Volcanoes
Types, silica & viscosity, hazards
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Reading: Volcanoes
Volcanoes
Both of these are volcanoes, but they look completely different. One pours out rivers of glowing lava. The other blasts rock and ash 20 miles into the sky. Today we find out why, and it comes down to one thing: how sticky the magma is.
What Controls a Volcano's Eruption Style?
The single most important variable is silica content, which determines the magma's viscosity (resistance to flow):
Think of it like this: low-silica magma is like ketchup, it pours easily. High-silica magma is like cold peanut butter, it barely moves, and when something finally gives way, it goes violently.
The Three Main Volcano Types
Shield Volcano: Broad, gently sloping, dome-like shape. Made from low-silica (basaltic) lava that flows far and wide. Eruptions are quiet and effusive. Example: Mauna Loa, Hawaii (built above a hotspot).
Cinder Cone: Small, steep, symmetrical cone built from ejected lava fragments (cinders). Short, explosive eruptions. Example: Paricutin, Mexico (famously erupted through a cornfield in 1943).
Composite Volcano (Stratovolcano): Tall, steep-sided, built from alternating layers of lava and ash. High-silica magma. Can have both effusive and explosive phases. Extremely dangerous. Examples: Mt. St. Helens (USA), Mt. Fuji (Japan).
Caldera: A large depression formed when the magma chamber empties and the volcano collapses inward. Not a cone-shaped mountain. Can produce catastrophic super-eruptions. Example: Yellowstone, Long Valley.
Where Do Volcanoes Form?
Most of the world's volcanoes are in the Ring of Fire: a belt of subduction zones around the Pacific Ocean. When oceanic crust subducts, water in the crust lowers the melting point of rock, producing silica-rich, explosive magma.
Hotspot volcanoes (like Hawaii, Yellowstone) form above a fixed plume of heat in the mantle, independent of plate boundaries. They produce basaltic (low-silica) magma and tend to be less explosive.
Volcanic Features
Volcanic Hazards
Lava flows are rarely the deadliest hazard. These are more dangerous:
Pyroclastic flow: A fast-moving cloud of hot gas, ash, and rock fragments. Temperatures over 700°C; travels up to 700 km/h. No escape. The deadliest volcanic hazard.
Lahar: A volcanic mudflow mixing ash and water. Can travel far from the volcano, triggered by rain or melting snow. Buries entire towns.
Ash fall: Fine volcanic ash falling over wide areas. Collapses roofs, clogs engines, ruins crops, causes respiratory problems.
Volcanic gases: SO₂, CO₂, H₂S released during eruptions. Toxic; SO₂ causes acid rain.
Tsunami: Ocean waves triggered by volcanic collapse or submarine eruption.
The 1815 Tambora eruption killed ~70,000 people and caused the "Year Without a Summer" in 1816, crop failures and famine globally due to ash blocking sunlight.