The English explorer, Captain James Cook sailed past the east coast of the South Island, in the famous bark Endeavour, in 1770, he noted a raised area of land that was ". . . seemingly detached from the coast we were upon". Because of time considerations and the wind at the time, Cook was not able to go closer, but he felt he was looking at an island, and named this land Banks Island. This was in honour of the expedition's young benefactor and botanist, Sir Joseph Banks.
From where he sailed, some 10 miles out, the 1200 square kilometre land mass does seem like an island. And in another sense, Captain Cook was not too far wrong, because until quite recently (geologically) it was indeed separate from the mainland. In fact geologists think the link with the mainland was extablished only 20,000 years ago to form a genuine peninsular.
Today, the highest point, Mt Herbert, is almost 1000 m high but in it's early days the highest point was 1500 m.
That was in a time of unprecdented volcanic activity. From about 15 million years ago, for a period of around 9 million years, the peninsular was the scene of repeated volcanic eruptions. the harbours, valleys and hills we see today are the eroded remains of a great field of volcanoes.
The biggest two were the Lyttelton and Akaroa volcanoes. When these two mountains became extinct they eroded, the sea crept in and formed Lyttelton and Akaroa harbours. And we can see the remains of these volcanoes in the steep hills that rise from the water of the harbours and, on the other side, slope gently away.