Eruption Types & Volcano Shape
Distinguish effusive lava flows from explosive fragmentation. Explain how magma resistance to flow, gas escape, and water affect eruption style. Use volcano shape and deposits as clues without treating shape as proof of one magma type.
What a learner can do afterwards
- Contrast effusive eruptions, where lava flows out without a major blast, with explosive eruptions that fragment magma or rock.
- Explain that eruption type depends on properties of the magma such as thickness and gas content
- Explain how repeated runny lava flows build broad shields, and why steep cones can contain lava or erupted fragments without proving magma thickness.
1 · Read
You know magma is melted rock underground. It can contain crystals and gas. Lava is magma that reaches the surface. In an effusive eruption, lava flows out without a major blast. In an explosive eruption, expanding gas or steam breaks rock and magma into fragments such as ash. These are eruption styles. Dormant and extinct describe activity history and whether another eruption is expected.
Runny magma lets bubbles escape more easily. This often favors effusive lava flows. Sticky magma resists flowing and can hold gas bubbles. If much gas stays trapped, pressure can build and lead to an explosive eruption. Magma thickness and gas content matter, but water can also cause explosions. Lava flow speed varies with slope and other conditions.
Gas escape and magma's resistance to flow affect how it erupts.
Runny lava can spread far before cooling. Repeated flows build broad shield volcanoes with gentle slopes. Material that collects near a vent can build a steep cone. Ash and rock fragments can pile there; some steep volcanoes also contain layers of lava. Shape alone does not prove magma thickness. Magma that resists flowing and holds many gas bubbles is thick and gas-rich, so it can erupt explosively.
Hawaiian shield volcanoes often grow through lava flows. They can also erupt explosively. At Mount St Helens in 1980, a landslide removed the northern flank. This released pressure and triggered a sideways blast. Gas expansion helped drive the blast, but thick magma alone does not explain the whole sequence.
Eruption style depends on magma, gas, and water. Volcano shape gives clues, but it does not prove one eruption history.
2 · Watch
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