|View of the eastern end of the newest tomo at Earthquake Flat. Photo: Colin Tremain
A spectacular sinkhole the length of two football fields and the depth of a six-story building has opened up on a New Zealand farm.
The huge chasm appeared after several days of heavy rainfall near the North Island town of Rotorua, reactivating a series of collapsed holes and causing a 500 meter crack along a fault line.
Farm manager Colin Tremain told RT.com he stumbled upon the sinkhole on Monday morning, guessing it opened up quickly some time on Sunday. Locally known as ‘tomo’ the hole is about 180 meters long, 20 meters wide and 20 meters deep (591ft x 66ft x 66ft).
The holes most commonly form when water seeping into the ground washes away finer particles, creating an underground cavity or tunnel – a path for water to flow in. The eroded soil is carried away into the porous volcanic rock underneath.
"Faults make easy pathways for water to flow into the ground, so these cavities will often form along faults, or at the bottom of fault scarps," says GeoNet volcanologist Brad Scott.
"Eventually a cavity can get so big that the overlying land falls into it, and boom – a collapse hole or tomo. This process can happen quickly, over the course of a few hours."
Before the cavity collapse there might not be any evidence, or only very subtle evidence that this underground erosion is happening.
Many are discovered by tractors or fertiliser trucks driving across what appears to be solid ground, says Brad.
"The collapse holes most often appear during heavy rain, when water ponds in low lying areas or old craters, and existing collapse holes can reactivate and get bigger during heavy rain."
Earthquake Flat, where the most recent collapse hole has formed, is a unique area about 15 km southeast of Rotorua.
|View looking east of the newest tomo to form at Earthquake Flat. Photo: Colin Tremain
It is the summit crater of a volcano that erupted about 60,000 years ago, flinging 50 cubic kilometres of pumice across the surrounding countryside.
"The Earthquake Flat crater is also criss-crossed with faults of the Taupo Fault Belt, and it is these faults that provide a convenient path for water to seep into, eroding away the soil as it goes.
"Last weekend's downpour – almost 170mm of rain in 38 hours – created the perfect storm for a collapse hole to form," says Brad.
"While last weekend's collapse hole is particularly large, and inconvenient for the farmer who must now work around it, these sorts of features aren't unusual around Rotorua."
In 1967 part of State Highway 5 about 1 km east of this latest tomo collapsed under similar circumstances, again along one of the faults in Earthquake Flat crater.
The one good thing about collapse holes is that they give our volcanologists a fascinating view back through tens of thousands of years, providing a few more pieces in the puzzle of the Taupo Volcanic Zone's history.
"We can see the layers of lake sediments, quietly formed on old lakebeds over long quiet periods, punctuated by layers of ash and pumice from relatively short by violent eruptions.