Lagoon Reveals Secret
 |
Photo: Rarity Exposed; Panmure Lagoon has a buried volcano beneath its calm exterior. (Fiona Greasley/East & Bays Courier) |
Lagoon Reveals Secret
|
Photo: Rarity Exposed; Panmure Lagoon has a buried volcano beneath its calm exterior. (Fiona Greasley/East & Bays Courier) |
February 27, 2008
Stuff, New Zealand
Beneath the mirror-glass surface of Panmure Lagoon lies a rare geological find – a buried volcano within a volcano mouth.
Geologists from GNS Science, a government-owned research and consultancy company, and Auckland University discovered the buried volcano while drilling into the basin from a barge last week.
The exploration of the lagoon is part of a study to learn more about the North Island’s volcanic history.
The hope is to better understand the clustering of volcanic eruptions in the Auckland volcanic field, to prepare for future activity.
The lagoon was the fifth location drilled in three years.
The structure of the Panmure Lagoon is called a maar, a volcanic crater that is produced by an explosion in an area of low relief. It is generally more or less circular and often contains a lake, pond, or marsh.
The lagoon maar was created about 28,000 years ago but the newly discovered volcano within it could be as young as 10,000 years, volcanologist Dr Graham Leonard says.
Dr Leonard says it is extremely rare for two eruptions to have occurred at the same site in the Auckland volcanic field.
The age of the newly discovered volcano means the two eruptions would have happened independently, he says.
He says Rangitoto is possibly the only other volcano in Auckland to have erupted twice.
Dr Leonard, project leader with GNS Science, says when the drills reached the 16-metre mark in the middle of the basin, they hit scoria.
Scoria is an igneous rock or mineral that has solidified from molten or partly molten material.
The material is formed when magma, rich in dissolved gasses, erupts to the earth’s surface.
At the surface the magma is under less pressure and the gasses escape to form bubbles.
These bubbles are trapped as the magma cools and solidifies.
Dr Leonard said the exploration showed the top of a scoria cone volcano, maybe about 30 metres high, buried under the mud that filled the basin when the sea level rose.
This volcano was formed by "fire fountaining" like the volcanos in Hawaii, where the magma spits up to the surface.
Dr Leonard says fire fountaining is "quite beautiful, like a sprinkler of hot lava".
Another example of a volcano within a volcano is the Auckland Domain, a maar containing a volcanic cone. But both those features are thought to have formed at the same time.
Dr Leonard said another important find involved Mt Wellington.
At 40 metres deep in the Panmure Lagoon was a 2.6-metre ash deposit from the Mt Wellington eruption.
Only 1.5km from the basin, Mt Wellington erupted about 9200 years ago.
It is thought to be the second-most recent Auckland eruption behind Rangitoto which erupted about 600 years ago.
The GNS Science website says Auckland’s volcanoes are unlikely to become active again since they all follow a pattern of short eruptions.
But the volcanic field itself is young and still active, meaning new volcanoes could appear with little warning in a new location.
The site states that in a city of a million people, even a small and brief eruption would be a major event.
Much of Auckland is built on a potentially active volcanic field. Between Manurewa in the south and Takapuna in the north, there are about 50 volcanoes.
http://www.stuff.co.nz/stuff/sundaystartimes/auckland/4416324a6497.html