The main Akaroa volcano seems to have first erupted about 9 million years ago, and over the next 1 million years, a big, classic volcanic cone reared and pushed its way up into the blue sky. It must have been an impressive sight.
Looking at the rocks that are left behind, we see they are very similar to the other major volcano on Banks Peninsular. They range from basalt to trachyte. Geologists point out a larger number of strombolian cinder cones, and especially at Onawe Peninsula and Scenery Nook where they can be seen in spectacular 200 m beach cliffs. But in more easily accessible part of Akaroa harbour, you can see the oldest rocks from the original volcano; the trachyte lavas and pyroclastics.
The volcanic outflows from the Akaroa volcano buried parts of the south eastern slopes of the Lyttelton Volcano and mixed with lava flows from Mt Herbert. This shows geologists that the two centres were both erupting at the same time for a period.
The Akaroa Volcano formed what is called a radial dike swarm. (The Lyttleton Volcanoe did the same thing.) You see the centre of the swarm to the south east of the Onawe Peninsula. And there is a good example at Panama Rock, where the feeder dike can clearly be seen running up to the base of the trachyte dome.
Actually geologists say the Onawe Peninsula remains with us because of a huge section of erosion-resistant syenite and gabbro at its southern tip. These plutonic rocks crystallised deep in the earth are are quite possibly the top of a magma chamber or a main volcanic pipe.
There came a time when all this volcanic activity in the Akaroa Volcano died away. When this happened, new eruptions of lava began from fissures in the Akaroa crater and on the sides of the nearbyLyttelton Volcano.
These are olivine-rich basalts, known as the Church Formation, and are the earliest lavas of the Diamond Harbour area across from the modern Port of Lyttleton. They were spewed out over a period of about 700,000 years.