Science5 publishers2 min readPublished Updated
Hawaii's 550-year-old Hōlei Sea Arch has collapsed into the Pacific Ocean
Hawaii's 90-foot Hōlei Sea Arch, formed about 550 years ago, fell into the Pacific between 23 and 27 September, the National Park Service says. No one knows the exact moment, so the case against Tropical Storm Nolo rests on timing.
The Scientist · Science desk
Drafted by a language model from the sources cited here and checked against its claim ledger before publication. How we use AISend a correction
What happened
- New cracks and instability in the cliffs led the park to close the original viewing area in January 2020, and a viewing area at a safer distance opened in March 2020.
- Surf from an earlier tropical storm, named by PopSci as Darby, broke a chunk of rock off the lower part of the arch.
- PopSci reports that Nolo brought more than a foot of rain to some places on the Big Island.
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Why it matters
- constraint Because nobody recorded when it failed, the collapse cannot be used to estimate how much storm loading the damaged cliffs can take before the next section fails.
- precedent In 2020 the park responded to cracking cliffs by moving visitors to a safer distance, and it would probably do the same if the coastline beside the old arch keeps failing.
- cost Visitors to the end of Chain of Craters Road lose a popular, heavily photographed destination on a stretch of coast that has been closed and rerouted before.
Four days passed between the last confirmed sighting of the arch and the first confirmed absence [1]. Nolo is the suspect because it passed through that window. According to the BBC, the arch is believed to have fallen while the park was temporarily closed for the storm [5]. PopSci, citing the park, says only that it may have [11].
The two outlets also label the storm differently. The BBC calls it Tropical Storm Nolo [5], while PopSci describes a hurricane that reached Category 3 but did not strike the islands at that strength [10]. Neither report includes wave heights at Hōlei during the storm. Waves are what made the arch in the first place: it formed about 550 years ago as Pacific surf pounded into an ancient lava flow [9].
Park service material cited by PopSci attributes the lifespan of these cliffs to differential erosion, in which softer rock wears away faster than stronger rock [12]. I'd read Nolo and the 2018 earthquake damage as late steps in that one process. PopSci's own summary is that five centuries of waves and earthquakes caught up with Hōlei [15]. The thing this record doesn't tell you is how to divide the blame. A collapse during a storm, on cliffs already damaged, fits the storm as the final trigger. It does not show that a calm week would have spared the arch for long, or that the arch would have outlasted Nolo without the earlier damage.
The park predicted the outcome back in 2021, though without a date. "This formation is beautiful, but temporary, and has a limited life span," the park service said that year. "The sea arch will eventually crumble into the sea. However, others will replace it as the cliff slowly migrates inland." [4] In its statement on the collapse, it said: "The coastline flanking the sea arch has shown signs of instability for years." [3] PopSci notes that replacement arches will take some time [16].
Hōlei is named for a small endemic plant in the milkweed family [18]. "Hōlei Sea Arch is a very special place to so many of our visitors!" Hawaii Volcanoes National Park wrote on Facebook, asking people to post their photos [17].
What to watch
- A park survey, imagery or footage that pins the collapse to a specific day inside the 23 to 27 September window, the direct test of the Nolo link.
- Any change to the viewing area at the end of Chain of Craters Road if the park finds new cracking in the cliffs that flanked the arch.