In Kinds of Volcanoes you met shield volcanoes, cinder cones and composite volcanoes. Their shapes come from how they erupt, and how they erupt comes down to two properties of the magma: its viscosity and its gas.
Why it works
Deep underground, magma holds gases (mostly water vapor and carbon dioxide) dissolved under pressure, like the carbon dioxide in an unopened soda. As magma rises, the pressure drops and bubbles form. In runny basalt the bubbles rise and escape easily, so lava fountains and flows: a gentle, effusive eruption. In sticky andesite, dacite or rhyolite, bubbles can't escape. Pressure builds until the magma is torn apart into ash and pumice by expanding gas: an explosive eruption. Water expands about 1,700 times when it turns to steam, so even basalt explodes when it meets groundwater or the sea.
Key idea
Viscosity and gas decide the style: runny, gas-poor basalt oozes out as lava; sticky, gas-rich felsic magma explodes as ash. The Volcanic Explosivity Index (VEI) rates eruptions by erupted volume, about ten times more for each step from VEI 2 up.
| VEI | Erupted volume | Example |
|---|---|---|
| 0 | under 10,000 m³ | most eruptions of Kīlauea, Hawaiʻi |
| 2 | 0.001 km³ or more | small explosive eruptions |
| 4 | 0.1 km³ or more | Eyjafjallajökull, Iceland, 2010 |
| 5 | 1 km³ or more | Mount St. Helens, 1980 |
| 6 | 10 km³ or more | Pinatubo, Philippines, 1991 |
| 7 | 100 km³ or more | Tambora, Indonesia, 1815 |
| 8 | 1,000 km³ or more | Yellowstone, about 2.1 million years ago |
Worked examples
Worked example: Comparing eruptions
About how many times more material does a VEI 6 eruption throw out than a VEI 4?
- Two steps, each about ×10.
- $10 \times 10 = 100$ times.
Answer: About 100 times.
Worked example: Rating an eruption
An eruption throws out about 3 km³ of ash and rock. What is its VEI?
- VEI 5 starts at 1 km³ and VEI 6 at 10 km³.
- 3 km³ is between them: VEI 5.
Answer: VEI 5.
Common mistake
The volcano with the biggest lava flows is not the most dangerous. Kīlauea pours out huge volumes of runny lava, but gently, and people can usually move out of its way. Mount St. Helens erupted far less lava, but its sticky, gas-rich magma blasted out explosively in 1980. Check the magma, not the volume: sticky plus gassy means explosive.