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Sea arches collapse because waves keep eroding the rock bridge and its supports after the opening forms. Wave impact, compressed air and sediment abrasion can loosen fractured rock; over time, the weakened roof or supports fail. The process is gradual, but the exact timing and immediate trigger of a particular collapse may be unknown.
How a sea arch forms—and what happens next
- Waves exploit weaknesses. Fractures, faults, bedding planes and softer layers give waves and carried sediment routes into a rocky headland. Erosion enlarges these weak zones into sea caves. The National Park Service’s coastal geology overview describes this process.
- A cave opens through the headland. Continued erosion can break through the rock, leaving a natural bridge: a sea arch.
- Erosion continues. Waves still strike the arch, its roof and the rock supporting it. Hawaiʻi Volcanoes National Park notes that rough surf can strike the back of caves and reflect toward the headland, helping enlarge openings; the process does not stop once an arch forms. NPS: Sea Arches
- The bridge fails. As rock is removed and weakened, the roof or a support eventually gives way. A detached column of rock may remain as a sea stack, while waves continue reshaping the coast.
How waves weaken the rock
Impact and compressed air
Breaking waves can trap air in cracks and compress it against the rock. Together, air pressure and water impact can dislodge fractured rock and loose particles, a process called quarrying.
Abrasion by sediment
Waves also move sand, gravel and other particles against the shoreline. This abrasion wears rock away. How much damage these mechanisms cause depends on wave energy and the material being attacked; large swell and storms can increase energy, but their occurrence alone does not establish why a specific arch collapsed. NPS: Coastal Processes
Why collapse timing varies
There is no single lifespan or universal trigger for a sea arch. Rock composition and structure affect how readily it erodes; fractures and weak layers can focus wave action. Local climate and wave patterns also shape rocky coasts. Headlands are exposed to waves, wind and storms, but the conditions and rock at each site differ. NPS: Rocky Coasts
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- ENGINEER SOLUTIONS TO EROSION – Students design, test, and improve shoreline protection systems to reduce erosion caused by waves and water movement.
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- Rock structure and material: Look at fractures, bedding and the resistance of the rock.
- Wave exposure and energy: Ordinary wave attack and high-energy swell both matter, but a storm occurring near a collapse does not prove it caused it.
- Observed deterioration: Cracks, earlier rock loss or unstable nearby cliffs can show a history of change.
- Causal certainty: Separate what was observed from mechanisms considered likely and triggers that remain unknown.
Hōlei Sea Arch: a recent collapse with an unknown exact trigger
Hawaiʻi Volcanoes National Park reported on October 1, 2026, that Hōlei Sea Arch—previously reported as 90 feet (27.4 meters) high—was standing on Wednesday, September 23, and had collapsed by Sunday night, September 27. The exact time is unknown. The collapse may have occurred while the park was temporarily closed during Tropical Storm Nolo, but the park did not identify the storm as the cause. NPS: Hōlei Sea Arch collapses
The arch’s setting had a documented history of instability. Earthquakes during the 2018 Kīlauea eruption and summit collapse damaged nearby cliffs. New cracks and instability prompted the park to close the original viewing area in January 2020; a safer viewing area opened in March 2020. Later, massive surf during Tropical Storm Darby broke a chunk from the arch’s lower part. These facts document prior damage and instability, not a definitive cause of the 2026 failure. NPS: Hōlei viewing area
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Why careful attribution matters: Double Arch at Lake Powell
Glen Canyon National Recreation Area confirmed that Double Arch in Rock Creek Bay collapsed on August 8, 2024. The feature was Navajo sandstone at Lake Powell, not an ocean-coast sea arch. NPS said weathering, wind and rain had affected it, and that changing water levels and wave erosion were suspected contributors; the agency said it did not know what caused the collapse. It illustrates why possible erosion mechanisms should not be reported as a proven immediate trigger. NPS: Double Arch collapse
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Stay clear of unstable coastal cliffs
At Hōlei, NPS directs visitors to stay out of closed areas and away from cliff edges because they are unstable and can collapse without warning. The park also identifies the rough, wave-swept coast as a falling and drowning risk. Follow current closures and keep well back from unstable edges; no viewing point is worth entering a hazardous area.
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