Sea arches form when waves exploit cracks and soft rock in a headland, carving caves that eventually meet and break through. They collapse because the same erosion never stops, and a 2026 example — Hawaii's Hōlei Sea Arch — shows the process ending in days after centuries of wave action.
A sea arch is a rock bridge left standing after waves erode a passage through a headland. The feature looks permanent on a human timescale, but geologists treat every arch as temporary: the same forces that built it keep weakening it until the span fails, usually without warning.
How an arch is carved
According to an oceanography course text from Diablo Valley College hosted on the LibreTexts platform, wave energy concentrates on headlands because of refraction — waves bend as they move through water of different depths and focus their force on protruding rock while losing energy in adjacent bays. Over time this uneven attack straightens an irregular coastline.
Within a headland, waves and longshore currents exploit cracks, joints and softer rock layers, hollowing out a sea cave. If erosion reaches all the way through a narrow headland — or if two caves on opposite sides meet — the result is an arch, per the LibreTexts explanation. Non-resistant rock is removed first, leaving harder rock standing as the arch's frame.
Why arches collapse
The mechanism that creates an arch is the same one that destroys it. Waves keep attacking the thinning rock from both the sea-facing and inland sides, widening cracks and undercutting the span until gravity wins. When the roof of an arch finally gives way, what remains standing in the water is called a sea stack — an isolated pillar of the harder rock that resisted erosion longest, as described in the LibreTexts coastal-landforms material.
Earthquakes, tropical storms and human damage can speed up a collapse that erosion alone would take longer to finish. The Azure Window in Gozo, Malta, illustrates this: the limestone arch fell into the Mediterranean on 8 March 2017 during a storm with rough seas, after years during which authorities had already banned visitors from walking on it due to visible erosion, according to Euronews. Maltese Prime Minister Joseph Muscat said at the time that the landmark had always faced the risk of natural erosion and that the day it finally collapsed had arrived, according to Euronews.
A 2026 example: the Hōlei Sea Arch
The clearest recent illustration is the Hōlei Sea Arch in Hawaiʻi Volcanoes National Park on Hawaii's Big Island. The park service says the 90-foot arch was cut into the cliff of an ancient lava flow roughly 550 years ago through differential erosion — softer rock wearing away faster than the surrounding, more resistant basalt — according to reporting by Popular Science that cited the park's description.
The arch had shown instability for years before it fell. Nearby cliffs were damaged during the 2018 Kīlauea eruption and summit collapse, and new cracks in January 2020 led the park to close the original viewing area, reopening a safer one farther from the edge in March 2020, according to the National Park Service. Tropical Storm Darby broke off a chunk of rock from the arch's lower section that same year, Big Island Now reported.
The arch was confirmed standing on Wednesday, 23 September 2026, but had collapsed by Sunday night, 27 September 2026, the park service said. Officials said the exact moment of failure is unknown and that it may have happened while the park was closed during Tropical Storm Nolo, according to Big Island Now.
Visitors are required to stay out of closed areas and away from cliff edges which are unstable and can collapse without warning.
— Hawaiʻi Volcanoes National Park, National Park Service statement
Key stages compared
From headland to stack: stages of arch formation and loss
Stage
What happens
Driving process
Headland
Waves focus energy on a protruding rock mass
Wave refraction
Sea cave
Cracks and soft rock are hollowed out
Differential erosion
Sea arch
Erosion breaks through the headland, leaving a bridge
Continued wave attack on both sides
Collapse
The arch roof fails under its own weakening structure
Ongoing erosion, often hastened by storms or quakes
Sea stack
An isolated pillar of harder rock remains
Removal of the weaker arch material
The Hōlei Sea Arch case fits this sequence: a 550-year-old arch, already thinned by storms and seismic damage, failed completely within a four-day window once a tropical storm added fresh stress, based on the timeline given by the National Park Service and Big Island Now.
Questions
What exactly is a sea arch?
It is a rock bridge formed when wave erosion cuts a passage all the way through a headland, leaving an opening with rock still standing above it, as described in coastal-landform material from LibreTexts.
What makes sea arches eventually fail?
The same wave action and erosion that created the arch keeps thinning the rock from both sides until it can no longer support its own weight; storms, earthquakes and cracking accelerate the failure, as seen with both the Hōlei Sea Arch and Malta's Azure Window.
What happens to the rock after an arch collapses?
The harder rock that resisted erosion longest is left standing alone in the water as a sea stack, according to the LibreTexts coastal erosion overview.
How old was the Hōlei Sea Arch when it collapsed?
The National Park Service and Popular Science reported the arch had formed roughly 550 years ago, cut into an ancient lava flow by Pacific Ocean waves.
When did the Hōlei Sea Arch collapse and why?
It was still standing on 23 September 2026 and had collapsed by 27 September 2026; the National Park Service said years of cliff instability, 2018 earthquake damage and a 2026 tropical storm closure likely contributed, though the exact moment of failure is unknown.
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