Volcano structures study guide notes

Volcanic structures can be categorized into several main types, including:


1. Three Main Types of Volcanic Peaks

Shield Volcanoes

  • Tectonic Setting: Oceanic hotspots (e.g., Hawaiian hotspot) and divergent plate boundaries (e.g., mid-ocean ridges, Iceland).

  • Magma Composition: Mafic / Basaltic magma with low silica content (∼45–52%∼45–52%), low viscosity, high temperature, and low gas content.

  • Relative Size: Massive in volume and horizontal area; largest volcanic structures on Earth.

  • Shape: Broad, gently sloped dome profile with shallow angles between 2∘2∘ and 10∘10∘.

  • Structure: Layered sheets of solidified, fluid basaltic lava flows.

  • Eruptive Style: Effusive ("flow"); non-explosive continuous lava streams and fountains.

  • Real-World Examples: Mauna Loa and Kīlauea in Hawaii.

Composite Cones (Stratovolcanoes)

  • Tectonic Setting: Convergent plate boundaries with subduction zones (e.g., Pacific Ring of Fire, Cascade Range).

  • Magma Composition: Intermediate to felsic magma (andesitic to dacitic/rhyolitic) with high silica content, high viscosity, and trapped gas.

  • Relative Size: Large, tall mountain structures rising thousands of meters above sea level.

  • Shape: Steep-sided, symmetrical conical profile with summit slopes reaching up to 30∘30∘.

  • Structure: Alternating composite layers of hardened lava, volcanic ash, tephra, pumice, and cinders.

  • Eruptive Style: Explosive ("blow"); violent eruptions driven by trapped gas pressure.

  • Real-World Examples: Mount Fuji (Japan), Mount St. Helens (USA), Mount Vesuvius (Italy).

Cinder Cones (Scoria Cones)

  • Tectonic Setting: Flanks of larger volcanoes or continental rift zones.

  • Magma Composition: Gas-rich basaltic to intermediate magma.

  • Relative Size: Smallest volcanic peak type, typically under 300 m300m to 400 m400m in height.

  • Shape: Steep, symmetrical cone with slope angles between 30∘30∘ and 40∘40∘ and a bowl-shaped summit crater.

  • Structure: Loose accumulations of cooled pyroclastic debris, scoria, cinders, and volcanic bombs.

  • Eruptive Style: Explosive ("blow"); short-lived gas-driven Strombolian eruptions.

  • Real-World Examples: Parícutin (Mexico), Sunset Crater (Arizona).

2. Igneous Rock Composition & Tectonic Settings

Color Guide & Composition

  • Felsic: <25%<25% dark minerals (shows pink, white, translucent gray quartz).

  • Intermediate: 25–45%25–45% dark minerals.

  • Mafic: Very dark minerals.

  • Ultramafic: Black and green minerals.

Tectonic Settings & Magma Generation

  • Allowed Settings: Oceanic-Continental convergent, Oceanic-Oceanic convergent, divergent boundaries, and hotspots (not CC convergent or transform).

  • Partial Melting: Releases lighter (felsic) material, shifting magma composition toward higher silica.

  • Hotspots & Divergent Boundaries: Melt peridotite mantle to produce mafic magma.

  • Subduction Zones: Melt basaltic plate to produce intermediate/felsic magma.

3. Viscosity & Eruption Style

  • High Viscosity: Intermediate/felsic magma traps gases, creating violent explosive eruptions at subduction zones.

  • Low Viscosity: Mafic magma allows gases to escape easily, producing smooth flow eruptions (e.g., Pahoehoe and A'a lavas).

4. Volcanic Hazards & Case Study: Mount St. Helens

Primary Composite Cone Hazards

  • Pyroclastic Flows: Superheated gas and ash traveling at 100+ mph100+ mph and temperatures up to 1,300 ∘F1,300∘F, incinerating everything in path.

  • Lahars: Volcanic mudflows; Mount Rainier poses high danger due to proximity to Seattle/Tacoma.