Volcano Eruption Energy Calculator

What Is the Volcanic Explosivity Index (VEI)? The VEI was proposed by Christopher Newhall and Stephen Self in 1982 (United States). It is a logarithmic scale (like the Richter scale) measuring the explosiveness of volcanic eruptions. VEI increases by 1 for each 10-fold increase in erupted volume. Scale: 0 (gentle lava flow) to 8 (supervolcano, a civilization-scale event).

VEI Scale and Volume VEI 0: < 10,000 m³ (Hawaiian lava flows, happening somewhere almost constantly). VEI 1: 10,000–1 million m³ (Stromboli, Italy, near-continuous since ancient times). VEI 2: 1–10 million m³ (Mount Unzen, Japan, 1990). VEI 3: 10–100 million m³ (Soufrière Hills, Montserrat, 1995–ongoing). VEI 4: 0.1–1 km³ (Eyjafjallajökull, Iceland, 2010, which shut European airspace). VEI 5: 1–10 km³ (Mount St. Helens, USA, 1980, 1.2 km³ ejected). VEI 6: 10–100 km³ (Pinatubo, Philippines, 1991, the largest of the 20th century). VEI 7: 100–1,000 km³ (Tambora, Indonesia, 1815, which caused the year without a summer). VEI 8: >1,000 km³ (Yellowstone supervolcano, last eruption ~640,000 years ago).

Energy Calculation Thermal energy per unit volume of magma erupted: approximately 1 GJ/m³ (depends on magma type). Basaltic lava: ~1–2 GJ/m³. Rhyolitic eruptions: ~1.5 GJ/m³. This calculator uses 1.5 GJ/m³, so the energy is simply the erupted volume in cubic metres times 1.5 billion joules.

Two different numbers both called “the energy of an eruption”

This trips up everyone, so it is worth being blunt about it.

Explosive yield is the energy of the blast itself, the part that shatters rock and throws it into the sky. That is the figure encyclopedias quote: Krakatoa at roughly 200 megatons of TNT, which is where “the loudest sound in recorded history” comes from.

Total thermal energy is the heat carried by all the magma that came out, blast or no blast. Most of an eruption’s energy is this, and most of it simply radiates away over the following weeks as the deposits cool. For Krakatoa’s 25 km³ that works out near 9,000 megatons, about 45 times the explosive figure.

Neither is wrong. They answer different questions. This page reports total thermal energy, because that is what erupted volume tells you, so expect it to be one to two orders of magnitude above the headline number you may have read elsewhere for the same eruption.

Comparison Scales 1 kiloton TNT = 4.184 × 10¹² J. 1 megaton TNT = 4.184 × 10¹⁵ J. “Little Boy” (Hiroshima bomb, 1945): ~15 kilotons TNT. Tsar Bomba (largest H-bomb, USSR, 1961): 50 megatons TNT. World primary energy consumption: about 6.3 × 10²⁰ J a year, which a VEI 7 eruption exceeds several times over.

Famous Eruptions Tambora (Indonesia, 1815): VEI 7. 160 km³ ejected. ~71,000 killed. Global cooling 0.5°C. Krakatoa (Indonesia, 1883): VEI 6. 25 km³. Tsunami 30 m high. Heard around the world. Pinatubo (Philippines, 1991): VEI 6. 10 km³. Lowered global temp 0.5°C for 1–2 years. Mount St. Helens (USA, 1980): VEI 5. Lateral blast at 900 km/h. 57 killed. Eyjafjallajökull (Iceland, 2010): VEI 4. Disrupted air travel for weeks across Europe.