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A buried volcano discovered inside another volcano in Auckland has excited scientists, who say it is extremely rare for two eruptions to have occurred at the same site in the Auckland volcanic field.
Geologists found the buried volcano inside Panmure Basin - which is the mouth of a volcano that erupted about 28,000 years ago - while drilling into the basin from a barge.
It was the fifth location the geologists, from the Government-owned research and consultancy company GNS Science and the University of Auckland, had drilled in the past three years to find out more about the volcanic history of volcanoes in the upper North Island.
Project leader Graham Leonard, a volcanologist with GNS Science, said when they drilled down to 16 metres in the middle of the basin they hit scoria.
Dr Leonard said the drill core indicated the top of a scoria cone volcano, possibly about 30 metres high, buried under the mud, which had filled the basin as the sea level rose following the last glaciation.
The volcano would have been formed as a result of "fire fountaining" (where the magma spits up to the surface), he said.
"It is quite beautiful, like a sprinkler of hot lava."
The Panmure Basin by comparison was a maar, the open mouth of a vent and created by a big explosion.
Dr Leonard said it appeared the volcanic cone could be about 10,000 years old, much younger than its host, so the volcanic events would have been independent of each other.
Of the 49 vents in the Auckland volcanic field, only Rangitoto was thought to have erupted twice.
There was, however, another example of a volcano within a volcano at the Auckland Domain - a maar which contained a volcanic cone within.
But that was thought to have formed at the same time.
Dr Leonard said another important find was a 2.6m deposit about 40m deep in the basin from the Mt Wellington eruption, thought to be the second-most recent Auckland eruption behind Rangitoto, which erupted about 650 years ago.
"It appears to be about 9000 years old. This is a large amount of ash in a small period of time."
Dr Leonard said the eruption of Mt Wellington, 1.5km from the basin, occurred about 9200 years ago.
It was important to understand the clustering of volcanic eruptions from the Auckland volcanic field in time and space to prepare for future volcanic activity in the Auckland region, he said.
Dr Leonard said the thick ash layers found in the Panmure Basin had complicated layering.
