Northland's Displaced Land Explained by Obduction
Northland's displaced land explained
Most of Northland is composed of giant slabs of rock that were pushed up out of the ocean and slid hundreds of kilometres southwest into the region says Auckland geologist Bruce Hayward in his new book Out of the Ocean into the Fire. This process known as obduction, is rare in Earth history. The Northland event, which occurred between 25 and 20 million years ago, is the largest known example of this process to have occurred in the Southern Hemisphere.
Hayward’s new book is the first written for general readership to explain how and when this happened. The rocks involved were erupted and deposited on the ocean floor between 100 and 25 million years ago. The undersea lava flows were displaced the furthest and slid onto Northland last and now form much of the high country blocks in Northland, such as the Mangamuka, Herekino, Warawara, Whirinaki, Mangakahia and Tangihua Ranges. The deep-sea sedimentary rocks underlie much of the rolling country of Northland extending as far south as Silverdale. Some of these displaced mudstones contain swelling clays that are responsible for many of the unstable roads north of Auckland. The deep-sea muddy limestone is the main raw material for New Zealand’s only cement works at Portland, near Whangarei.
The huge slabs of displaced rock were each 500 metres to 2 kilometres thick and up to hundreds of square kilometres in area. “Together the volume of displaced rocks would bury all of Northland to a depth of about 2 km”, says Hayward. After the displaced rocks stopped moving, northern New Zealand was uplifted and became land. This land was wracked by hundreds of violent eruptions over the next 20 million years and their story is also documented in this new book on the 265 million-year history of Auckland, Northland and Coromandel.