Types of Volcanoes

Why This Matters

Volcanoes aren't just dramatic geological features—they're windows into Earth's interior processes and key players in the rock cycle, atmospheric composition, and landform development. When you study volcano types, you're really learning about magma viscosity, plate tectonic settings, and eruption dynamics—concepts that connect to everything from seafloor spreading to climate change to hazard assessment. The AP exam loves to test whether you understand the relationship between a volcano's structure and its behavior.

Don't just memorize names and locations. For each volcano type, ask yourself: What kind of magma creates this? What tectonic setting produces it? How does its eruption style affect nearby communities and global systems? When you can answer those questions, you're thinking like an earth scientist—and you're ready for any FRQ they throw at you.

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Low-Viscosity Eruptions: The Gentle Giants

When magma has low silica content, it flows easily and releases gases without explosive buildup. These eruptions build broad structures and can last for extended periods.

Shield Volcanoes

• Broad, gentle slopes (2-10°)—formed by highly fluid basaltic lava that travels long distances before cooling
• Non-explosive eruptions produce massive lava volumes over time, making these the largest volcanoes by volume on Earth
• Hotspot association—Mauna Loa and Kilauea in Hawaii demonstrate how mantle plumes create volcanic chains as plates drift overhead

Fissure Volcanoes

• Linear vent systems erupt along cracks in the crust rather than from a central cone, producing extensive lava fields called flood basalts
• Massive lava output—the 1783 Laki eruption in Iceland released enough sulfur dioxide to cause crop failures across Europe
• Divergent boundary connection—Iceland's location on the Mid-Atlantic Ridge makes it a prime example of rifting volcanism

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High-Viscosity Eruptions: The Explosive Builders

Silica-rich magma traps gases and resists flow, creating pressure that leads to violent eruptions. These volcanoes build steep structures and pose significant hazards.

Stratovolcanoes (Composite Volcanoes)

• Steep, symmetrical cones built from alternating layers of lava, ash, and pyroclastic material—hence "composite"
• Explosive eruption potential due to high-viscosity andesitic or rhyolitic magma that traps volcanic gases until pressure overcomes resistance
• Subduction zone indicators—Mount St. Helens and Mount Fuji mark convergent boundaries where oceanic crust melts beneath continental plates

Lava Domes

• Bulbous, steep-sided structures form when extremely viscous lava piles up near the vent instead of flowing away
• Collapse hazards—domes can become unstable and generate deadly pyroclastic flows, as seen in the Mount St. Helens dome growth after 1980
• Often form inside craters of larger volcanoes during post-eruption activity, like Novarupta in Alaska

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Small-Scale and Short-Lived Features

Not all volcanoes build massive structures. Some form quickly from single eruptions and represent localized volcanic activity.

Cinder Cone Volcanoes

• Steep slopes (30-40°) built from loose pyroclastic fragments (cinders or scoria) that pile up around a single vent
• Short eruption lifespans—Parícutin in Mexico famously grew from a cornfield to 336 meters in just one year (1943-1944)
• Often appear on flanks of larger volcanoes or in volcanic fields, representing the simplest and smallest volcano type

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Collapse and Catastrophic Features

Some volcanic landforms result not from building up, but from dramatic collapse following massive eruptions.

Caldera Volcanoes

• Large circular depressions form when a magma chamber empties rapidly during eruption and the overlying rock collapses inward
• Supervolcano association—Yellowstone Caldera sits atop a massive magma reservoir capable of continental-scale eruptions
• Post-collapse activity can include resurgent domes, hot springs, and new volcanic cones within the caldera, as seen at Santorini, Greece

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Underwater Volcanism: Hidden Builders

Most of Earth's volcanic activity occurs beneath the oceans, where eruptions create new seafloor and can eventually build islands.

Submarine Volcanoes

• Pillow lava formations result from rapid cooling when hot basalt meets cold seawater, creating distinctive rounded shapes
• Island-building potential—Loihi Seamount near Hawaii will eventually emerge as the newest Hawaiian island in 10,000-100,000 years
• Mid-ocean ridge dominance—the majority of Earth's magma output occurs at submarine spreading centers, driving the rock cycle and seafloor spreading

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Quick Reference Table

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Self-Check Questions

1. Which two volcano types are most associated with low-viscosity basaltic magma, and how do their vent structures differ?

2. A volcano has steep sides, alternating layers of ash and lava, and is located near a subduction zone. What type is it, and what makes its eruptions dangerous?

3. Compare and contrast calderas and cinder cones in terms of formation process, size, and eruption history.

4. An FRQ asks you to explain how plate tectonic setting influences volcano type. Which three examples would you choose to represent divergent boundaries, convergent boundaries, and hotspots?

5. Why do lava domes pose ongoing hazards even after the initial eruption ends, and how does this relate to magma viscosity?