Volcanism and Plate Boundaries

Zones of Volcanism

• Volcanism occurs at convergent plate boundaries, divergent plate boundaries, and hot spots.
• Oceanic-continental subduction zones cause mantle melting and magma rising, forming volcanoes. Most land volcanoes result from this type of subduction, known as explosive eruptions.
• Divergent boundaries typically produce nonexplosive eruptions. Pillow lava is formed at divergent boundaries on the ocean floor.
• Hot spots are unusually hot areas in Earth's mantle. High-temperature plumes of mantle material rise to the surface in these locations.

Hotspots and Plate Motion

• Chains of volcanoes over stationary hotspots show plate motion.
• Flood basalts are formed when lava flows out of long cracks in Earth's crust, called fissures.

Anatomy of a Volcano

• Crater: A bowl-shaped depression at the volcano's summit surrounding the vent.
• Caldera: Larger depressions at the summit of a volcano.
• Magma Chamber: A region below Earth's surface that stores molten material known as magma.
• Vent: The opening where lava emerges.
• Conduit: A tube-like structure that transports magma from the magma chamber to the vent.

Types of Volcanoes

• The appearance of a volcano depends on the materials that form the volcano and the eruptions.
• Volcanoes are classified as shield volcanoes, cinder cones, and composite volcanoes.

Shield Volcanoes

• Shield volcanoes have broad, gently sloping sides and a nearly circular base.
• They form from layers of lava accumulated during nonexplosive eruptions.
• Shield volcanoes are the largest type of volcano.

Cinder Cone Volcanoes

• Cinder cones have steep sides and are the smallest type of volcano.
• These volcanoes are formed when eruptions eject small pieces of lava into the air, forming tephra.
• Tephra falls back to Earth and piles up around the vent.

Composite Volcanoes

• Composite volcanoes are formed from layers of ash and hardened lava chunks from violent eruptions, alternating with layers of lava.
• These volcanoes form cone-shaped structures with concave slopes before solidifying.

Eruptions

• A volcano erupts when pressure from the magma forces its way to the surface.

Making Magma

• Pressure increases with depth due to overlying rock weight.
• As pressure increases, temperature also increases.
• This explains why most rocks in Earth's lower crust and upper mantle are solid, while those in the core are liquid.

Types of Magma

• Volcano explosivity depends on magma composition.
• Basaltic: Forms when upper mantle rock melts. Associated with quiet eruptions.
• Andesitic: Found along oceanic-continental subduction zones. Intermediate explosivity.
• Rhyolitic: Results when molten material mixes with the overlying continental crust. Extremely explosive.