"We know that it wasn't just a hydro-thermal eruption, it wasn't just a shallow explosion, there was definitely magma gas released and that's why we're saying it was a phreatic eruption, driven by volcanic gas pressure. But at this stage the cause is a little bit speculative."
Cloudy weather meant it would not be possible to do a flyover of the mountain today, or to venture up it, as Mr Rosenberg did on Friday.
He and volcanologist Brad Scott wore protective clothing and hard hats as they gathered rocks that were flung into the air during the explosion.
Mr Rosenberg said the experience made him nervous.
"You are very, very aware of the situation around you, looking over your shoulder all the time to see what's coming out of the vents. You're very conscious about not spending a lot of time making your measurements and observations. It's exciting but you're definitely nervous - you know that anything could happen at anytime."
Meanwhile, GNS Science believes the raft of pumice found near the Kermadec Islands last week came from a previously inactive volcano.
Principal scientist Cornel de Ronde said the pumice came from Havre Volcano, near Curtis Island at the south of the Kermadec Islands.
Between July 17 and 18 there were 157 earthquakes, ranging in magnitude from 3.0 to 4.8 near where the pumice was first spotted.
The volcanoes in this area were "pretty deep", he said.
"At these sorts of depths, no, those eruptions are unlikely to affect anyone here in New Zealand."
