Underlying the volcanics of Banks Peninsula are the basement rocks. On Banks Peninsula these rocks are found exposed at the Teddington and Gebbies Pass. The oldest are part of the Rakaia Terrane and are between 252 – 201 million years old. Torlesse rocks are brought to the surface here due to faulting and the erosion (removal) of volcanic rocks from above.

The Mt Somers Volcanic Group erupted during the breakup of Zealandia from Australia. Rifting – the pulling apart of continents – allowed magma to makes its way to the surface and erupt. Some of these eruptions occurred at Gebbies Pass and McQueens Valley, Banks Peninsula forming the Gebbies Rhyolite (98.5 – 79.0 million years) and McQueens Andesite (94.0 – 88.0 million years)

The main period of volcanism that formed Banks Peninsula erupted from 14 to 5.8 million years ago. Each area of the peninsula represents a different stage and phases of volcanic activity. Primarily basaltic magma was erupted, but more evolved volcanic rocks erupted throughout, like trachyte and rhyolite.

Volcanic activity stages:

Lyttelton Volcano / Volcanic Complex - 11 – 9.7 mya

Mt Herbert Volcanic Group - 9.7 – 8.0 mya

Akaroa Volcano / Volcanic Complex - 9.4 – 8.0 mya

Diamond Harbour Volcanic Group - 8.0 – 5.8 mya

Erosion has been occurring to the landscape ever since it formed. The volcanic complexes of Lyttelton and Akaroa rapidly incised, sculpting the harbours. Whakaraupō Lyttelton Harbour was formed by 8.0 million years ago with younger lavas flowing into the harbour.

Over the last 100,000 years wind blown glacial dust has mantled the eroded volcanic landscape. For long periods, this was Banks Island, and only recently (last 10,000 years) connected to the Mainland by the formation of the Canterbury Plains.