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Masterton’s hidden ‘volcanoes’

With Gisborne District Council seeking to have mud volcanoes included in Land Information Memorandum [LIM] reports, reporter GRACE PRIOR went on a mission with geologist Chris Hollis to track down local examples.

A mud volcano is formed when methane and other gases are pushed up in fractures near tectonic faults, interacting with mud and groundwater and bubbling to the surface.

A “field” of mud volcanoes can be found on a Mangapakeha farm just northeast of Masterton.

The geological phenomenon is likely to attract increased official interest following Gisborne District Council’s Civil Defence Emergency Management Group noting this week that the hazard risk associated with mud volcanoes has only recently been assessed.

The council has identified specific areas of highest risk where dwellings have been constructed, as well as a broader area where there are no dwellings currently, and suggested a prohibition on construction would be advisable.

“Areas at risk with existing dwellings should have LIM reports flagged to include the mud volcano risk and a zone defined where new builds are prohibited.”

The council’s report said an “eruption” of a mud volcano in 2018 was “short-lived but violent and capable of throwing large boulders over 100m”.

A significant volume of debris was deposited over an area of over 2ha within a few hours, it said.

“Such an eruption below or close to a dwelling would pose an extremely elevated risk to life and property.”

Carterton-based geologist Chris Hollis said it is unlikely that any mud volcanoes in Masterton would be hazardous, and those that he knew of are quite far from dwellings.

“Generally, the mud is cool, and eruptions don’t result in rocks being thrown more than a few metres.”

While showing the Times-Age around the Mangapakeha mud volcano field, Hollis noted that there wasn’t a rock in sight.

He said hot mud would only occur if the mud volcano was near hot springs, which the Masterton ones aren’t.

Hollis added that the Mangapakeha mud volcano field is fenced off from stock but otherwise has little impact on farm operations.

The area is also not accessible to the public.

Hollis said mud volcanoes are a “rare but expected phenomenon” on the east coast of the North Island.

“The gas is likely sourced from organic-rich sediments buried at depth. An increase in the rate of expulsion might be caused by an earthquake.”

The area near the Mangapakeha mud volcano has the oldest rock at the core, and the younger and softer rock around the outside, which is where the natural gas has been able to escape.

Hollis said the “Whangai Formation” that the mud volcanoes have arisen in, also has within it the “Waipawa Formation”, which was thought to be the significant petroleum “source rock” for the East Coast.

“So it is quite likely that the methane [that is coming out of the vents] is coming from the Waipawa formation.”

Hollis said the source rock is rich in organic matter, which breaks down into methane and other hydrocarbons.

“Normally, the gas would be trapped but here, with so many faults running through this area, it’s escaping through fractures in the fault zone.”

“The ‘mud’ bit of the mud volcano is made up of a large part of the mud that is naturally around here, but it will be particularly muddy because the substrate – which is the rock beneath us – is also mud.”

When it comes to the Mangapakeha mud volcanoes, Hollis said that they aren’t likely to be a hazard to people unless they step in them, and even then, “you’ll just get wet and muddy”.

Hollis said the volcano and its emissions are not toxic and hardly flammable.

“Certainly, some gas vents you can light with a match; possibly you could ignite it, but there would be a little puff. It wouldn’t burst into flames.”

The East Coast has been of interest to petroleum geologists because there has been a suggestion of hydrocarbons deep in the ground.

However, Hollis said that further investigation is unlikely to lead them to much because the gases are being released through vents like the mud volcanoes.

Greater Wellington Regional Council senior policy advisor for hazards and coasts Dr Ian Dawe said the council keeps an eye on hazards such as mud volcanoes.

“In terms of land management and hazards, mud volcanoes are not suitable to be built on and this would be prevented under the Building Act as the land would not be considered stable and would not meet building consent requirements.”

Dawe said building consents are issued by the local council, but GWRC could become involved to help improve land stability through tree planting and
erosion control programmes.

He said if the mud volcanoes became more of a problem, similar to the scale seen in Gisborne, GWRC’s Regional Policy Statement would require them to be mapped and included in the Wairarapa Combined District Plan with rules to avoid inappropriate development in the area.

“If another mud volcano occurred, and it was threatening the general public, it could trigger an Emergency Management response.

“This would probably be dealt with at a local level but with GWRC regional support if needed.”

In 2001, the Wairarapa Midweek reported a former landowner ploughing over the mud volcanoes, only for them to reappear days later.

Hollis explained that with tectonic movement [earthquakes], it is possible that the fractures could open or close, prompting more mud volcanoes to form or others to disappear.

Grace Prior
Grace Prior
Grace Prior is a senior reporter at the Wairarapa Times-Age with a keen interest in environmental issues. Grace is the paper’s health reporter and regularly covers the rural sector, weather, Greater Wellington Regional Council, and coastal stories.

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