The island of Krakatoa basically deleted itself in 1883, and when it did, the ocean noticed. Hard.
We tend to think of tsunamis as earthquake things—tectonic plates grinding and lurching beneath the seafloor, displacing ungodly amounts of water in the process. And sure, that’s the standard playbook. But volcanoes? They’ve got their own bag of tricks for generating waves that can obliterate coastlines, and honestly, some of them are weirder than you’d expect. The Krakatoa eruption killed roughly 36,000 people, and most of them drowned. Not from lava. Not from ash. From waves that shouldn’t have existed in the first place.
Here’s the thing: volcanoes don’t need to follow earthquake rules.
When an Entire Mountain Collapses Into the Sea Like a Drunk Uncle
Mount St. Helens taught us something crucial in 1980—volcanic edifices are basically unstable Jenga towers waiting for the wrong piece to get pulled. When a volcano’s flank suddenly decides gravity is a thing and slides into the ocean, it displaces water the same way a cannonball does in a bathtub. Except the bathtub is the Pacific, and the cannonball weighs several billion tons. This is what happened during Krakatoa’s grand finale: the volcanic cone collapsed into its own magma chamber, and the ocean rushed in to fill the void. Instant tsunami. The largest wave reached 120 feet—higher than most buildings you’ve ever been inside.
Anak Krakatau pulled a similar stunt in 2018, shedding about 64 acres of its southwestern flank into the Sunda Strait. The resulting tsunami killed over 400 people in Indonesia, and the truly unsettling part? There was no earthquake warning. Seismic monitoring systems designed to catch tectonic rumbles completely missed it because this was a diffrent beast entirely.
Pyroclastic Flows That Hit Water Like a Meteorite Made of Anger
Imagine a superheated avalanche of gas, rock, and ash screaming down a mountainside at 450 miles per hour. Now imagine it hits the ocean.
Turns out that’s a spectacularly efficient way to generate waves. When pyroclastic flows—those hellish clouds of volcanic material that incinerate everything they touch—plow into water, they transfer their momentum instantly. The 1902 eruption of Mount Pelée in Martinique sent pyroclastic surges into the Caribbean Sea, creating waves that added insult to injury for the 30,000 people who’d already been vaporized by the flows themselves. The port city of Saint-Pierre was annihilated in minutes, and the harbor wasn’t spared either.
Wait—maybe the most disturbing part is how fast this happens. Unlike earthquake-generated tsunamis that give you maybe 15 minutes of warning if you’re lucky, volcano-induced waves can arrive within minutes of the triggering event. You’re dealing with a threat that’s both immediate and unpredictable, which is basically a nightmare scenario for coastal communities.
Underwater Eruptions Nobody Saw Coming Because They’re Literally Invisible
Submarine volcanoes are the ocean’s dirty secret. There are thousands of them, and we’ve barely mapped a fraction of the seafloor where they lurk. When one of these underwater mountains decides to erupt, it can displace massive volumes of water from below—like someone detonating a bomb in a swimming pool. The Hunga Tonga-Hunga Ha’apai eruption in January 2022 was so violent it sent atmospheric shock waves around the planet multiple times. The tsunami it generated crossed the Pacific, causing damage as far away as Peru and California. Two people drowned in Peru from waves that originated 6,000 miles away. That’s about as dramatic as planetary-scale cause-and-effect gets.
The explosion was heard in Alaska.
Caldera Collapses That Rewrite Coastlines in an Afternoon
When a volcano’s magma chamber empties out during a massive eruption, the ground above it sometimes just… gives up. The entire summit area collapses inward, creating a caldera—a giant crater that can span miles. If this happens near or under water, you get displacement on a scale that makes normal landslides look quaint. Santorini did this around 1600 BCE during the Minoan eruption, possibly generating waves over 100 feet tall that slammed into Crete and may have contributed to the collapse of an entire civilisation. We’re still finding pumice deposits from that event scattered across the eastern Mediterranean.
The geological record suggests this has happened repeatedly throughout Earth’s history, and there’s no particular reason it won’t happen again. Yellowstone’s last caldera-forming eruption was about 640,000 years ago, and while it’s landlocked now, previous eruptions in the Yellowstone hotspot track occurred when the continent was positioned differently. The math is uncomfortable: big volcano plus water equals bad news for anyone downstream.
The Ones We Haven’t Thought of Yet Because Nature Loves Plot Twists
Volcanic tsunamis can also result from mechanisms we’re still trying to understand. Limnic eruptions—where dissolved CO2 suddenly bubbles out of volcanic lakes—could theoretically generate waves if they occurred violently enough. Lava entering water can create steam explosions that send out shock waves. Volcanic earthquakes themselves, triggered by magma movement, can shake the seafloor just like tectonic quakes. The 1975 Kalapana earthquake in Hawaii, caused by volcanic activity, generated a tsunami that reached 47 feet locally.
And then there’s the truly speculative stuff: what happens if a massive lava dome collapses into a crater lake? What if glacial meltwater from a subglacial eruption gets displaced catastrophically, like what happened during Iceland’s Eyjafjallajökull eruption in 2010, except bigger? We’re basically cataloging disaster mechanisms as we discover them, which is a polite way of saying we keep getting surprised by the creative ways volcanoes can kill people from a distance.
The ocean doesn’t care whether your tsunami came from an earthquake or an exploding mountain. The wave arrives either way, and it won’t wait for you to understand the difference.
