Story: Geology – overview
Page 3 – New Zealand’s oldest rocks
Earth is estimated to have formed about 4,600 million years ago. All of earth’s major continents contain extensive regions of bedrock that formed during the planet’s earliest history, a time known as the Precambrian era. Many of these rocks are thousands of millions of years old. New Zealand, however, has no rocks from this early period.
The beginning of the Cambrian period, about 540 million years ago, is marked by the appearance of the first widespread fossil evidence of life. At this time earth looked very different. Australia, Antarctica, India, Africa and South America were all parts of a single, huge supercontinent called Gondwana, which spanned the south of the southern hemisphere.
New Zealand is a geological newcomer – its bedrock has formed since the beginning of the Cambrian period. Much of that time it was under construction on the fringes of Gondwana. It is only in the last 85 million years that New Zealand broke free of Gondwana and moved into the Pacific Ocean.
The formation of New Zealand’s landmass
The greater part of the landmass of New Zealand – the area above sea level and its extensive continental shelves – is built from recycled material. The rivers of Gondwana carried sediment into the ocean. These sediments built up offshore for millions of years, until movements of the sea floor carried them towards the land. There the sediments were plastered onto the edge of the continent, creating new coastal mountains that enlarged the land area of Gondwana. The main part of the New Zealand landmass, sometimes called the ‘basement’ rocks, was formed on the margins of Gondwana during several of these cycles of deposition and mountain building.
The oldest rocks – Cambrian to Devonian periods
The oldest section of New Zealand’s landmass was formed during the Cambrian through to the Devonian periods, some 540 to 360 million years ago. They originally consisted of sediments deposited on the sea floor offshore from the parts of Gondwana that would later become Antarctica and Australia. Volcanic activity on offshore islands also produced volcanic rocks and sediment.
The oldest sedimentary rocks in New Zealand, found in the Cobb valley, north-west Nelson, were deposited about 510 million years ago, during the Cambrian period. Their age is known from the fossils they contain, including animals called trilobites.
One of the most widespread older rocks, found throughout the western side of the South Island, is a greenish-grey greywacke called the Greenland Group. These greywackes were deposited in early Ordovician times, about 480 million years ago. Greenland Group sediments have been heated and metamorphosed to dark-grey gneiss. Rocks similar to the Greenland Group are found in other parts of former Gondwana, including Antarctica and eastern Australia.
During the late Devonian and Carboniferous periods, the sediments were disrupted by movements of the earth’s plates. Sea floor movement carried them towards the Gondwana margin, where they were squeezed and folded to form land that eventually became part of Australia, Antarctica and New Zealand. Many of the sediments that had been deposited in the ocean were altered by heat and pressure to form metamorphic rocks such as schist and gneiss. The heat was sufficient in some areas to completely melt the rock, which recrystallised to form large masses of granite and diorite. These rocks today can be found along the West Coast of the South Island from Fiordland to Nelson. The crystalline rocks are resistant to erosion, and can form steep-walled valleys such as those in Fiordland.