FORMATION OF THE AUCKLAND WEST COAST

FORMATION OF THE AUCKLAND WEST COAST

Geological Evolution of the Waitakere Ranges/West Coast Natural Landscape Stage 1 Underwater Waitakere Volcano 24-17 million years ago Stage 2 Tectonic Uplift and Small Eruptions 17-14 million years ago Stage 3 Period of Erosion 14-5 million years ago Stage 4 (a) Tectonic Uplift: Range-building (b) Deposition: Sand Dune Development along Coast 5 million years ago to the present day 1.5 million years to the present day

Stage 1: Underwater Waitakere Volcano (24-17 million years ago) The Waitakere Ranges originated about 24 million years ago when what is now the Auckland region was under water in a deep-sea trough. Geologists call this deep trough the Waitemata Basin. It stretched from Dargaville in the north to Hamilton in the south and was edged by a landmass to the east that stretched from Whangarei to the present Coromandel Peninsula. To the west of the trough a giant andesite volcano was actively growing on the floor of the sea. The centre of this volcano was 20kms west of the present-day Wait4ere Ranges. Periodic eruptions of lava, pumice and ash spewed on to the ocean floor and, eventually, the volcano rose above sea level to form one of a line of active volcanic islands.

The ancient Waitakere Volcano was similar to the andesite volcanoes of Ruapehu, Ngauruhoe and Tongariro, but it grew to around five times the size of all three of these volcanoes combined. It was active for six to seven million years and built to 3000m in height with a 50km diameter. The present Waitakere Ranges are the highly eroded remains of just the eastern slopes of the huge Waitakere Volcano.

Most of the Waitakere Ranges today consists of weakly layered volcanic conglomerates and breccia that swept down the sides of the growing volcano in undersea láhars (also called turbidity flows). These lahars came to rest on the ancient volcano's lower slopes, 2000m below sea level. Conglomerates are pebbles and boulders that have been rounded by the sea and then held together in a cement-like grit; breccia is formed in the same way as conglomerates but with angular rocks. We can see evidence of these rock-types in the cliffs along Auckland's west coast beaches. Occasionally, much larger undersea lahars flowed 30 to 50kms eastwards into the submarine trough before coming to rest on the sea floor. Today, we can see some of these thick, uplifted and exposed beds of volcanic breccia and grit in the cliffs beside Auckland's Parnell Baths and in the cliffs of North, Shore's East Coast Bays.

Stage 2: Tectonic Uplift and Small Eruptions (17 14 million years ago) About 17 million years ago, as its volcanic activity was drawing to a close, the Waitakere Volcano was pushed up out of the sea by deep tectonic forces to form an uplifted block.

Two lines of small volcanic vents began erupting on the eastern side of this uplifted block. Today, the remains of one line of vents are located near Scenic Drive in the Waitakere Ranges. The remains of the second, more active line of vents and volcanic dykes can be seen in the cliffs along Auckland's West Coast beaches. Lion Rock at Piha, is one of these eroded volcanic vents. It is composed of scoria, volcanic bombs and lava flows that collapsed back into the vent. Other volcanic remains are explosion craters filled with lava flows or domes of lava, such as Watchman Island at Karekare and the cliffs of White's and O'Neill's beaches further north. The two lines of vents poured extensive lava flows over the surrounding land. Today, these ancient lava flows form a thin eroded cap along the ridges of the Waitakere Ranges. They can be seen as the weathered red and purple clays in many of the road cuttings along Piha Road.

Stage 3: Erosion Period (14-5 million years ago) Volcanic activity ceased about 14 million years ago. A period of about five million years followed where the Tasman Sea eroded all parts of the ancient volcano that were above sea level. This long period of erosion resulted in an extensive low- lying coastal plain.

Stage 4(a): Tectonic Uplift: Range-building (5 million years ago - present day) Following the period of erosion, tectonic forces pushed up the coastal plain to form a plateau that was gently tilted to the northwest. In the last one or two million years, erosion by streams has dissected the plateau into relatively steep-sided valleys and, formed narrow floodplains where these streams flow out to the coast. Erosion by the sea has created high coastal cliffs, stacks, caves and blowholes on the western edge of the Waitakere Ranges.

Stage 4(b): Deposition: Sand Dune Development along Coast (1.5 million-years ago - present day) Sand dunes have developed along the west coast of.the Waitakere Ranges since the last Ice Age. The sources of sand have been volcanic material discharged by the Waikato River and Taranaki iron sands. Sea currents have transported this sand northwards; waves have moved the sand onto beaches; westerly winds have blown it inland. At Muriwai, the Kaipara South Heads is a major sand barrier that has been built by this process.