This past weekend, a large portion of an Australian beach suddenly collapsed into the ocean. Initial reports indicated it was a sinkhole, but geologists say it’s more likely to be the result of a unique near-shore landslide.

The Inskip Sinkhole, as it was first described, appeared on Saturday September 26 at the Inskip Peninsular at MV Beagle Point, north of Rainbow Beach. Thankfully, there was no loss of life, but witnesses say a caravan, a car, and some tents were flushed into the sea. Check out some first-hand accounts of the incident here and an image gallery here.

The cavity measures about 655 feet long (200 meters), 164 feet wide (50 meters), and up to 30 feet deep (9 meters). That’s bigger than a football field. Around 300 campers were evacuated from Inskip Point after the incident. The hole now appears to be stable, but there is still concern that more trees may fall down, and that other sections of the beach may soon collapse.

Image credit: Leonie Mellor/ABC News

At first, experts said the phenomenon was caused by a sinkhole, which is essentially a collapsed cave. Over the last few days, however, realization has set in that the hole was probably caused by a submarine landslide, or what’s also known as a near-shore landslide. A report from the Sunshine Coast Daily explains:

A Queensland Parks and Wildlife spokeswoman said the event was unlikely to be related to or caused by earthquake activity.

“Rather, it’s most likely a natural phenomenon caused by the undermining of part of the shoreline by rapid tidal flow, waves and currents,” she said. “When this occurs below the waterline, the shoreline loses support and a section slides seaward, leaving a hole, the edges of which retrogress back towards the shore.’’

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Peter Davies, a geologist at the University of the Sunshine Coast, added that the liquefaction visible at the site might be similar to processes seen at events triggered by earthquakes, where buildings fall into soft, loose, wet dirt that was once firm.

According to geologist Ted Griffin, a large channel between Inskip Pt and Fraser Island builds up a “shoulder” of sand, and then falls away, describing it as “a very unstable cliff of sand,” reports the Sunshine Coast Daily.

Geologist Dave Petley from the University of East Anglia in the UK says that it “would be fascinating to see some high resolution bathymetry data for this area as these slope failure events should leave a very distinctive deposit on the seabed,” adding that “the exact mechanism of failure, and its causes, are not clear to me and would be worthy of further investigation.”

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Via AGU Blogosphere’s Landslide Blog!


Email the author at george@io9.com and follow him at @dvorsky. Top image by Kieren Hudson/Facebook

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