JH

Volcanos: A Comprehensive Study Guide

Definition and Formation

  • A volcano is a vent in Earth's crust through which lava, rock fragments, hot vapor, and gas erupt.

  • Volcanoes erupt when a portion of the Earth's upper mantle or lower crust melts, forming magma.

    • Magma: Molten rock formed from partial melting of Earth's mantle and crust. Composed of silicon, oxygen, minerals, and dissolved gases (especially water), with gas pressure driving its ascent.

    • Magma and gases create fissures (cracks), rise through conduits, and erupt through vents.

    • Erupted molten rock is called lava, appearing as fountains or explosive eruptions.

  • Most lava flows occur through fissures along the volcano's sides, not just the main top vent.

Types of Volcanos
1. Shield Volcano

  • Name Origin: Resembles a warrior's shield.

  • Size: Usually the largest volcanoes.

  • Magma/Lava Properties:

    • Temperature: Hottest, ranging from 1,000^{\circ}C to 1,250^{\circ}C.

    • Gas Content: Lowest (0.5-2\%).

    • Silica Content: Lowest (50\%).

    • Viscosity: Low viscosity due to low gas and silica content, allowing lava to escape easily.

    • Flow Type: Characterized by very fluid (water-like), gentle flows.

    • Nature: Mafic/basaltic.

    • Tephra: Materials flowing from the volcano, also referred to as tephra in this context.

  • Location: Typically found in oceans.

  • Gentle Flow Kinds:

    • Aa: Clinky and jagged-looking lava.

    • Pahoehoe: Smooth, with a look of ropy braids. 90\% of basaltic lava flow is Pahoehoe.

    • Pahoehoe can transform into Aa flow, but Aa flow cannot revert to Pahoehoe flow.

2. Composite / Stratovolcano

  • Nature: Mostly intermediate, except for their eruptions.

  • Danger Level: Most dangerous eruptions of all volcano types.

  • Magma/Lava Properties:

    • Temperature: Intermediate, ranging from 800^{\circ}C to 1,050^{\circ}C.

    • Make-up: Andesitic.

    • Gas Content: Intermediate (3-4\%).

    • Silica Content: Intermediate (60\%).

    • Flow Type: Produces both lava flow and pyroclastic tephra (fire pieces).

  • Location: Usually situated on the perimeter or edge of continents.

3. Cinder Cone / Scoria Cone

  • Size: Smallest type of volcano due to short eruptive cycles, forming quickly.

  • Magma/Lava Properties:

    • Temperature: Lowest eruptive temperature, ranging from 650^{\circ}C to 900^{\circ}C.

    • Nature: Felsic (sialic/granitic/rhyolitic).

    • Gas Content: Most gas (5-8\%).

    • Silica Content: Most silica (70\%).

    • Eruption Type: Extremely explosive eruptions with pyroclastic tephra.