The first volcanoes erupted on Bank Peninsular around 15 million years ago, geologists tell us. Then, for the next 9 million years a series of volcanos burst into life, pushing up millions of tons of material and gas.
But eventually something under the ground changed and about 7 million years ago the last volcanoes exploded and surged up in a final period of activity that seems to have lasted just over 1 million years.
These last volcanoes pushed out the olivine basalts (known today as the Kaioruru and Stoddart Formations) from fissures around the old Lyttleton Volcano. It is possible to see how lavas flowed down deep valleys cut into older rocks and, above what is now Halswell Quarry, where a prominent lava cone formed.
From another vent, this one near the top of Mt Herbert, lava flows ran down into the Lyttleton crater and across to Quail Island. And today we have counted more than 60 flows lying under the easy slope between Mt Herbert and Diamond Harbour. the last of these flows is 5.8 million years old. It was the last time the peninsular felt the power and heat of a volcanic event.
During these millions of years of volcanic activity Banks Peninsula was an island, but there is no sign of any submarine volcanoes. That is interesting. Also, because there are no so-called 'raised' beaches on the peninsula, it is likely the island is sinking rather than being lifted up.
So, after 9 million years of more-or-less continuous activity, the volcanoes on Banks Peninsula died. Why? What was it that stopped the melting of the earth deep down (around 80 kilometers) below the ground? Even more to the point, why did the melting start down there in the first place?
Simple questions, perhaps, but ones the geologists have not been able to answer for us. The volcanoes of Akaroa and Banks Peninsular are a mystery.