Muriwai lava cliff

In Early Miocene time, a huge submarine volcano erupted mainly west of the present coast; the onland Waitakere Ranges are a part of it. The base of the volcano covered about 60 km by 40 km, larger than Ruapehu, Ngauruhoe and Tongariro combined. The pebbly sandstone with minor conglomerate forming cliffs at Maukatia and Fisherman's Rock, Muriwai were deposited deep beneath the sea, on the flanks of the volcano. At Maukatia the sandstones are overlain by andesite lava. Behind the car park in the old quarry face and in the cliffs to the south are pillow lavas erupted on the sea floor, large lava feeder dikes with fan-shaped cooling joints, and a sill with regular, vertical columnar jointing.

This location has world class exposures of pillow lavas. A feeder dike and pillows are well exposed around the edges of the car park but the lavas are best seen from the walking path to the gannet colony and Fisherman's Rock. Large fallen blocks of fresh andesite with regular cooling cracks can be seen on the beach. There is volcaniclastic sandstone (a sandstone made up of volcanic rock fragments) and some conglomerate at Maukatia and Fisherman's Rock. They show parallel bedding but also channelling and scouring. These features indicate high energy submarine flows and currents.