Extinct volcanic neighbours

Press, 24 April 1986, Page 14

Extinct volcanic neighbours

“Because it is there,” Christchurch people tend to take Banks Peninsula for granted. It is a topographical feature that relieves the monotony of the Canterbury Plains, and which provides two magnificent natural harbours. City dwellers and visiting tourists alike appreciate the variety of scenery and walks to be found in the Peninsula, while sailing enthusiasts revel in the easily accessible harbours and bays, with their guaranteed easterly wind. But quite often their knowledge of the past is no greater than that of the first people to visit there. The early pioneers, laboriously trudging over the Bridle Path from Lyttelton, may have had other things on their mind than to consider they were climbing out of a decayed volcano; or when standing at the top of the Port Hills gazing down at the wide expanse of the Canterbury Plains could not have known that they were seeing coalesced gravel fans that had spread from the distant mountains to bury that volcano’s flanks, converting it from a volcanic island to part of the mainland. Even today, many who take advantage of the pleasures that Banks Peninsula has to offer are unaware of its fascinating and violent geological origin. Though incorrect, Captain Cook’s mapping of Banks Peninsula in 1770 as an island off the coast of southern New Zealand was only 20,000 years out of date. Banks Peninsula owes its

origins to the development of two large, consecutive volcanoes between 15 and eight million years ago, known as the Lyttelton and Akaroa cones. Some earlier volcanic activity occurred in the McQueens Valley area as much as 90 million years ago, and may have been related to large vents near Mount Somers, but these early remnants are insignificant compared to the volumes of basaltic lava and ash that were erupted later. Eruption from the Lyttelton centre began in Miocene times, about 15 million years ago. Pale coloured rhyolite lavas were poured out on to older sediments in the Governors Bay, Head of the Bay, Quail Island, and Gebbies Pass region of Lyttelton Harbour. But the main building phase of the Lyttelton Cone began about 12 million years ago, around a centre near Charteris Bay, and a volcanic island composed of outward dipping lava flows and ash beds grew to an estimated height of 1500 metres. Drainage gullies developed on the slopes in the same way that they have developed more recently on the sides of Mt Egmont, and as activity dwindled after several million years, a crater was deeply eroded and a crater lake formed, in which sediments and plant debris accumulated. Lesser eruptions occurred later from subsidiary vents on the side of the Lyttelton Cone near Port Levy and Mt Herbert. These produced the dramatic flows that cap the Mt Bradley and Mt Herbert Table-

land, some of which flowed down into the eroded Lyttelton Cone. At the same time, about nine million years ago, a separate volcanic cone began to grow around a vent near the head of Akaroa Harbour. This developed along comparable lines to the Lyttelton Cone, which it eventually joined. As the Akaroa volcanic activity died down, several new centres broke out on the slopes of the Lyttelton Cone, with a series of lava flows pouring down into the old erosion crater to form the slope above Diamond Harbour, and along outward facing stream valleys to form the flows at Halswell Quarry and Ahuriri. The youngest of these lavas is 5.8 million years old, and there has been no volcanic activity

since then. Both the Lyttelton and Akaroa cones were eventually breached by the sea to form the two well known harbours. This is a very much simplified summary of volcanic activity in Banks Peninsula, and a more detailed account can be obtained from an appealing booklet being sold at Canterbury Museum. The booklet, called "Extinct Volcanoes,” is the latest in a series of guidebooks published by the Geological Society of New Zealand. It has been written by Dr Stephen Weaver, a senior lecturer at the University of Canterbury, and two senior research students, Mr Rod Sewell and Mr Chris Dorsey. At a cost of $6, this booklet is excellent value and a must for all local amateur geologists.

By

MARGARET BRADSHAW