The Indonesian village of Armero vanished in less than an hour on November 13, 1985. More than 23,000 people died when Nevado del Ruiz erupted and sent lahars—volcanic mudflows that move like wet concrete—racing down the mountain at 60 kilometers per hour.
Here’s the thing: scientists had warned officials. They’d mapped the hazard zones. They’d predicted exactly what would happen if the volcano woke up. But the maps sat in offices while people slept in the path of destruction.
When Cartography Becomes a Matter of Life and Several Thousand Deaths
A volcanic hazard map is essentially a fortune-telling document written in the language of probability and geology. It shows where pyroclastic flows might roast everything in their path, where ash could collapse roofs, where lahars will follow river valleys like heat-seeking missiles made of mud and rock.
These aren’t simple drawings.
Scientists spend months—sometimes years—studying a volcano’s past behavior, running computer simulations of potential eruptions, analyzing topography to predict where deadly materials will flow. They’re trying to forecast chaos, which is a bit like predicting which way a drunk person will stumble, except the drunk person weighs several billion tons and can melt steel.
The Colors That Tell You Whether to Pack a Bag or Write a Will
Most hazard maps use a color-coded system. Red zones mean “catastrophic and unsurvivable”—places where pyroclastic flows or lava will annihilate everything. Yellow zones indicate “probably survivable but your house won’t be.” Green zones are relatively safe, though “relative” is doing heavy lifting when discussing volcanic eruptions.
Mount Pinatubo in the Philippines erupted in 1991 after sleeping for 500 years. Hazard maps created just weeks before the eruption guided the evacuation of 58,000 people from high-risk zones. An estimated 5,000 lives were saved because cartographers worked faster than magma could rise.
Turns out—drawing lines on paper can prevent apocalypses.
The Problem With Mapping Something That Hasn’t Happened Yet
Wait—maybe the real challenge isn’t creating these maps but getting people to believe them. Vesuvius, that famous mountain that burried Pompeii in 79 CE, currently has about 600,000 people living within its red zone. They know the hazard maps exist. They know what happened to Pompeii. They live there anyway because, well, humans are spectacularly bad at processing abstract future threats.
Hazard maps for Mount Rainier in Washington State show that lahars could reach suburbs of Seattle and Tacoma, areas where hundreds of thousands of people live and work. The maps have existed since the 1990s, updated regularily with better data and more sophisticated modeling. Construction continues in mapped hazard zones.
Why Your Volcano Map Might Be Lying to You
No map can predict everything. Volcanoes are creative in their destruction.
When Mount St. Helens exploded in 1980, the lateral blast—a sideways explosion that wasn’t vertically directed like most eruptions—wasn’t fully anticipated by hazard assessments. The blast zone extended 19 kilometers north, destroying 600 square kilometers of forest. Previous hazard evaluations had focused on vertical eruptions and lahar paths.
Volcanic hazard maps also struggle with timescales. A volcano might erupt tomorrow or in 500 years. Both scenarios could warrant the same hazard zones, but they create vastly different levels of urgency. Try telling someone they can’t build a house in a spot because it might be incinerated in a few centuries. They’ll build the house.
The Mapmakers Who Race Against Magma Chambers
Creating hazard maps for newly restless volcanoes becomes an emergency operation—geological cartography as crisis response. When Mount Agung in Bali showed signs of reawakening in 2017 after 54 years of dormancy, scientists rushed to create and update hazard maps while also monitoring seismic activity and gas emissions.
The maps had to be simple enough for emergency managers to use immediately but detailed enough to be accurate. They had to account for seasonal weather patterns that would affect ash dispersal. They had to consider that some areas had been built up significantly since the last eruption in 1963, when 1,100 people died.
These cartographers work knowing their maps might cause massive evacuations that disrupt economies—or might save thousands of lives. Sometimes both. The 2017 Agung evacuation displaced over 140,000 people for months. The volcano produced several small eruptions but never the catastrophic event that hazard maps suggested was possible.
Were the maps wrong? Or did they work exactly as intended, preventing deaths by erring on the side of caution? Nobody knows, which is the maddening thing about predicting disasters that might not happen. You only get credit for the catastrophes you prevent, and prevented catastrophes are invisible.
