50km diameter) formed by collapse (e.g., Crater Lake).

Types of Volcanoes

  • Appearance depends on material and eruption type.

Shield Volcanoes

  • Broad, gentle slopes, circular base, basaltic magma, nonexplosive, low viscosity.

Cinder-Cone Volcanoes

  • Steep sides, small, intermediate viscosity, more explosive.

Composite Volcanoes

  • Layers of volcanic fragments and lava, large, steep, violent explosions.

Size and Slope

  • Size: Shield > Composite > Cinder Cone.

  • Slope: Cinder Cone > Composite > Shield.

Volcanic Material

  • Tephra: Rock fragments ejected during eruptions.

    • Dust: Smallest fragments.

    • Ash: Medium-sized fragments.

    • Lapilli: Larger fragments (little stones).

  • Pyroclastic flows: Clouds of gas, ash, and tephra moving rapidly down slopes, very dangerous (over 700°C).

Where Volcanoes Occur

  • Most at plate boundaries (80% convergent, 15% divergent, 5% far from boundaries).

Convergent Volcanism

  • Subduction at plate boundaries, common in the Pacific Ring of Fire.

Divergent Volcanism

  • Magma forced into fractures as plates move apart (rift volcanism).

Hot Spots

  • Volcanoes far from plate boundaries, caused by mantle plumes (e.g., Hawaii).

  • Chain of islands indicates plate movement direction over the hot spot.