Measurement

VEI (Volcanic Explosivity Index)

"A logarithmic scale (0-8) used to measure the explosive magnitude of a volcanic eruption, similar to the Richter scale for earthquakes."

The Volcanic Explosivity Index (VEI) is the standard measuring stick used by volcanologists to quantify the power and magnitude of an eruption. Developed in 1982 by Chris Newhall and Stephen Self, it provides a framework for comparing the tiny “burps” of daily volcanic activity with the planet-altering cataclysms of the geological past. It is not determined by the destructive impact on humans, but by pure physical output—the volume of material ejected and the height of the eruption column.

How VEI is Calculated

The index is determined primarily by two measurable factors:

  1. Volume of Ejected Material: How much tephra (ash, pumice, and rock) is thrown out? This is measured in cubic kilometers of dense rock equivalent (DRE)—meaning the volume is corrected for the porosity of pumice to give the equivalent volume of solid rock.
  2. Plume Height: How high does the eruption column reach into the atmosphere? The plume height is measured in kilometers above the vent.

The scale ranges from VEI 0 (non-explosive, effusive) to VEI 8 (mega-colossal). Crucially, the scale is logarithmic from VEI 2 upwards. This means that each step represents a tenfold increase in the volume of ejected material. A VEI 5 eruption produces ten times more material than a VEI 4, and a VEI 6 produces one hundred times more than a VEI 4. The differences between the extremes of the scale are therefore almost incomprehensibly large.

The Scale Breakdown

VEI 0 (Effusive): Gentle lava flows with no explosion. Eruption plume less than 100 m. Volume less than 0.0001 km³.

  • Examples: Kīlauea (Hawaii, most activity), Stromboli (minor activity), Fagradalsfjall (Iceland, 2021).

VEI 1 (Gentle): Small, rhythmic explosions producing minor ash. Column 100 m – 1 km.

  • Examples: Stromboli (Italy, typical explosions), Mount Etna (minor eruptions).

VEI 2 (Explosive): Columns 1–5 km high. Significant local ashfall. Volume 0.001–0.01 km³.

  • Examples: Whakaari/White Island 2019, Galeras 1993, Sinabung (typical).

VEI 3 (Severe): Columns 3–15 km high. Significant regional effects. Volume 0.01–0.1 km³.

  • Examples: Nevado del Ruiz (1985), Soufrière Hills Montserrat (major phases), Ruapehu (1996).

VEI 4 (Cataclysmic): Columns 10–25 km. Regional disruption including to air traffic. Volume 0.1–1 km³.

  • Examples: Eyjafjallajökull 2010, Mount Pelée 1902, Mayon 1814.

VEI 5 (Paroxysmic): Columns >25 km. Significant global effects possible. Volume 1–10 km³.

  • Examples: Mount St. Helens 1980, Mount Vesuvius 79 AD, El Chichón 1982.

VEI 6 (Colossal): Columns >30 km. Major global climate impact; potential volcanic winter. Volume 10–100 km³.

  • Examples: Krakatoa 1883, Mount Pinatubo 1991, Hunga Tonga 2022 (at the high end of VEI 5–6).

VEI 7 (Super-Colossal): Events that rewrite history. Volume 100–1,000 km³.

  • Examples: Mount Tambora 1815 (caused the “Year Without a Summer”), Samalas/Rinjani 1257 (largest eruption of the last 7,000 years).

VEI 8 (Mega-Colossal): Extinction-level events with massive global consequences. Volume >1,000 km³.

  • Examples: Yellowstone (~640,000 years ago, ~1,000 km³), Toba (~74,000 years ago, ~2,800 km³ DRE), La Garita (~27.8 million years ago, ~5,000 km³).

Comparing the Extremes

To appreciate the logarithmic nature of the scale:

  • The 2010 Eyjafjallajökull eruption (VEI 4) that grounded European air travel for weeks ejected roughly 0.18 km³ of tephra.
  • The 1991 Pinatubo eruption (VEI 6) ejected roughly 10 km³—about 55 times more material.
  • The 1815 Tambora eruption (VEI 7) ejected roughly 160 km³—about 880 times more than Eyjafjallajökull.
  • The Toba super-eruption (VEI 8, ~74,000 years ago) ejected roughly 2,800 km³—over 15,000 times the volume of Eyjafjallajökull.

Frequency and the Rarity of Large Eruptions

VEI eruptions follow a rough inverse relationship between magnitude and frequency:

  • VEI 0–2: Hundreds of eruptions occur globally every year. These are the “background hum” of planetary volcanism.
  • VEI 3–4: Several per year to several per decade.
  • VEI 5: Roughly 1–2 per decade.
  • VEI 6: A few per century.
  • VEI 7: Approximately 1 every 1,000 years. The last was Tambora in 1815.
  • VEI 8: Approximately 1–2 every million years. The last occurred at Toba ~74,000 years ago.

This frequency distribution is why large eruptions are so consequential when they do occur—they are rare enough that human civilization has never experienced a VEI 8 event, but they have clearly shaped the trajectory of life on Earth throughout geological time.

Historical Impact of Large VEI Events

  • VEI 7 – Tambora (1815): The 1815 eruption killed approximately 71,000 people directly. The following year, 1816, became known as the “Year Without a Summer” across the Northern Hemisphere—crop failures caused widespread famine from Europe to North America, cholera pandemics spread, and the climate disruption inspired literary works including Mary Shelley’s Frankenstein and Lord Byron’s poem Darkness.
  • VEI 6 – Pinatubo (1991): One of the best-monitored eruptions in history. The eruption column reached 35 km; approximately 20 million tons of SO₂ were injected into the stratosphere, reducing global average temperatures by ~0.5°C for approximately two years. The eruption also partially masked the ongoing warming trend from greenhouse gases for the period, making 1991–1993 cooler than the background trend.
  • VEI 8 – Toba (~74,000 years ago): This super-eruption deposited ash across South Asia, the Indian Ocean, and Africa. The “Toba Catastrophe Hypothesis” proposes that the resulting volcanic winter reduced the global human population to as few as a few thousand individuals, creating a genetic bottleneck visible in the relative lack of genetic diversity in modern humans compared to other primates. While the severity of the population crash remains scientifically debated, there is no doubt that the Toba eruption was a globally significant event.

Limitations of the VEI

While the VEI is widely used and useful, it has important limitations:

  • Duration not captured: A lava eruption (VEI 0) like the 2021 La Palma event lasted 85 days, destroying over 3,000 buildings and displacing 7,000 people. A VEI 3 explosion might last only a few minutes. Both cause damage, but in very different ways—VEI captures only the explosive magnitude.
  • Not a hazard measure: VEI measures magnitude, not hazard or risk. A VEI 4 eruption in an unpopulated area causes less human harm than a VEI 2 eruption on the flank of a densely populated mountain.
  • Difficulty of historical measurement: For ancient eruptions, VEI is estimated from deposit volumes and column heights inferred from deposit distribution patterns. These estimates carry significant uncertainty, especially for older events where erosion has removed much of the deposit.
  • Gas emissions not directly measured: The climate impact of an eruption depends heavily on the quantity of SO₂ injected into the stratosphere, which correlates only imperfectly with the volume of material ejected. Some smaller-VEI eruptions inject disproportionately large amounts of sulfur.

Alternative indices, such as the Eruption Intensity (mass discharge rate) and the Magnitude scale (log₁₀ of erupted mass), are used by researchers who need more precise quantification of specific eruption parameters.

Plinian eruption is the eruption style associated with the highest VEI ratings (VEI 5–8). Tephra is the material whose volume is used to calculate VEI. Volcanic winter is the climate phenomenon associated with VEI 6+ eruptions. Caldera formation typically accompanies VEI 7–8 events.