Sunken Zealandia splits up
About 25 million years ago, a shift in plate movements began to wrench apart the largely submerged New Zealand continent, Zealandia. In the north, sections of ocean floor of the Pacific Plate began to sink beneath continental rocks of the Australian Plate. Within the continent, pressure caused major cracks to develop. These cracks would eventually join to become New Zealand’s great Alpine Fault, splitting the continental mass in two. New Zealand now lay across two separate plates. These plates began to rotate. A sideswiping collision began, with the plates sliding past and running into each other. New land began to rise above the sea along the plate margins as colliding sections began to crumple. Volcanic activity and uplift increased, and substantial mountain building began about 5 million years ago.
While many areas were being uplifted, parts of the New Zealand landmass were warped downward, creating large basin areas. As more land was pushed above the sea it began to erode and shed more sediment into the surrounding ocean. Layers of soft, grey mudstones and fine sandstones were deposited, with particularly thick accumulations along the east coast of both islands and in large subsiding areas such as the Taranaki and Whanganui basins along the North Island’s west coast.
Rocks of the Taranaki basin contain oil and natural gas derived from the organic material in the region’s older coal beds. The lighter gas and oil seeped upward, becoming trapped in the overlying layers of sediments that accumulated later in the Taranaki basin.
As the land rose, the surface layers of younger rocks such as limestones, sandstones and coal were fractured and folded. In the rising ranges of the Southern Alps, however, most of the younger rocks were eroded away, exposing the underlying Torlesse rocks.
The soft sediments deposited during this period are locally known as papa or papa rock. Papa rocks have been uplifted and now make up many of the hill areas of the North Island. These soft rocks are prone to landsliding and are easily eroded during downpours, especially where the native forest has been cleared from steep hillsides to create pasture.
Building big volcanoes
During this period, volcanoes erupted in areas now far removed from current volcanic activity. Huge basaltic volcanoes formed the Banks and Otago peninsulas. Dunedin sits on the eroded remains of a volcano that first exploded to life about 13 million years ago. Volcanic activity continued there intermittently until 10 million years ago. Banks Peninsula is the eroded remnants of two large volcanoes. Lyttelton volcano began to erupt around 12 million years ago. It was later eroded, then partially buried by lava flows from the larger Akaroa volcano, which started building around 9 million years ago. Volcanic activity at Banks Peninsula finally died out around 6 million years ago.
The Coromandel Peninsula has seen numerous periods of volcanic activity, beginning around 18 million years ago and continuing to about 2.5 million years ago.