The Banks Peninsula Volcano is an extinct volcanic complex to the east of Christchurch on New Zealand's South Island.Sewell 1986 While the volcano is highly eroded it still forms the majority of Banks Peninsula with a highest point of . It is a composite of two main eruptive centres one originating at Lyttelton Harbour, the other at Akaroa Harbour. The eruptions were predominantly basaltic, with associated andesite and trachytes, with minor rhyolite. The volcanic activity occurred in the Late Miocene and possibly extended into the Early Pliocene. There are four volcanic groups, all of which are within the Māui Supergroup.Mortimer et al. 2014 The Christchurch earthquakes led to rumors of a possible eruption, however, there is no known magma chamber beneath the volcano and there has not been any sign of volcanic activity in the last 5 million years.

Origin and relationship to other volcanoes New Zealand's South Island has many extinct volcanic centres with no obvious tectonic mechanism of formation.Timm 2009Hoernle 2006 et al. Because of this the cause of these intraplate volcanoes is debated. They extend in age from the Cretaceous to the Pliocene and outcrop throughout Otago, Canterbury and on the Chatham Islands.Hoernle 2006 et al. The largest of these is the Banks Peninsula Volcano followed by the Dunedin Volcano which is half its size, but part of a much larger volcanic area, the Dunedin Volcanic Group. Also the Cretaceous Mount Somers volcanics occur throughout Canterbury including Banks Peninsula resulting in adjacent volcanics say near McQueens Valley having very different ages being rhyolites at 98.0 ± 1.2 million years agoHoernle 2006 et al. and basaltic and trachytic lava flows at 11.59 ± 0.08 million years ago.

These volcanic centres can be dormant for tens of millions of years between eruptions. This implies that the mechanism of formation is connected to the lithosphere unlike some other intraplate volcanoes such as the Hawaii island chain, which are rooted in the asthenosphere. One mechanism for the creation of these volcanoes is the flaking off of the base of Zealandia's lower lithosphere into the asthenosphere. Zealandia has a thin lithosphere as it has been extended while rafting away from Australia. If large sections of this already thin lithosphere sank into the asthenosphere, it would be replaced with hotter rock leading to decompression melting. This theoretically could cause volcanic activity that is locked to the moving lithosphere over many millions of years.Hoernle 2006 et al.

Geomorphology and structure The shape of the volcano was controlled by a north-west south-east fault system that controlled magma flow.Ring 2012 Local extension formed the Lyttelton Volcanic Complex. Erosion has formed streams radially around the volcanic cones and more significant valleys have formed where these radial patterns meet.Hampton 2010 The erosion and destruction of one wall of these cones has led to an increase in catchment size and associated erosion. Therefore, forming the significant natural harbours of Banks Peninsula. By this mechanism Lyttelton Harbour had started to form by 8.1 Ma.Hampton 2010 The large mass of resistant volcanic rock that now makes up Banks Peninsula has significantly controlled the shape of the Canterbury Plains.Ballance 2009 p.252

Volcanic subunits Volcanic activity started at Lyttelton Harbour (c.11 Ma)Forsyth 2008 p.29 then extended through the Mount Herbert Volcanic Group to Akaroa Harbour (c.9 Ma).Forsyth 2008 p.29 During the latter phase of the volcano it was simultaneous active from both its major magma chambers, at Akaroa and Lyttelton. The complex geochemically evolved over time and has cyclic eruptive phases. Each progressive cycle has an increase feldspar with time, which created more blocky lava flows due to increased viscosity.Hampton 2010

Lyttelton Volcanic Group The oldest eruptions of the Banks Peninsula occurred between 11–9.7 Ma around the modern town of Lyttelton. These volcanic rocks rest unconformably on Mount Somers Volcanic Group and Rakaia Terrane. The Lyttelton Volcanic Group is primarily basalt and it has been constructed by the formation of five overlapping volcanic cones.Forsyth 2008 p.29Sewell 1986

The Allandale Rhyolite is composed of rhyolite and dacite banded flows with rare tuffs and obsidian (10.8 Ma). it is conformably overlain by Governors Bay Andesites in the Lyttelton Harbour area.Forsyth 2008 p.29

Mount Herbert Volcanic Group The Mount Herbert Volcanic Group (9.7–8 Ma) represents the movement of volcanic activity towards the east.Forsyth 2008 p.29Sewell 1986Hampton 2010

Akaroa Volcanic Group The Akaroa Volcanic Group (9–8Ma) are basaltic to trachytic lava flows centred on Akaroa Harbour.Forsyth 2008 p.29Sewell 1986

Diamond Harbour Volcanic Group

The Diamond Harbour Volcanic Group represents a return of volcanic activity to the western side of the peninsula (7–5.8Ma). The group is mostly made up of basalt, with rare interbedded carbonaceous mud.Forsyth 2008 p.29Sewell 1986

See also List of rock formations of New Zealand List of volcanoes in New Zealand Stratigraphy of New Zealand South Island surface volcanism Volcanology of New Zealand Tombolo (coastline landform)

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