In 1868, a tsunami rolled over the tops of coconut trees on Hawaii's southeast coast -- "probably 60 feet high," one witness reported -- sweeping houses, livestock, and 46 people out to sea. The source was not a distant earthquake across the Pacific. It was the island itself, breaking apart. The Hilina Slump, a colossal block of Kilauea's south flank, had lurched seaward, and the ocean responded accordingly. More than 150 years later, the slump is still moving -- roughly 10 centimeters per year, pushed by magma injection and pulled by gravity -- and the question of what it might do next remains one of the most consequential in Hawaiian geology.
The Hawaiian Islands are shield volcanoes, built by eruptions from a hotspot deep in Earth's mantle. Their broad, gently sloping profiles belie a structural instability that only becomes apparent over time. As each volcano grows, its own weight causes its flanks to stretch and sag seaward, sliding across a nearly horizontal detachment fault -- a decollement -- where volcanic rock rests atop the older oceanic crust, roughly 8 to 10 kilometers below the surface. On Kilauea's south flank, this process has created the Hilina Slump: a massive, largely intact block of rock that extends from the thousand-foot-high cliffs of the Hilina Pali down to depths of 5,000 meters on the ocean floor. Kilauea's rift zones -- the Southwest and East Rift Zones, which channel lava parallel to the coast -- act as wedges, driving the south flank further downslope with each injection of magma.
The slump moves both silently and violently. Much of its seaward creep is aseismic, unnoticed by anyone on the surface. But periodically the friction holding the block in place gives way all at once, producing earthquakes of magnitude 6 or greater. The 1868 event -- estimated between magnitude 7.25 and 7.75 -- triggered a landslide on Mauna Loa that killed 31 people, plus the tsunami that killed 46 more and devastated the villages of Punalu'u, Ninole, and Honuapo. In 1975, a magnitude 7.2 earthquake sent a 40-mile section of the slump sliding 11 feet into the ocean, widening the gap at the headscarp by 26 feet. The resulting tsunami reached 47 feet at Keauhou Landing. Two campers died at Halape, and 19 others were injured. In 2018, a magnitude 6.9 earthquake moved the slump about two feet, generating a small tsunami but no casualties.
Systematic mapping of the Hawaiian seafloor in the late 1980s revealed 17 areas around the islands that appear to be remnants of enormous ancient landslides. The Nuuanu Slide alone is estimated at 5,000 cubic kilometers in volume, with blocks "tens of kilometers" in size transported more than 50 kilometers from their origin. These discoveries raised the specter of a catastrophic failure of Kilauea's south flank generating a megatsunami that could threaten the entire Pacific Rim. But geologists draw a critical distinction between debris avalanches -- fast-moving, fragmented flows that release energy suddenly -- and slumps, which are slower, wider, largely intact blocks. The Hilina Slump is the latter. Its seaward movement has been partly halted by submarine seamounts and a "Midslope Bench" that acts as a natural buttress. As researcher Pararas-Carayannis concluded, dire predictions of a future breakaway "may be overstated."
What makes Kilauea's south flank particularly prone to failure is a process called magma-jacking: fresh magma injected into pre-existing fractures or weak rock, with the pressure literally prying the flank apart from within. Smith and Malahoff, in a 1999 paper, noted that Kilauea's position as a secondary volcano on the larger Mauna Loa makes it especially susceptible -- a pattern they observed across many of Hawaii's historic landslides. The interplay between volcanic construction and gravitational destruction continues today. Kilauea's south flank comprises about 13.7 percent of the Big Island, and the submarine portion of the slump makes up roughly 10 percent of the island's total volume. Model results suggest that earthquake accelerations of 0.4 to 0.6 g could exceed the static friction holding the block in place. Whether the buttressing provided by the Midslope Bench and surrounding seamounts is enough to prevent a catastrophic detachment remains an open question.
The Hilina Slump occupies Kilauea's south flank, centered around 19.27°N, 155.16°W. The Hilina Pali cliffs are visible as a series of northeast-trending scarps at roughly 2,000 ft elevation. Chain of Craters Road descends through the slump zone to the coast. Nearest airport: PHTO (Hilo International Airport). The submarine extent of the slump is not visible but extends to 5,000 m depth offshore. Best viewed from 8,000-12,000 ft altitude, looking south from above Kilauea's summit.