Wandering N.Z. for 500 million years

Press, 7 December 1985, Page 20

Wandering N.Z. for 500 million years

Landa in Collision: Discovering New Zealand’s Past Geography. By Graeme Stevens. Science Information Publishing Centre, D.5.1.R., 1985. 118 pp. Index. $18.95.

(Reviewed by

Howard Keene)

The theory of continental drift and its modern version, plate tectonics, has provided a dynamic history of the Earth. The story is one of the splitting apart of land-masses and .the creation of ocean basins, and the control this had on animal and plant evolution. This book explores. the changing geography and biology of the New Zealand region during the last 570 million years and it follows on from Graeme Stevens fine “New Zealand Adrift” published in 1980.

The book begins with a survey of continental drift theory and the advances of the last 20 years in fields such as deep sea drilling, seismology, and palaeomagnetism, which have led to the general acceptance of a theory which, up to the late 19605, had only a few dedicated devotees. Much of the book consists of an account of the New Zealand region during each geologic period, beginning in the Cambrian (570-500 million years

ago) when New Zealand was actually situated in the Northern Hemispheric degrees-50 degrees N). By the Silurian period (430-400 million years ago) New Zealand had crossed the equator to be 20 degrees-30 degrees S. New Zealand, of course, did not exist at this time, but was part of the supercontinent Gondwanaland (made up of South America, Africa, Australia, New Zealand, Antarctica, and India). It is only in the last three million years or so that there has been any resemblance to the New Zealand of today. This is frequently stressed throughout the book. The palaeogeography of each period is accompanied by a description of life at the time, and a brief account of rocks from that period found in New Zealand today. The relationship between the changing continents and the migration paths which were opened or closed to fauna and flora is treated well. Readers discover the likely arrival times of many of the ancestors of New Zealand’s native fauna and flora. For example the Ratites — that group of flightless birds of which the kiwi, ostrich, emu, rhea, cassowary, and extinct moa are members — were fairly widely distributed across Gondwanaland, and it seems likely that

ancestral forms walked into the New Zealand area before Gondwanaland split up, perhaps in late Jurassic time (160-135 million years ago). The relationship between the positions of the land-masses and world climate during past eras is traced throughout the book. Widespread glaciation and frigid conditions occurred several times before the current cycle of ice ages when an upland land-mass had drifted astride the South Pole. Conversely when there was no land-mass at the South Pole world temperatures were much warmer, and New Zealand at 70 degrees-80 degrees S in the late Triassic (208-192 million years ago) probably • experienced a cool, temperate climate like that of Stewart Island today. This book is written for the general reader and for students, and fulfils its purpose well. A step-by-step historical text such as this could easily become dull, but the book’s editing and length avoids this. The problem of treating conflicting ideas in a general text is partly overcome by the technical notes at the end. The text is accompanied by clear maps and artist’s reconstructions, some of which are overly dramatic. A few minor errors occur, but they do not detract from the flow.