Reading the Rocks

Forest and Bird, Issue 293, 1 August 1999, Page 42

Reading the Rocks

by

and

Ann

Basil Graeme.

eyond the ken of most of us, and beyond our imagination, great forces within the Earth shape our land. The surface of the Earth lies on crustal plates, which bump and slide and buckle against each other, riding over or descending beneath the adjacent mass. The gradual effects of mountain building are so slow that we do not notice them. The violent episodes of earthquake and eruption are long remembered but, when the earth is still, we overlook the marks they leave. Yet all around us is a landscape bearing witness to the subterranean power. Our country lies on the boundary where the Pacific Plate to the east meets the Indian-Australian Plate to the west. The plate boundary skirts the North Island, curves west to cross the South Island about Kaikoura, and then turns south again down the west coast of the South Island. East of the North Island the Pacific Plate is being pushed deep under the IndianAustralian Plate, while south of the South Island, the situation is reversed: the Indian-Australian Plate graunches into and under the Pacific Plate. The plates carry sediment, washed over aeons to the sea floor from the erosion of previous land masses. As one plate goes down, its sediment is shorn off and rucked up by the over-riding plate. It doesn’t happen quickly — a mere centimetre or two a year, yet this merry-go-round of erosion and rebuilding has created range after range of mountains, from the Raukumara of East Cape, to the Kaweka, Kaimanawa, Tararua and Ruahine ranges, and the Southern Alps. From high above, it looks as if a wave of wrinkles is travelling across the land. The foothills on the margins of these uplifted mountains are often made of the most recent sediment raised from the sea. They are soft rocks — mudstone and sandstone — and easily eroded, as seen in the hill country of Taranaki, Taihape and Gisborne. From Taranaki north, the forces of volcanism also mould the land. These forces overlay the processes of erosion and mountain building. Imagine that the Pacific Plate is diving steeply beneath the Indian-Australian Plate, like a conveyor belt going down. Sometimes the belt

jams, then moves again, and an earthquake rocks the land above. Then the conveyor drives on, carrying deep into the Earth its harder crustal plate of ocean floor, and sometimes sediment too. At a depth of about 85 kilometres, the crustal rock becomes molten. Directly above is the zone of volcanic activity, where molten rock rising through cracks and weak places bursts out as volcanoes. At present, the zone of volcanic activity is about 250 kilometres west of, and parallel to, the boundary line where the Pacific Plate is pushing under the IndianAustralian Plate. This volcanic zone runs from the mountains of Tongariro

National Park northward to the thermal area of Waimangu and the craters of Tarawera, to White Island, off the coast, and beyond to the string of undersea volcanoes named "Rumble 1’ to "Rumble 5". There are older extinct volcanoes to the west of this, such as the Coromandel Ranges. A theory that explains this pattern is that the orientation and position of the plate boundary has moved over time. The youngest volcanoes, which arise in the Rotorua and Taupo regions, are made of andesite or rhyolite. Andesite forms from lava derived from a mixture of land sediments and heavier, basalt material from the ocean floor. Rhyolite rock is dif-

ferent; it is a melt mainly of land-based sediments, which have rafted down the conveyor belt deep into the earth and become molten lava. Far back to the west of the active fronts of andesite and rhyolite volcanoes are the basalt volcanoes. Basalt comes from melted and recycled sea floor. Some of the basalt rock of Northland was erupted at the same time that the Coromandel volcanoes have been active. The basalt volcanoes of Auckland were recently active, though they are ‘resting’ at the moment.

Rangitoto erupted just 600 years ago. It isn’t always easy to recognise a volcano. The shape depends on the type of rock. The classic cones of Taranaki and Ngauruhoe and the jagged peaks of Ruapehu are typical of andesitic volcanoes. Rhyolite domes such as Ngongotaha and Mount Maunganui occur where thick lava has bulged up and flowed out like treacle, or sometimes never flowed at all. Other rhyolitic volcanoes are so explosive that they blow themselves out as vast clouds of hot pumice and then collapse, leaving a sunken basin or caldera. One such ‘master blaster’ created the deep caldera of Lake Taupo. Another resulted in Lake Rotorua. These eruptions can be so hot that the loose pumice and ash are welded together when they come to rest on the ground to form flat, ignimbrite plateaux such as the Mamaku. In other forms of volcano, runny basalt lava builds up low profiles such as Rangitoto Island, while gassy basalt explodes as in the red scoria of Auckland’s Mount Eden.

Not all volcanoes fit these shapes; nor do they retain their shapes after millions of years of wind and rain have worn them down and dissected them into hills and gullies. Between 12 million and six million years ago, low cones of basalt rose over the sites of the Otago and Banks peninsulas. Today the gullied crater walls form the hills of the peninsulas, with the sea making harbours of the eroded craters. From the eroding mountains of our island chains, the rivers have carried sediment and the winds have blown dust to build the plains of Canterbury, Southland, Hawkes Bay and Hauraki. We now farm these rich spoils, in the brief flicker of geological time which we humans occupy, before they return to the sea to begin the cycle of mountain building again.

BASIL and ANN GRAEME live in Tauranga. Basil is a former Forest and Bird conservation officer, while Ann is national organiser of Forest and Bird’s Kiwi Conservation Club.

Interpreting our mountains is further complicated by the plates sliding sideways against each other along the Alpine Fault through the South Island. Once, Dun Mountain near Nelson and the Red Mountains of Fiordland were side by side. Their identical and dramatic red rock composition betrays their early relationship.