The few active volcanoes of modern New Zealand represent the dying phase of an earlier period when volcanic activity was more widespread and vastly greater in scope than at present. As early Middle Tertiary times there were outbreaks of explosive violence, for in several places, notably in the Oamaru district and in the Trelissick basin Canterbury, great piles of volcanic ash lie between layers of limestone of that period. Lava-flows including the remarkable pillow-lavas of Oamaru are also found.

The great period of volcanic activity was, how­ever, the Upper Tertiary. At this time great forces were setting in motion the Kaikoura movements of mountain-building. In all probability the same forces, working beneath the earth’s crust, forced to the surface great quantities of molten material, or ‘magma.’ The same thing occurred throughout the world, and New Zealand is but a part of the so-called ‘ girdle of fire’ that encircles the Pacific Ocean.

In late Tertiary time the Dunedin district was the site of vigorous volcanic activity. Vast out­pourings of lava flowed from several sources, while huge masses of material were ejected and fell as ash showers. Flagstaff, Mount Cargill, Signal Hill, and many other parts of the district from Saddle Hill to Karitane, are mainly or entirely of volcanic origin.

Farther north Banks Peninsula is formed of two volcanoes, the centres of eruption being at Quail Island in Lyttelton Harbour and at Onawe Peninsula in Akaroa Harbour. Both are surrounded by well preserved crater rims, and in each case part of the rim has been broken down, thus allow­ing the sea to flow in over the old crater floor.