The greater Te Pātaka o Rākaihautū Banks Peninsula is comprised of the eroded remnants of two large volcanic complexes (Lyttelton & Akaroa) made of the igneous rock, basalt. The Lyttelton and Akaroa Volcanoes are termed strato-shield volcanoes, which means their flank deposits alternate between layers of large lava flows with more explosive features, ash layers and changing mineral compositions.
The Banks Peninsula volcanoes were active during the Mid- to Late-Miocene (12.4 to 5.8 million years ago) with multiple eruption vents. These eruptions formed an island about 10 km off the coast that later became connected to the mainland due to uplift along the Southern Alps increasing erosion rates, resulting in deposition of the gravels that became the Canterbury Plains. These volcanoes are characterised by a number of local volcanic centres, each with differing eruptive styles, chemistry and volumes. Both the volcanoes have sustained significant erosion since formation dramatically reshaping the volcanic complex into what we see today, with the prominent harbour features of Whakaraupō and Whakaroa.
The older of the two volcanoes is Lyttelton at around 12 to 9.7 million years old. At an age of 9.4 to 5.8 million years and with an original diameter of about 50km, Akaroa is the younger and larger volcano of the two. It is thought that the Akaroa Volcano evolved in a two-stage process with a lower level magma chamber replenishing and mixing vigorously with an upper level magma body, resulting in the variety of rocks types we observe in the landscape.
