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As we spend time at Coast beaches, it is possible to take the rocks, cliffs and other natural structures around us for granted.
However, it is worth paying attention, as the Hibiscus Coast features some geological wonders that feature in the latest book by geologist, marine ecologist and author Dr Bruce Hayward.
In his book – Mountains, Volcanoes, Coasts and Caves; origins of Aotearoa New Zealand’s Natural Wonders – Dr Hayward describes Whangaparāoa Head, at the eastern end of the peninsula, as one of the top 100 natural wonders of NZ – a “mind boggling” example of folded, faulted and tilted strata.
“Walking around the base of the cliffs at the end of the peninsula at low tide, you can see a wide variety of folded, faulted and chaotically deformed strata which record landslides and slumps on the seafloor,” the section on Whangaparāoa Head says.
Dr Hayward, who last month was guest speaker at Hibiscus Coast Forest & Bird’s meeting, says taking a closer look at rocks and landforms around us is rewarding, as they provide a record of the Earth’s history.
He says the cliffs of Whangaparāoa Peninsula and Waiwera are go-to places for many international and NZ geologists.
“This is because of the beautiful fresh exposures of rock and the intense deformation of these relatively young deep-water strata,” Dr Hayward says. “These are some of the most folded and faulted, relatively soft, sedimentary strata in NZ and internationally.”
“Most of the disruption occurred when the strata were still only partially hardened and able to be folded plastically in places and broken apart in other places as a result of seafloor slumping and a massive seafloor failure that occurred soon after they were deposited.”
“At Waiwera, there is a thick, dark layer of cobbles, grit and sandstone that is quite different to the other thinner layers of sandstone. It is composed of broken up pieces of volcanic rock and ash and slid downslope from an active volcano into the deep sea, 20 million years ago.”
Dr Hayward says other local rock features of note are the spherical concretions that occur in the mouth of Ōrewa Estuary and were excavated from the Whangaparāoa Road turnoff and placed in the gardens by Silverdale Medical Centre. They are larger versions of the better known Moeraki boulders.
“They grew about 70 million years ago inside sandstone beds when buried beneath the ocean floor. Then, between 25 and 20 million years ago, they were progressively uplifted and slid here from 300km away to the north, as part of the Northland Allochthon,” Dr Hayward says.
Coastal cliff history: The rocks that form the coastal cliffs around the Hibiscus Coast record what happened over the Auckland region during one small period of time about 20 million years ago, when the region lay about 2000m under the sea. Every few hundred years a severe earthquake would trigger large submarine landslides of sand and mud that flowed down submarine canyons from the north and spread a layer of sand (0.1-2m thick) over the deep-sea floor. The cliffs contain many of these sand layers, now hardened to sandstone, separated by softer layers of mudstone. Since they were deposited, these strata have been pushed up out of the sea and partly eroded to form the hills and valleys of much of the Auckland region, with marine erosion allowing us to see their detailed structure in coastal cliffs.
Dr Bruce Hayward’s latest book, Mountains, Volcanoes, Coasts and Caves, is available from bookstores or can be ordered online from many distributors.
