Tuesday, 3 May 2022

Guest post by Chris Twemlow and Karoly Nemeth. The ghosts of old volcanoes in Coromandel.

Karoly Nemeth is Professor of Geology at Massey University in Palmerston North, and Chris Twemlow is Kauri Dieback Ranger for the Department of Conservation, Whitianga District. Recently we spent a weekend exploring some spectacular geological sites on the Coromandel Peninsula, as background research to a recently published paper exploring the geodiversity and geoconservation values of this area. To take a  journey through this geologically and culturally rich area read on.......


Chris Twemlow and Karoly Nemeth get up close to an outcrop of the Tahanga Basalt
on the Kuaotunu Peninsula, Coromandel. Source: Gravis (2020).

Map of the central North Island of New Zealand
showing Kuaotunu on the eastern coast of the Coromandel
Peninsula. Source: Backpack New Zealand (2020).

Geological map of the Kuaotunu Peninsula showing the main geological formations and rock types in the area. Source: Karoly Nemeth (2020).


Our own Giant's Causeway.....

Columnar jointed basalt forms as the lava cools more quickly at the surface than in the interior of the lava mass. This in turn builds up stress and this stress is dissipated through the formation of cracks running branching at 120 degrees. One of the most famous examples is The Giant's Causeway in northeastern Ireland. These formations can also be found in New Zealand, with one of the most spectacular examples forming Motutu Point, between Whangapoua Beach and Wainuiototo Bay (New Chums). This headland shows striking examples of columnar basalt, with the remains of a Māori Pā seen on the grassed slope above the columns. Another example of columnar basalt can also be seen at Mt. Cargill in Dunedin, and are known as "The Organ Pipes" due to their columnar shape.

Above and below: Basalt columns are clearly visible from New Chums Beach,
where the grassy top of Motutu Point is also visible, the site of a Pā site.
Source: Gravis (2020).

Source: Karoly Nemeth  (2020).

Above: Over time basalt columns erode and collapse leaving the foreshore
littered with boulders, while the relentless action of salt water and waves
produces distinctive features. Source: Gravis (2020)

Tahanga Basalt.

Basalt in the Coromandel  generally occurs as the Mercury Basalts, erupted 5 -8 MYA. As well as forming the columns at Motutu Point, on the Kuaotunu Peninsula the Mercury Basalt forms Mt Tahanga, and can be seen outcropping on the beach. Mt. Tahanga is a highly significant maunga, as this was the source of a particularly fine-grained basalt highly prized for tool making and widely traded throughout New Zealand prior to European settlement. The maunga is the site of several pre-European quarries, and examples of the fine grained-basalt can also be found on the beach.

Above: Rubbly outcrops of Tahanga Basalt can be seen on the foreshore below Mt. Tahanga,
with flow features visible in the eroded basalt of the shore platform. Significant
pre-European quarries where this highly prized basalt was worked are situated
further up the slopes of Mt. Tahanga. Source: Gravis (2020).

Above and Below: This detail shows three samples of Mercury Basalts all found within
a one kilometre area. The sample on the left shows a coarse grain with clearly
visible feldspars, while the sample on the right shows the very fine grained and dense
Tahanga Basalt, with no crystals visible to the naked eye. Source: Gravis (2020).


Kuaotunu Peninsula

This peninsula sits on the eastern coast of the  Coromandel Peninsula and north of  Mercury  Bay. It is a geologically diverse and culturally rich area, with a dynamic history of human settlement from the earliest human arrivals in the country to the period of the Coromandel gold rush and beyond. 


One of the highest points on the peninsula is Blackjack's Hill, which looks out towards the Mercury Islands, and to Opito Point at the end of the peninsula. Blackjack's Hill is the site of a long extinct geothermal field with signs of well preserved quartz and geothermal deposits to be found around the tracks. The details below show example of quartz-based rocks and minerals that can be found inland, and also on beaches and in rivers in the area.

Otama Dunes and Wetland.

Otama Beach forms part of the north-facing coast of the Kuaotunu Peninsula, and is significant as one of the most undisturbed beaches and associated sand-dune formations in the Coromandel. The distinctive white sands are high in quartz and silica, while active dune formations with a back-dune wetland annd associated ecology are increasingly rare. Nearby Sarah's Gully is the site of one of the earliest sites of Māori settlement in the area, with archaeological evidence in the area showing the eastern coast of the Coromandel was a thriving area of settlement up until the arrival of the first Europeans. 

Source: Davidson (2018). In search of the North Island Archaic:
Archaeological excavations at Sarah’s Gully, Coromandel Peninsula,
New Zealand, from 1956 to 1960. Tuhinga 29: 90–164


In more recent times, this area has become the focus of a significant restoration project by the Otama Reserves Group.

Above and below: Distinctive dunes are formed by pale coloured
quartz-rich sands, formed by the weathering of silica-rich volcanic
materials. Source: Gravis (2020).


Above: This area is well known for its sparkling white sand beaches. This detail
shows the high content of quartz fragments, formed by physical weathering
of silicic volcanic materials. Other volcanoc products less resistant
to erosion have been broken down by chemical weathering. Source: Gravis (2020).

Above and below: The dunes harbour a diversity of
rare and unusual vegetation adapted to life in this
sandy and harsh coastal environment.
Source: Chris Twemlow -  Otama.org.nz (2020)

Trees surrounding the estuary behind
the dunes provide a haven for nesting shags
Source: Gravis (2020).

New Chums Beach. 

Wainuiototo, more commonly known by the name New Chum Beach, is justifiably well known for its unspoilt sands and specatcular coastal outlook. Less well-known are spectacular outcrops of volcanic breccia originating from one of the main eruption centres in this area.  Brightly coloured scoria and andesitic pebbles and boulders are preserved in a matrix of ash and tephra, with weathering leaving fresh surfaces well exposed.


Above: Coastal erosion processes continuously expose
fresh surfaces, allowing a view into the volcanic
features formed during unfolding eruptive processes.
Source: Gravis (2020).

Above and below: A range of bright colours
and rock types suggest a dynamic and rapidly
evolving volcanic environment shaped by magma
chemistry, environmental conditions, and physical
parameters. Source: Gravis (2020).

Above: Erosion exposes the inner workings of a
volcano, with a dyke and layers of ash and volcanic
debris visible here. Source:Karoly Nemeth (2020).


Coromandel Granite.

This distinctive igneous rock is well known as "Coromandel Granite", though strictly speaking it is more correctly defined as a tonolite due to its mineral composition. Granitic type igneous rocks form when magma does not erupt to the surface as lava, but slowly cools underground. This slow cooling, which may take thousands to millions of years, allow the large crystals to form which are so clearly visible in these types of rocks. Over time uplift and surface erosion expose the granite body (or pluton) where we find it at the present-day surface.

The Coromandel Granite is significant because it is a very rare outcrop of plutonic rock in the North Island, with most granites found at the surface in New Zealand in the South Island. For a time it was quarried and used as a building stone, including in the Old Parliament buildings in Wellington. 

This fascinating area tells a story of volcanic processes taking place deep underground millions of years ago.It is also significant as a site of a considerable stone extraction industry, utilising a locally unique stone resource with relative ease of access and proximity to local shipping routes. However, over time the extraction and supply became uneconomic, especially in the context of an increasingly globalised stone supply industry. 

Above: The stone quarry is an hour north of Coromandel
Township, a relatively remote location on the coast via road.
At it's height of production coastal shipping provided
convenient transport between the peninsula and Auckland,
making this a well-placed location for the times as seen in
this aerial phot of the coast from 1945. Source:Retrolens.nz (2020).

Above: This zoomed detail from the image above clearly
shows the stone wharf, and nearby houses for quarry
workers. To the left can be seen a distinctive tidal
shore platform formed by the hard erosion resistant
igneous rock. Source: retrolens.nz (2020).


Above: The structure of the wharf formed by blocks
of this very hard igneous rock remain clearly
visible here. Source: Gravis (2020).

Above and below: The large crystals forming
this granitic rock are clearly visible to the naked eye.
Also cleary visible on blocks and boulders in the area
are drill holes where drills were used to break up the rock
into blocks for shipping. Source: Gravis (2020).

Above and below: Near the site of the old quarry
on the beachfront can be found a type of rock called
hornfels. This forms when surrounding rock is "baked"
and undergoes structural and chemical changes due to the
heat of the intruding magma body. Source: Gravis (2020).


Above: Parliament buildings in Wellington, showing steps and
the bases of columns made of Coromandel Granite and facings and
colums made of Tākaka Marble. Source: Te Ara - The Encycopedia of
New Zealand (2020) Photograph by Alastair McLean.




Above: Inspiration for focusing on this area of the
North Island principally came from the book "Vanishing
Volcanoes" by Philip Moore and Homer Loyd, unfortunately
now out of print. However, our ongoing research aims to
build on this valuable work both in this region and other
areas of the country.

Above: Follow this link to a full copy of The Ghosts of Old
Volcanoes, A geoheritage trail concept for Eastern Coromandel
Peninsula, New Zealand.

"Out of the Ocean, Into the Fire" by Bruce Hayward provides
a good overview of the geological history of the Coromandel
in the context of the upper North Island.


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