A volcano is an opening (or vent) in the surface of the earth or sea floor where magma comes out. This volcanic debris piles up or spreads out to produce many different kinds of volcanic rocks and landforms. Many of New Zealand’s landscape features have been shaped over several million years by volcanic activity.
Most volcanoes are formed by the movement of tectonic plates on the surface of the earth. These plates are basically huge pieces of rock that ‘float’ on the mantle. The tectonic plates are in very slow, constant motion. Sometimes they move towards each other, other times they move apart, and still other times one will sink while the other rises above it.
Beneath New Zealand, the Pacific tectonic plate is colliding with the Australian tectonic plate and is being forced down into the Earth’s mantle. This creates a zone of stress, called a subduction zone. Most of the North Island’s active volcanoes are associated with this process.
As the Pacific plate is forced down into the mantle, the oceanic crust, which caps the Pacific plate, begins to heat up. Under increasing pressure and temperature, the former crustal material undergoes chemical reactions and molten magma is formed. This magma is very hot and contains gases making it lighter than the surrounding mantle. It rises towards the surface of the Earth at weak points in the Earth’s crust.
Once this magma reaches the surface at the vent it is called lava (if it erupts passively) or tephra (if it is blasted apart explosively in the vent).
Taupo volcanic zone
The volcanic region of the central North Island is called the Taupo Volcanic Zone. It is made up of a line of active and potentially active andesite stratovolcanoes (volcanic cones) stretching from Ruapehu in the south to White Island in the north.
Auckland volcanic field
Unlike the Taupo Volcanic Zone, the volcanoes of Auckland are not related to a plate boundary. Instead, they have formed in the middle of a plate. This sort of activity is known as ‘intraplate’ volcanism. Sometimes these areas of intraplate volcanism are referred to as “hotpots” or “mantle plumes”. The source of magma in these areas is the mantle. The type of magma produced in these areas is called basalt.
The Auckland volcanic field is made up of monogenetic volcanoes, which means they use up all of their magma during the eruptive period, and each new magma batch or 'bubble' produces a new volcano. That is why there are so many individual volcanoes in the Auckland volcanic field.
Frequently active versus dormant volcanoes
In New Zealand, volcanoes can be divided into two groups:
Frequently active volcanoes such as White Island, Tongariro-Ngauruhoe, Ruapehu and Raoul Island in the Kermadecs.
Dormant (sleeping) volcanoes such as the Auckland Volcanic Field, Mayor Island, Rotorua, Okataina, Taupo, Taranaki and the volcanic fields in Northland.
The North Island is home to all of New Zealand’s active volcanoes today. The South Island’s volcanic heritage belongs to a more distant past but is still evident today at sites such as the Takitimu Mountains, Mount Tapuaenuku, Mount Somers, Oamaru, Otago Peninsula, Banks Peninsula, Timaru and Solander Island.
Intra-plate volcanism: Not all volcanism is associated with a plate boundary. The Auckland and Kaikohe volcanic fields are active intra-plate or 'hot-spot' volcanoes, although not all geologists support hot spots as an explanation for these. Past instances of intra-plate volcanism in NZ include the Banks Peninsula volcanoes, basalts near Timaru, basalts on the Chatham Islands and the Otago Peninsula. Intraplate volcanism is dominated by basalt. This type of volcanism will be discussed in more detail later
So what does all that mean? Well what it basically says to me is that the volcanoes that form Banks Peninsula have formed on top of the layer of rock that underlies the Canterbury Plains. The volcanic action spewed up volcanic rock through cracks or fissures in that layer, and formed a pile of volcanic rock now known as Banks Peninsula. This underlying layer of hard rock is part of the crust of the earth that floats above the earth's mantle and molten magma that lies beneath, as depicted in this graphic.
Putting all this together, the scenario is that after the volcanic activity that formed Banks Island, there was tremendous downward pressure exerted on the Torlesse rock layer below. Just simply the force of gravity on this new mass of rock - area 450 square miles, average height (now) around 300 metres. This force over time has forced the layer below, which is floating on molten magma, downward. Like a boat floats deeper when more people jump into it. (This accounts for the observations of geologist Davis - above). The underlying rock layer sank till some sort of steady state was reached over time. Or maybe it got jammed.
Banks Peninsula first emerged as an island thrust out of the sea by volcanic eruptions estimated to have started between 10 and 15 million years ago. The two harbours of Lyttelton and Akaroa, which indent the coastline, are eroded remnants of those eruptions and the highest peak in the district, Mount Herbert, is also an ancient volcanic cone. Many of the rocky outcrops which form some of the most striking landscape features on the Peninsula are a result of the volcanic activity which ceased about 6 million years ago. Since the time of the first eruptions, the erosive action of the sea and streams has shaped the land to form the pattern of valleys, coastal cliffs and indented bays that characterises the Peninsula today. Glacial action during the ice ages of the last two million years has also contributed to the form of the Peninsula. As the glaciers ground away the Southern Alps, fine silt was produced which was carried by the prevailing northwesterly winds and deposited as loess over the existing volcanic landform. At the same time, rock eroded by the glaciers was carried by rivers and deposited at the coast which, in turn, gradually moved eastward forming the Canterbury Plains. Some 20,000 years ago, the plains met the volcanic island and linked it to the South Island. The out-washing of eroded gravels and sands from the erosion of the Alps also enclosed and formed the two lakes of the district, Waihora/Ellesmere and Wairewa/Forsyth. The long and intricate coastline is a legacy of the geological processes of uplift, erosion and deposition which have shaped the Peninsula. Rocky cliffs and headlands punctuated by intimate sheltered, sandy bays and the dramatic harbours of Akaroa and Lyttelton form a spectacular link between the land and the sea and contribute to the scenic value of the District.
The Akaroa Volcanic Group is defined as comprising all the volcanic products of central, flank and parasitic vent eruptions in the south-eastern two-thirds of Banks Peninsula, which collectively form Akaroa Volcano. Field mapping has shown that the lavas and pyroclastics of which Akaroa Volcano is constructed can be grouped into an Early Phase and a Main Phase. Early Phase rocks (?11-9 Ma) are restricted in outcrop to the inner shoreline of Akaroa Harbour. The oldest exposed basaltic lava flows of Akaroa Volcano are assigned to Early Phase I. Early Phase II comprises extensive trachyte tuffs, breccias, agglomerates, flows, sills, and a large dome, with minor basaltic tuffs, and appears to represent a major episode of eruption of trachytic lava marking the end of the construction of a proto-Akaroa Volcano.
Weathered basaltic flows, tuffs, lahars, scoria cones and pyroclastic breccia of Early Phase III unconformably overlie rocks of Early Phase II.