What this lesson teaches
I can explain how tectonic processes decide the explosiveness of a volcanic eruption and how it is measured.
Syllabus K229, Topic 3.2, Key Question 2. How do tectonic processes affect the magnitude of volcanic eruptions?
Make a guess
Two volcanoes hold the same amount of gas. One has runny magma, the other thick, sticky magma. Which is more likely to explode?
- Neither, because gas has nothing to do with eruptions.
- The one with sticky magma.
- The one with runny magma.
- Both equally, because the gas is the same.
Show the answer
The one with sticky magma.
Sticky, viscous magma traps gas, so pressure builds until the magma bursts apart. Gas escapes more easily from runny magma.
Runny magma lets gas escape easily, while thick, sticky magma traps gas and can erupt explosively.
Magma is molten or partly molten rock, containing crystals and dissolved gases. It is usually lighter than the rock around it, so buoyancy and pressure push it up through weak areas in the crust. A pipe called a conduit carries it to a vent. At the surface, it is called lava. Magma chambers are regions of partly molten rock, not empty caves.
Viscosity means resistance to flow: how thick and sticky a liquid is. As magma rises, pressure falls and dissolved gases form expanding bubbles, like opening a fizzy drink. In runny magma, gas escapes easily. Viscous magma resists flow and traps the gas, so pressure builds until the magma and rock shatter explosively. The amount of gas, how fast magma rises, and contact with water also affect the eruption.
Silica-rich magma tends to be more viscous, though temperature and crystals also matter. Stratovolcanoes usually have viscous magma. They build steep cones from layers of lava and ash, and can be explosive. Shield volcanoes usually erupt runny basalt lava that spreads into wide, gentle slopes. These are typical patterns; neither type always erupts the same way.
Sort these
Which type of volcano does each clue usually point to?
Groups: Shield volcano, Stratovolcano.
- Steep cone built from layers of ash and lava
- Thick, sticky magma rich in silica
- Runny basalt lava that flows a long way
- Gas escapes easily, so eruptions are usually gentle
- Wide base with gentle slopes, like Kilauea in Hawaii
- Trapped gas builds pressure until the eruption is explosive, like Pinatubo in 1991
Show the answer
Shield volcano
- Runny basalt lava that flows a long way Low-viscosity lava spreads out before it cools, building wide, gentle slopes.
- Gas escapes easily, so eruptions are usually gentle Runny magma lets gas bubbles out, so pressure does not build up to an explosion.
- Wide base with gentle slopes, like Kilauea in Hawaii Layer after layer of runny lava spreads far, so the cone is broad and low.
Stratovolcano
- Steep cone built from layers of ash and lava Viscous lava and ash pile up near the vent instead of spreading, so the sides are steep.
- Thick, sticky magma rich in silica High-viscosity magma resists flow and traps gas, which is typical of stratovolcanoes.
- Trapped gas builds pressure until the eruption is explosive, like Pinatubo in 1991 Sticky magma holds the gas in until it bursts out, shattering rock and magma into ash.
The Volcanic Explosivity Index (VEI) compares explosive eruptions. It uses the volume of material thrown out, the height of the eruption cloud and how long the eruption lasts. Each number on the scale sums up how explosive an eruption was.
VEI does not measure deaths, damage or every hazard. A gentle lava eruption can still destroy buildings, and a lahar can threaten people far downstream. Eruptions can also change behaviour over time. So when judging risk, identify the hazards, not just the VEI.
- Viscosity
- How thick and sticky magma is.
- Stratovolcano
- Steep cone, viscous magma, often explosive.
- VEI
- Uses volume ejected, cloud height and duration.
Step by step
Magma rises
It is lighter and moves up through weak rock.
Gas expands
Falling pressure forms gas bubbles.
Gas escapes or builds up
Runny magma lets gas out; sticky magma traps it.
Real examples to use
Mount Pinatubo, Philippines, 1991
Sticky, gas-rich magma produced one of the largest eruptions of the 20th century. Its ash column rose about 35 km high.
Kilauea, Hawaii, 2018
Runny basaltic lava flowed out gently rather than exploding, but the flows destroyed over 700 homes.
Worked example: Compare two magmas
Suppose two magmas hold the same amount of gas. Gas escapes more easily from the runny one. The sticky one traps gas and pressure, so it is more likely to explode. This compares processes; it is not a forecast.
Watch out for this
A low VEI means a volcano cannot cause serious harm.
VEI only measures explosiveness. Lava flows, gases and lahars can still destroy homes and kill people.
Check your understanding
Which combination tends to make an eruption more explosive?
- Runny magma with a lot of gas escaping freely.
- A high death toll near the volcano.
- Viscous magma with gas that cannot escape.
Show the answer
Viscous magma with gas that cannot escape.
Yes. Trapped gas builds pressure until the magma bursts.
Check your understanding
Why was Mount Pinatubo's 1991 eruption explosive, while Kilauea's 2018 eruption was gentle?
- Pinatubo is taller, so magma had farther to rise and built up more pressure.
- Pinatubo's magma was sticky and gas-rich; Kilauea's let gas escape.
- Kilauea's eruption was much smaller in magnitude, so it was gentler.
Show the answer
Pinatubo's magma was sticky and gas-rich; Kilauea's let gas escape.
Viscosity and gas content control explosivity.