Volcanic Eruptions

A volcanic eruption happens when magma escapes from inside the Earth and reaches the surface. Eruptions come in many different forms, from gentle fountains of glowing lava to catastrophic explosions that send ash 30 km into the sky. Vulcanologists (volcano scientists) classify eruptions in two main ways: by their style (effusive, explosive, etc.) and by their size on the Volcanic Explosivity Index (VEI), a logarithmic scale from 0 (gentle) to 8 (super-eruption). Roughly 50 to 70 volcanoes erupt each year worldwide.

  • Eruptions per yearApproximately 50 to 70Worldwide
  • Main eruption stylesApproximately 6Hawaiian, Strombolian, Vulcanian, Pelean, Plinian, Surtseyan
  • Volcanic Explosivity IndexVEI 0 to 8Logarithmic; each step is 10x bigger
  • Largest known historical eruptionTambora, 1815VEI 7
  • Caused "year without summer"1816Following Tambora
  • Deadliest in modern timesMount Tambora, 181592,000 deaths

The main eruption styles

  • Hawaiian: gentle eruption of runny basalt lava. Spectacular fountains and slow-moving rivers of glowing rock. Lets tourists get close.
  • Strombolian: small frequent bursts that throw blobs of lava into the air. Named after Stromboli, an Italian volcano that has been doing this almost continuously for 2,000+ years.
  • Vulcanian: short violent explosions that shatter cooled lava plugs. Named after Vulcano, the Italian island that gave us the word "volcano".
  • Pelean: dangerous explosive eruptions that produce pyroclastic flows: super-heated avalanches of ash and rock. Named after Mont Pelee, which destroyed the town of Saint-Pierre on Martinique in 1902 and killed 30,000 people in minutes.
  • Plinian: the most violent eruptions, producing huge ash columns kilometres tall. Named after Pliny the Younger, who described the eruption of Vesuvius in 79 AD.
  • Surtseyan: underwater eruptions where lava meets seawater explosively. Named after Surtsey, an Icelandic island that emerged from the sea in 1963.

The Volcanic Explosivity Index (VEI)

The VEI is a scale from 0 to 8 that measures the size of an eruption based on how much material it produces.

  • VEI 0: tiny. Less than 10,000 cubic metres of ejected material.
  • VEI 1: small. Like ordinary Hawaiian eruptions.
  • VEI 2: explosive. Approximately one per year worldwide.
  • VEI 3: severe. Approximately one per year worldwide. Mount Etna routinely.
  • VEI 4: catastrophic. Approximately one per decade worldwide. Mount St Helens 1980, Eyjafjallajokull 2010.
  • VEI 5: huge. Approximately one per century. Mount Vesuvius 79 AD, Mount St Helens 1980 (actually borderline 5).
  • VEI 6: colossal. Approximately one per few centuries. Krakatoa 1883, Pinatubo 1991.
  • VEI 7: super-colossal. Approximately one per millennium. Tambora 1815.
  • VEI 8: super-eruption. Approximately one per 50,000 to 500,000 years. Yellowstone 640,000 years ago, Toba 74,000 years ago.

The scale is logarithmic: each step is approximately 10 times bigger than the one below.

The dangers of a volcanic eruption

Different eruption styles produce different dangers.

  • Lava flows: usually slow and easy to escape, but unstoppable. Can destroy property and infrastructure.
  • Ash fall: heavy fine ash can collapse roofs, ruin crops, ground aircraft and damage lungs.
  • Pyroclastic flows: super-heated avalanches of ash and gas reaching 700 degrees C, moving at 100+ km/h. By far the deadliest volcanic hazard. Wiped out Pompeii.
  • Lahars: fast-moving rivers of mud and rock, often triggered when ash mixes with melting snow or heavy rain.
  • Volcanic gases: invisible clouds of toxic gas can suffocate people and animals.
  • Tsunamis: underwater eruptions or volcano collapses can produce giant ocean waves.
  • Climate effects: large eruptions inject sulphur into the stratosphere, where it can reflect sunlight and cool the planet for years.
Fact The eruption of Mount Tambora in Indonesia in April 1815 was the largest in recorded human history (VEI 7). It killed approximately 71,000 people in the immediate aftermath. The huge cloud of sulphurous gas it injected into the stratosphere then caused global cooling, leading to 1816 being known as "the year without a summer". Crops failed across Europe and North America. Famines and disease epidemics caused an estimated 100,000 more deaths worldwide. Mary Shelley was famously trapped indoors that summer in Switzerland because of the constant cold rain, and used the time to write Frankenstein.

Pyroclastic flows: the deadliest threat

The most dangerous part of an explosive volcanic eruption is the pyroclastic flow: a super-heated avalanche of ash, gas and rock that pours down the volcano's flank at up to 700 km/h and temperatures up to 700 degrees C. Pyroclastic flows have killed more people than any other volcanic hazard. They are what wiped out Pompeii in 79 AD and Saint-Pierre in 1902.

The hot ash and gas in a pyroclastic flow ignite everything flammable in seconds. Pompeii's residents were not buried alive by ash; they were instantly killed by the heat of the flow before being entombed in the falling ash. There is no escape from a pyroclastic flow once it has started. The only protection is to evacuate before the eruption begins.

Did you know? The 2010 eruption of Eyjafjallajokull in Iceland was relatively small (only VEI 4), but it grounded almost all aircraft in Europe for 6 days because the ash cloud could damage jet engines. Over 100,000 flights were cancelled, affecting 10 million passengers. It was the biggest air travel disruption in Europe since World War II, all from one moderately sized eruption.
Deeper dive: how volcanic eruptions affect global climate

Large volcanic eruptions can have surprising and dramatic effects on the global climate, sometimes lasting years.

The mechanism is straightforward. Big explosive eruptions inject huge amounts of sulphur dioxide gas high into the stratosphere (above the weather), where it reacts with water to form fine droplets of sulphuric acid called aerosols. The aerosols spread around the world within months and stay aloft for several years before slowly falling out. While in the stratosphere, they reflect a small percentage of incoming sunlight back into space, cooling the Earth's surface.

Some examples:

  • Tambora 1815: caused the "Year Without a Summer" in 1816. Snow fell in New England in June and crops failed across Europe.
  • Krakatoa 1883: lowered global temperatures by approximately 1.2 degrees C the following year. Created brilliant red sunsets around the world for years.
  • Pinatubo 1991: dropped global temperatures by approximately 0.5 degrees C for 2 to 3 years. Provided a real-world test of climate models.
  • Toba 74,000 years ago: a super-eruption that may have caused a 1,000-year volcanic winter and almost wiped out early humans (the "Toba bottleneck" theory).

Some scientists have proposed deliberately injecting sulphur into the stratosphere as a way to fight global warming (called stratospheric aerosol injection). The idea is controversial and has many risks, but it is taken seriously enough that several countries are now studying it. Nature's biggest experiments in this technique are the volcanic super-eruptions.

For more, see types of volcano, the Ring of Fire and Mount Vesuvius.