St Kentigern’s pyroclastic flow

In the heart of Auckland, in the playing fields of St Kentigern College, there is an unusual white rocky outcrop. Dr Darren Gravley explains that this came from a volcanic eruption in the central North Island travelling at supersonic speed along the ground to end up in Auckland.

Transcript

DR DARREN GRAVLEY At St Kentigern’s College here in Auckland, we have quite a thick, about 3–4 metre thick ash deposit that has come from one of these caldera volcanoes in the central North Island.

Now, it’s not a deposit that actually went up in the air and sort of moved laterally through the atmosphere and fell in the Auckland area. It’s actually material that fell probably fairly close to the source where the caldera was erupted, and from that energy of falling and the eruption pushing it out, it actually travelled across the landscape as what we call a pyroclastic flow.

That pyroclastic flow is made up of ash, pumice, rock fragments moving at close to supersonic speeds across the Earth’s surface. It was able to travel over 200 kilometres from its caldera source to end up in Panmure Basin, which is where St Kentigern’s College is.

A truly remarkable, widespread pyroclastic flow deposit – the most widespread that’s been found anywhere in the world.

Acknowledgements: Google Earth Natural History New Zealand Ltd.