Emeritus Professor Alan Cooper, of the University of Otago geology department, explains the origins of Otago Harbour.
The basement rock underlying Dunedin, known as Otago Schist, was formed approximately 150 million years ago by a process of metamorphism (burial, deformation and recrystallisation) of more ancient sediments during a period of mountain building referred to as the Rangitata orogeny.
Subsequently, during the Cretaceous, approximately 110 million to 90 million years ago, the schist was uplifted, faulted, eroded and overlain by a series of terrestrial sediments.
• Our harbour - the heart of a city
• Harbouring all kinds of life
At this time the New Zealand crust was being stretched, prior to, and during our continent's separation from Australia and the Antarctic. As Zealandia's crust cooled it sank, and the sea advanced across the land surface, depositing a series of sandy, muddy and limey sediments.
The transgression reached a peak approximately 25 million years ago, in the late Oligocene period, when the first magmas relating to the Dunedin volcano were intruded through the crust. Some of them gained access along earlier Cretaceous faults, others appearing randomly as isolated volcanic vents scattered over East Otago.
Most of the volcanic events were short-lived, with vents active only long enough to erupt a single batch of magma. In contrast, approximately 16 million years ago, during the early Miocene, a more voluminous supply of magma migrated along structural weaknesses in the crust along the northern extension of the down-faulted depression of the Taieri Plains.
Eruption began near what is now Sandfly Bay on Otago Peninsula. Initially submarine, the volcanic cone soon grew above sea level. Continued subaerial volcanism over the next 6million 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 final magmatic events in Dunedin now form the prominent peaks and rounded hills on the north side of the volcano. These have been dated at approximately 10 million years. Collectively, the various lava flows produced during the 6million-year lifespan of the Dunedin volcano formed a circular edifice approximately 25km in diameter that exceeded the height of Mt Cargill (676m above sea level).
The death 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.
Previously, the crust had been extended by stretching, allowing magma to make its way relatively easily to the surface. However, in the past 10 million years plate tectonic forces have changed and the crust is now being compressed, making it difficult for magma to find a way to the surface.
In the past 10 million years the volcano has undergone extensive marine and terrestrial 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. These stream valleys now provide a geological cross-section through the volcano.
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 interglacials, 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 120m, 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 sea-level stand.
