Heavy Rotorua rain creates 'inconvenient' new sinkhole, reactivates others

Friday, 4 May 2018

A fissure more than 150m long opened up on a farm near Rotorua in 2018, and immediately drew crowds. (This video was first published in May 2018.)

A cavernous sinkhole has made its problematic presence known on a Rotorua farm after all the heavy rain at the weekend.

Geonet volcanologist Brad Scott visited the region after the deluge because he knew heavy rain often caused sinkholes along fault lines to reactivate. The influx of water had, in fact, caused the underground cavities to swell and reactivate some old sinkholes, and even created a new one.

'If high volumes of water go down the faults, the water gets trapped and the only way it can escape is by going through fractures,' he said.

'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. Eventually a cavity can get so big that the overlying land falls into it, and boom – a collapse hole or 'tomo'.'

Farm assistant Phillip Setters takes a look at the massive new sinkhole.

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Sinkholes formed when water washed away the soil and created underground cavities, which often happened near fault lines, he said.

This massive sinkhole opened up on a farm near Rotorua after heavy rain.

'They do tend to occur along earthquake faults, but this is because the ash or rocks along faults has been smashed up during movement on the fault over thousands of years, so it is more easily washed away during heavy rainfall. The sinkholes are not related to any recent earthquakes.'

Rotorua recorded its highest ever hourly rainfall rate at the weekend.

'Last weekend's downpour – almost 170mm of rain in 38 hours – created the perfect storm for a collapse hole to form,' Scott said.

The new sinkhole emerged at Earthquake Flat, about 15 kilometres southeast of Rotorua.

'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,' Scott said.

The crater intersected with Taupo Fault Belt faults, which made it particularly vulnerable to sinkholes forming from eroded soil.

This new sinkhole was 'particularly large' and 'inconvenient for the farmer who must now work around it', Scott said.

FARM ASSISTANT 'COULD HAVE DIED'

No animals or humans were harmed in the forming of this sinkhole.

Farm assistant Gabriel Lafalla discovered the huge crack that opened up on the dairy farm he worked at.

He 'could have died' if he had been riding his bike in the early morning a few inches to the right, he said.

Sinkholes in the area were known to 'take out some farmed land and fences', but otherwise had no societal consequences, Scott said.

'No houses, buildings, animals or people have ever been affected as far as I'm aware.

'To date, only roads, farm trails and pasture have been affected by these features.'

SINKHOLES NOT UNUSUAL

Scott said sinkholes were common in the Rotorua area.

'They've been happening as long as people have farmed in the area,' he said.

'The formation of collapse holes, locally known as 'tomo', is very common in the soft, easily eroded, pumice-based soils.

'In 1967, part of State Highway 5 - about one kilometre east of this latest tomo - collapsed under similar circumstances, again along one of the faults in Earthquake Flat crater.'

THE SILVER LINING

'[Sinkholes] give our volcanologists a fascinating view back through tens of thousands of years,' Scott said.

'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, but violent eruptions.'

Geonet volcanologists and landslide scientists would spend the next few weeks investigating the new and reactivated sinkholes and landslides, Scott said.