Runny magma lets gas escape more easily. Viscous magma can trap expanding gas and erupt explosively. VEI describes explosivity, not total risk.
Magma is molten or partly molten rock containing crystals and dissolved gases. It is often less dense than surrounding rock, so buoyancy and pressure help it rise through weak areas. At the surface, it is called lava. A conduit carries it towards a vent. Magma storage regions contain material; they are not empty caverns.
Viscosity means resistance to flow. As magma rises, falling pressure lets dissolved gases form expanding bubbles. Gas often escapes more easily from low-viscosity magma. High-viscosity magma resists flow and can trap gas, allowing pressure to build until rock and magma fragment explosively. Gas content, ascent and contact with water also affect the eruption.
Silica-rich magma tends to be more viscous, though temperature and crystals also matter. Stratovolcanoes commonly have relatively viscous magma and build steep cones from layers of lava and fragments. Shield volcanoes commonly erupt runnier basaltic lava that spreads into broad, gentle slopes. These are typical patterns; neither volcano type has one guaranteed eruption style.
The Volcanic Explosivity Index, VEI, compares explosive eruptions using the volume of ejected fragments, eruption-cloud height and duration. Its numbered categories summarise explosivity. Lava volume is not simply added as though all volcanic output were measured alike.
VEI does not measure deaths, damage or every hazard. A less explosive lava eruption can destroy buildings, while a lahar can threaten people far downstream. Eruptions can also change behaviour. Identify the hazards as well as the VEI when assessing risk.
Step by step
Magma rises
Buoyancy and pressure move magma through weak areas.
Gas expands
Falling pressure lets dissolved gases form expanding bubbles.
Gas escapes or builds pressure
Runny magma often releases gas easily. Viscous magma can trap it and erupt explosively.
Worked example: Compare two magma samples
In a teaching comparison with similar gas content, a more fluid sample allows gas to escape more readily than a viscous sample. The viscous sample is more likely to retain pressure. This is a mechanism comparison, not an exact forecast of an eruption.
Watch out for this
A low VEI means a volcano cannot cause serious losses.
VEI describes explosivity. Lava flows, gases and lahars can threaten exposed people and assets without a high explosive index.
Check your understanding
Which combination tends to favour a more explosive eruption?
- Viscous magma and expanding gases that cannot escape easily.
- Fluid magma with gas escaping freely always produces the largest explosion.
- A high death toll automatically causes magma viscosity to rise.