Mount Pinatubo sat quiet for 500 years before it exploded in June 1991, killing 847 people and forcing 20,000 families from their homes. The eruption ejected 10 cubic kilometers of magma—roughly enough to bury Manhattan under 30 feet of volcanic debris. Scientists had nine weeks of warning signs. Nine weeks to evacuate thousands. Nine weeks to watch seismographs scribble increasingly frantic messages.
That’s why we have alert levels.
But here’s the thing: volcanic alert systems aren’t like tornado warnings or hurricane categories. They’re more like asking your moody teenager if they’re upset—sometimes the answer is reliable, sometimes you get volcanic rage from nowhere, and sometimes the grumbling never amounts to anything. The United States Geological Survey uses a five-tier system ranging from “Normal” (green) to “Warning/Eruption Imminent” (red). Sounds straightforward until you realize volcanoes don’t read the manual.
When the Earth Decides to Ruin Your Afternoon with Zero Notice
The USGS alert system works like this: Normal means background activity, basically the volcano doing volcano things—steaming, minor earthquakes, generally existing. Advisory means elevated unrest above known background levels. Watch means escalating unrest with increased potential for eruption. Warning means hazardous eruption imminent or underway. Aviation codes run parallel because nothing says “bad Tuesday” like flying a 747 through an ash cloud at 35,000 feet.
Eyjafjallajökull—yes, that Icelandic volcano whose name broke newsreaders in 2010—grounded 100,000 flights across Europe for six days. The eruption itself wasn’t particularly violent by volcanic standards, but the ash cloud disrupted 10 million travelers and cost airlines $1.7 billion. Aviation alert went from Green (normal) to Red (eruption underway) in the span of hours on March 20, 2010.
Wait—maybe the real question isn’t how alert levels work but why they fail.
Nevado del Ruiz in Colombia showed textbook warning signs in 1985: increased seismic activity, sulfur dioxide emissions, small phreatic explosions. Scientists raised alarms. Local officials dithered about evacuation costs and logistics. On November 13, the volcano erupted, melting glacial ice that triggered massive lahars—volcanic mudflows that buried the town of Armero under meters of mud, killing 23,000 people in a single night. The alert system worked perfectly. Human response did not.
The Problem With Mountains That Lie About Their Intentions
Turns out volcanoes are spectacularly bad at consistency. Kīlauea in Hawaii has been erupting almost continuously since 1983—forty-two years of lava flows, with alert levels yo-yoing between Watch and Warning so often that locals barely blink. Then in 2018, it pulled a surprise move: draining its summit lava lake, triggering earthquakes up to magnitude 6.9, and opening 24 fissures in residential neighborhoods. That eruption destroyed 700 homes and displaced thousands.
The Japanese Meteorological Agency uses a six-level system because apparently five wasn’t enough granularity for a country with 110 active volcanoes. Level 3 means “Don’t approach the volcano” which seems like solid life advice regardless of eruption status. Mount Ontake sat at Level 1 (normal) when it suddenly erupted on September 27, 2014, killing 63 hikers. No significant precursors. Just a mountain deciding violence was the answer.
Here’s what makes volcanic monitoring genuinely terrifying: the best early warning systems in the world—seismometers, gas sensors, satellite thermal imaging, GPS deformation networks—sometimes detect nothing unusual before an eruption. Phreatic explosions, caused by groundwater flashing to steam when it hits hot rock, can occur with basically zero warning. They’re not driven by rising magma, so the usual signs don’t appear.
Why We’re Essentially Guessing With Extremely Expensive Equipment
The Volcanic Explosivity Index measures eruption size on a logarithmic scale from 0 to 8, but it’s backward-looking—tells you what happened, not what’s coming. Mount St. Helens reached VEI 5 on May 18, 1980, when the entire north face collapsed in the largest debris avalanche in recorded history. The lateral blast traveled at 300 miles per hour, flattening 230 square miles of forest. Alert levels had been raised weeks earlier, saving thousands of lives, but 57 people still died including volcanologist David Johnston who radioed “Vancouver! Vancouver! This is it!” moments before the blast killed him.
Indonesia uses a four-tier system and has more active volcanoes than any nation on Earth—127 at last count. When Mount Agung showed signs of unrest in September 2017, authorities raised the alert to Level 4 and evacuated 140,000 people. The volcano then proceeded to… sit there grumbling for weeks. Some evacuees returned home. Then on November 21, Agung finally erupted, though not catastrophically. Cost of false alarm versus cost of missed warning—volcanic monitoring’s eternal dilemma.
The volcano alert pyramid basically inverts during a crisis. Under normal conditions, scientists monitor, analyze, and advise. During eruptions, they’re simultaneously collecting data, updating forecasts every few hours, and explaining to emergency managers why probabilistic hazard assesments aren’t the same as predictions. Nobody wants to hear “40% chance of major eruption in the next 48 hours” when they need to decide whether to evacuate a city.
Santorini’s caldera—that picturesque Greek island with white buildings tumbling down cliffs—last erupted in 1950. A minor event. But 3,600 years ago, the Minoan eruption there ranked VEI 7, one of the largest in human history, possibly inspiring Atlantis legends. Current alert level: Normal. Current tourist population: millions annually. Sleep tight.
