The Mega Tsunami That Smashed Hawaii

You can stand hundreds of metres above the ocean on the dry, wind-scoured slopes of Lanai, pick up a rock under your feet, and find yourself holding coral ripped from a reef that once lived at sea level. Not beach sand, not shell fragments dusted inland by a storm, but heavy, wave-rounded marine rubble sitting impossibly high on a volcanic island. That single observation is where this story begins, because once you accept that those rocks are real, in place, and marine, the only remaining question is what kind of wave could have put them there.

The hidden truth is that Hawaiʻi, the postcard paradise we associate with gentle surf and trade winds, has been struck by some of the largest waves the planet is physically capable of generating. Not earthquake tsunamis like Japan or Sumatra, but something far rarer and far more violent. At some point in the late Pleistocene, a section of a Hawaiian volcano catastrophically collapsed into the Pacific, displacing so much water that the resulting wave ran hundreds of metres up neighbouring islands. Not ten metres. Not fifty. Hundreds.

To understand the scale, it helps to forget everything you know about “normal” tsunamis. The 2011 Tōhoku tsunami that devastated Japan had local run-ups of around 40 metres. That event involved thousands of kilometres of fault rupture and released energy on a continental scale. The Hawaiian event we’re talking about didn’t rely on a fault at all. It relied on gravity. When a mountain-sized chunk of volcanic island drops into the ocean almost instantaneously, the ocean doesn’t ripple. It heaves.

The idea first entered the scientific literature in a serious way in the early 1980s, when geologists mapping Lānaʻi noticed a peculiar unit known today as the Hulopoe Gravel. This deposit isn’t subtle. It’s a chaotic mix of coral fragments, limestone blocks, basalt clasts, and marine fossils, plastered across slopes and ridges up to roughly 300–370 metres above present sea level. These aren’t thin veneers either. In places, the deposit is metres thick, clast-supported, and shows clear signs of high-energy flow, including imbrication and grading that point inland, not downslope.

That directionality matters. If these were uplifted shorelines, you’d expect neat terraces, wave-cut benches, and in-situ reef structures. If they were storm deposits, you’d expect them to thin rapidly inland and cap out at elevations consistent with even the most extreme cyclones. Instead, what the field data show is something that looks like it arrived in a single, catastrophic rush, surged uphill, and then drained back toward the sea.

Similar deposits occur on Molokai, particularly along the island’s flanks facing the open Pacific. Molokaʻi is already famous for its sea cliffs, the tallest in the world, carved by massive slope failures in the island’s past. Sitting above those cliffs are scattered marine gravels and coral-bearing units at elevations that again make no sense in a storm or uplift framework. The simplest explanation tying Lānaʻi and Molokaʻi together is that they were struck by the same wave train.

So where did that wave come from? The answer lies offshore, hidden beneath thousands of metres of water. The Hawaiian Islands are only the exposed peaks of enormous volcanic edifices, and like all steep volcanoes built on weak foundations, they fail. Sometimes slowly, sometimes all at once. One of the largest known submarine landslides on Earth is the Nuʻuanu Landslide, which tore away the northeastern flank of Oʻahu. The debris field covers an area larger than the state of California, with individual blocks tens of kilometres long.

When that much rock detaches and accelerates downslope, it doesn’t politely settle onto the seafloor. It ploughs into the ocean basin, displacing water laterally and vertically. Numerical models based on the volume and geometry of these Hawaiian landslides consistently show that they are capable of generating initial wave heights well over 100 metres near the source. When those waves encounter nearby islands, especially steep volcanic ones, run-up can be dramatically amplified.

Dating these events is tricky, but multiple lines of evidence converge on a late Pleistocene age, roughly around 100,000 to 120,000 years ago. Coral fragments within the high-elevation gravels yield ages consistent with that window, and that timing broadly overlaps with periods when major Hawaiian flank collapses are thought to have occurred. There’s still debate over which exact landslide was responsible, with candidates including not just Nuʻuanu but other massive failures such as Wailau off Molokaʻi. What matters is that the timing and the physics line up.

The height of the wave is where things get uncomfortable, even for seasoned geologists. Conservative estimates, based purely on minimum elevations of confirmed marine material, put run-up on Lānaʻi well above 170 metres. More aggressive interpretations, incorporating the highest outcrops and flow indicators, push that number toward 300 metres or more. Even if you take the lowest defensible value, you’re still dealing with a wave several times taller than anything produced by historic earthquakes.

Scepticism is healthy in science, and this hypothesis has attracted plenty of it. The main alternative explanation has always been uplift. Hawaiʻi is not tectonically active in the plate-boundary sense, but volcanic loading and flexure can cause islands to rise and fall over long timescales. Could these deposits have formed near sea level and then been lifted? The problem is that independent constraints from submerged reef terraces and drowned shorelines show that Lānaʻi and Molokaʻi have, if anything, subsided over the relevant time period. There simply isn’t a mechanism to lift a former beach deposit hundreds of metres without leaving an abundance of other coastal markers along the way.

Storm waves are even easier to rule out. The physics of wave generation in the open ocean impose hard limits. Even the largest cyclones cannot generate the kind of sustained, coherent flow required to transport multi-tonne coral blocks hundreds of metres uphill, across complex topography, and leave them embedded in thick, laterally extensive deposits. The energy budget doesn’t work, and the sedimentology doesn’t match.

One of the most compelling aspects of the tsunami interpretation is how well it explains the weirdness. The patchy distribution of the deposits, their occurrence on windward and leeward sides, the mixture of shallow and deeper-water fossils, and the evidence for multiple surges all fit a scenario where a wave train wrapped around the islands, refracted by topography, and slammed into some slopes while barely touching others. That kind of selective chaos is exactly what tsunamis do.

Could this happen again? In principle, yes. Hawaiian volcanoes are still growing, still spreading, and still failing. The south flank of Kīlauea, for example, is creeping seaward today at measurable rates. Most of that motion is slow, accommodated by earthquakes and gradual deformation. But the geological record makes it clear that slow creep does not preclude sudden collapse. When these systems fail catastrophically, they do so without warning on human timescales.

That said, events on the scale required to generate a 200–300 metre wave are extraordinarily rare. We’re talking recurrence intervals of tens to hundreds of thousands of years. This is not a near-term hazard in the way that earthquakes or hurricanes are. But rarity doesn’t make it irrelevant. These events shape coastlines, reset ecosystems, and leave indelible marks in the stratigraphic record. They also remind us that the most extreme natural disasters don’t come from the processes we experience most often, but from the ones we barely ever see.

What makes the Hawaiian mega-tsunami especially important is that it’s not an abstract model. It’s written into the rocks, sitting there in plain sight, waiting for someone to ask the uncomfortable question of how they got there. Whether every detail of the original interpretation survives future research is almost beside the point. The core idea—that volcanic island collapses can generate waves far larger than earthquake tsunamis—is now widely accepted, supported by modern analogues like Lituya Bay and Vajont, just scaled up to an oceanic setting.

In the end, this story isn’t really about Hawaiʻi at all. It’s about the upper limits of what Earth can do when gravity, geology, and water collide. The islands just happen to be one of the few places where the evidence survived, perched high above the sea, quietly contradicting our intuition. And once you’ve seen coral at the top of a mountain, it becomes very hard to look at the ocean the same way again.

Here's the video we made on this on the OzGeology YouTube Channel: