An Australian geologist has taken exception to the media calling the Queensland “sinkhole” a sinkhole. Last Saturday night, approximately 100 square meters of sand literally fell into the sea, swallowing up a car and camping trailer on Inskip beach, a popular beach in Australia’s sunny state of Queensland. “Sinkhole” is what the world and Australia-based media outlets immediately began calling the phenomenon.
Stephen Fityus has taken exception to the label “Queensland sinkhole” and has presented a long and closely reasoned argument to Australian essay publication the Conversation. But it’s not just a dislike of the word sinkhole that is driving him. Stephen Fityus is a professor of geotechnical engineering at the University of Newcastle and, as such, there’s a good chance that he knows what he’s talking about.
Professor Fityus tells us that the event described could not possibly have been a sinkhole. Queensland coastal topography and geology are responsible, according to the professor. Firstly, he says, a sinkhole is a very specific kind of hole. According to the strict definition of a sinkhole, Queensland’s event did not pass muster.
“In its strict sense, a sinkhole occurs when there is movement of surface soil or rock downward to fill a cavity in the ground below it.”
What this means is that for a sinkhole to be a sinkhole, there must be a sort of “cap” of ground covering a pre-existing void. In the case of the Queensland sinkhole, Professor Fityus doubts that such a void existed. According to the professor, sinkholes are usually formed in what he calls “karstic” environments.
“These are where the percolation of groundwaters through limestones and dolomites over geological timescales causes them to dissolve, leading to the formation of underground cave systems.”
Water moves around through porous stones and carves out underground caves. When the caves get too large for the ground above them, the ground collapses into the resulting sinkhole. Poorly capped mineshafts and other human activity can also cause the formation of a sinkhole. Queensland, according to the professor, is not naturally rich in these kinds of environments, or at least not in the area where a massive chunk of beach disappeared.
So far, so simple. But then the professor goes on to explain what he thinks the Queensland sinkhole really was, at which point it all gets a bit multi-syllabic and technical. I’ll try to simplify it for us non-geologist types.
Basically, Professor Fityus argues that there is no explanation for what he calls the “Inskip event” that is consistent with a sinkhole. Queensland, or at least this part of its massive coastline, is, however, subject to strong and uniform currents and one of the world’s biggest dumps of fine quartz sand. The local river systems dump this sand at the seaside, which is then picked up by ocean currents until there is enough sand to form a deposit. Fityus points to Fraser Island, the world’s largest sand island, as an example. Fraser Island is off the coast of Queensland.
As well as this movement up and down the coast, tidal forces and river currents alternately dump, scour away, and swirl sand around. The site of the Queensland “sinkhole” is, according to the professor, at a juncture of all these forces. Professor Fityus theorizes that the lower portion of the sand deposit forming Inskip beach was eroded by the confluence of these forces, causing the natural “angle of repose” to become too steep to maintain its stability, and thereby causing the incident incorrectly described as the Queensland sinkhole.
This all seems to make sense, but I doubt it makes very much difference to those immediately concerned what name is given to the thing that swallowed their car.
[Picture via Getty Images/Christopher Furlong]