Cover me in your magma

OK, so one of the most obvious features of Dunedin is the harbour itself. I mean its pretty hard to not notice it. It’s home to Dunedin’s early industry and migrant history, Dunedin wouldn’t exist without the harbour and today the tourism from people exploring the harbour and surrounding areas is a major income earner for the city.

But the harbour itself was once a volcano, did ya know that?

volcano.png

The Dunedin volcano ( wow guys really? that’s one hell of an inventive name there!) began erupting around 16 million years ago in three main phases and was approximately 1000 metres high, the highest point today is Mt Cargill at 680m. Between phases the slopes of the volcano were forested and traces of these forests are preserved between lava flows in swamp and lake deposits found around the harbour.

Its form is called a ‘shield‘ volcano that is built up from fluid lava flowing down its gently sloping sides. These volcanoes occur away from plate boundaries and subduction zones, they arise from hot spots in the mantle and produce mainly basalt magma . The Dunedin City area, Otago Peninsula and Otago Harbour were all part of the volcano.

Shield Volcano

Eruption began near what is now Sandfly Bay on the Otago Peninsula. Initially submarine, the volcanic cone soon grew above sea level. Continued volcanism over the next 6 million years was concentrated in the Portobello and Port Chalmers area, but included other vents on the flanks of this growing shield volcano, and in satellite vents in East Otago, up to 100km distant from Dunedin. The Eruptive materials consist of pyroclastic flows, lava flows and fall deposits made mainly from basalt magma.

The death (R.I.P) of the volcano was either due to the cooling of the mantle source beneath Otago, or the result of failure of magma to reach the surface. The latter was possibly a result of the progressive change in the tectonic environment in Otago at this time.

In the past 10 million years the volcano has undergone extensive erosion. It has been suggested that two major streams were responsible for cutting down into the heart of the volcano. One flowed northeastwards towards what is now the mouth of Otago Harbour, the other southwestwards towards St Clair and St Kilda, with a watershed along the Portobello-Port Chalmers divide.

In the Quaternary period, from 2.58 million years to the present day, climate change has seen multiple periods of high latitude cooling, resulting in glaciation, alternating with shorter periods when temperatures were as high or higher than today. These warm periods, referred to as interglacial periods, are characterised by high sea level stands when Earth’s ice sheets and glaciers melted.

Since the end of the last glacial maximum, approximately 20,000 years ago, sea level has risen approximately 120 m, effectively flooding the two stream valleys cut into the Dunedin volcano.

Otago Harbour, created by sea level rise, is therefore an excellent example of a drowned valley system. Otago Peninsula, originally an island, has been joined to mainland Otago by deposition of sands and swampy sediments related to the present high sealevel stand.

Before you ask

The Dunedin Volcano is EXTINCT!

I feel this gif explains perfectly how we all feel right now, that the volcano is extinct AND that we are at the end of the piece (almost) . That man is everyone right now!

And finally, though not the most captivating video, this one below describes the Maori legend behind the harbour formation, if you are interested.

 

Till next time,

Images and Gif sourced under the creative commons license.

https://en.wikipedia.org/wiki/Shield_volcano

Video credits:

Shield Volcano Formation, Eric Hultgren, Published on Jan 23, 2015

How Otago Harbour was formed, Portobello School, Published on Oct 9, 2013

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