For a country that just witnessed the formation of the largest sinkhole in its history, New Zealand seems impressively chill. In early May, a farmer reported that a gaping hole, as long as two football fields and deep enough to swallow a six-story building, had opened up on his land near Rotorua, a city on the North Island. Rather than freaking out, volcanologists checking out the site seem merely curious about the sinkhole, its formation, and the traces of an ancient volcano within its depths.
GNS Science volcanologist Brad Scott, who visited the sinkhole shortly after it was discovered by a farm assistant who nearly drove his motorcycle into it, explained to Science Alert on Monday that sinkholes in the area weren’t that rare, partially because of the natural geology of the region, which is known as “Earthquake Flat.” Rain, Scott speculates, may have been eroding the limestone beneath the farm for up to 100 years, setting the stage for a massive collapse. The sinkhole finally caved in after a period of particularly heavy rainfall in the region at the end of April.
Peering into the sinkhole revealed a 60,000-year-old relic. At the bottom of the hole, Scott told TVNZ on May 2, is the “original 60,000-year-old volcanic deposit that came out of this crater.” He explained that the deposit was covered by 10 to 12 meters of sediment left over from ancient lakes that had formed and dissipated, then by three meters of volcanic ash. The volcano, fortunately, is a dormant one.
In 2017, a massive sinkhole opened up in New Lynn, a suburb of Auckland, some 143 miles from Rotorua. At the time, GNS Science engineering geologist Dr. Sally Dellow explained to the New Zealand Herald that limestone was also to blame, though she pointed out that one of the greatest factors leading to sinkholes in cities tended to be strong water flows, either naturally occurring ones or those resulting from broken pipes or water mains.
Dellow, like Scott, didn’t seem too concerned about the sinkholes, noting again that they aren’t uncommon in the area. She did, however, express some concern about the warming planet: “Climate change is envisaged to produce more storms and more intense storms so, again, this might see a few more sinkholes appear but being able to make a direct link will be difficult as sinkholes are still relatively rare, compared to, say, landslides.”
Colin Tremain, who manages a farm near the new sinkhole, was likewise nonplussed. According to Phys.org, he said he’d “put a fence around it and forget about it, waste of time filling it in.”