JuneauAn Alaskan slope shed 63.5 million cubic meters in August 2025.
Glacier retreat stripped Tracy Arm’s ice-stabilized slope, triggering a 500-meter wave — the second-highest tsunami recorded — a Science study found.
Tracy Arm sees 20-plus boats daily in summer; the team is developing seismic early-warning for similar slopes worldwide.
Sources: Ars Technica
WHAT DEBUTTRESSING MEANS
Glaciers don't just sit in valleys — they press laterally against the rock walls that contain them. When the ice thins or retreats, that pressure releases. Slopes that were stable for thousands of years suddenly aren't, because the buttress holding them in place has melted away.
WHY THE WAVES ARE SO TALL
A landslide tsunami is not a normal tsunami. Ocean tsunamis spread their energy across thousands of kilometers; a fjord landslide dumps tens of millions of cubic meters into a narrow basin with nowhere for the energy to go. The water piles vertically against the opposite wall — physics, not geology, sets the height.
THE LITUYA BAY RECORD
In 1958, an earthquake-triggered rockslide in Lituya Bay, Alaska, sent a wave that stripped trees off the opposite slope to 524 meters above sea level — the tallest tsunami ever recorded. Two fishermen anchored in the bay surfed it out and survived. Tracy Arm's 500-meter wave is the second-tallest ever measured, and the first attributed to glacier retreat rather than seismic shaking.
WHY FJORDS CONCENTRATE THE RISK
Fjords are U-shaped valleys carved by Pleistocene glaciers — steep walls, deep water, narrow width. The same geometry that makes them photogenic makes them tsunami amplifiers. Greenland, Norway, Patagonia, British Columbia and Alaska all share this hazard, and all are losing the ice that stabilizes their walls.
WHY EARLY WARNING IS HARD
A seismic detector can flag an earthquake before the shaking arrives because seismic waves travel faster than the destructive surface waves. A landslide tsunami offers no such window — the slide IS the source, and the wave reaches the far side of a 5km fjord in under a minute. The only viable warning detects the slope creeping in the days or hours before failure, not the slide itself.
THE COMING INVENTORY PROBLEM
Roughly 200,000 glaciers worldwide are retreating. Each one was buttressing some slope, somewhere. Identifying which of those slopes will fail — and which will fail catastrophically into water — is a hazard-mapping problem with no precedent at this scale. Tracy Arm wasn't on anyone's high-risk list before August 2025.