Lake Rotorua Q&A: What caused the eruption?

A hydro-thermal eruption in Te Ruapeka Bay, Lake Rotorua, this morning caused panic among residents. Photo / Alex Robertson

GNS volcano information specialist Brad Scott answers questions on the science behind the eruption which sent water shooting 20- to 30-metres into the air in Lake Rotorua early today.

What caused water to shoot into the air?
A hydro-thermal eruption which is a geyser eruption caused by steam and gas coming up through the lake bed. The floor of Lake Rotorua lifted up the water above it, causing it to shoot into the air. It happened in a very shallow part of the lake, which is between about 1 to 2-metres deep.

What caused the loud noise?
A combination of steam bubbles popping and rocks and gravel from the floor of the lake falling back into the water.

How unusual is this?
These eruptions have been occurring for tens of thousands of years. It is part of the territory and is normal. There has been nothing like this since 2001, which was one kilometre south of today's eruption at Kuirau Park. Hard to compare the two but they were probably of a similar size.

Could the eruption be related to this month's earthquake?
We can't eliminate the earthquake, however, it is difficult to connect the dots. I can't, however, put my hand on my heart and say 'no they aren't connected'.

Could this be a sign of more activity to come?
It is difficult to know if there is something to come. We used to have several incidents like this when the geothermal system was being exploited, however we haven't had any in the past 15 years since the bore closure [more than 300 bores within a 1.5km radius of the declining Whakarewarewa geysers were gradually closed down in 1987 and 1988].

Does this pose any danger to the public?
No. It is out in the lake well away from where people are, not in a part of the lake that is used by boats as it is so shallow, they could run aground.