Te Henga Beach south end

BY BRUCE HAYWARD (GEOLOGIST)
Accessibility: EASY
The sea cave in the south corner of Te Henga Beach.
A pillow lava pile that was extruded out onto the floor of the ocean and is a mix of lava lobes and glassy breccia (hyaloclastite).
A large sea cave.
The sea cave in the south corner of Te Henga Beach.
The Waitakere Ranges are the eroded submarine eastern slopes of New Zealand's largest stratovolcano. The rocks forming the cliffs at the south end of Te Henga erupted and were deposited on the sloping submarine sea floor. They consist of a 20 m thick pillow lava pile which was formed by molten magma erupting into the cold sea water. Some of the lava immediately solidified as glass and shattered into fine glass fragments that accumulated in a heap. This glass has been chemically weathered to a yellow-orange-coloured iron mineral. Some lava squeezed out onto the sea floor like molten toothpaste. The surface of these lava fingers immediately formed a cooled glassy envelope around them and when they slowly solidified the solid andesite lava inside them cooled and contracted forming radiating cooling joints.
At south Te Henga the pillow lava lobes and glassy breccia are mixed together often with fragmented glass forming the material between lobes.
The pillow lava pile overlies regularly layered cobble and pebble volcanic conglomerate that was left behind as lag deposits as submarine lahars flowed down the volcano's slopes.

The pillow lava and other rocks were buried by hundreds of metres of further rocks and lava and these have later been eroded off as the Waitakere Ranges were pushed up out of the sea.

In the corner of the cliffs a large sea cave has eroded out along weakened and fractured joint planes at a right angle to the waves.
View south down Te Henga/Bethells Beach to the cliffs at the south end.
Can you see the base of the pillow lava pile overlying layers of volcanic conglomerate?
Can you tell whether the pillow lava pile has been turned upside down or not?
Explore the cave at low tide - can you see the fractures that the sea eroded along to create the cave?
Directions/Advisory

Park in the large carpark adjacent to the surf clubhouse at the end of Bethells Rd.

Watch out for rogue waves that may surge further up the beach than the normal ones on the day. If you clamber onto the rocks they may be a bit slippery, especially where they have green algal cover. On the rocks watch out for big waves surging up onto them. Do not attempt this trip when the tide is high.

Google Directions

Click here for Google driving directions

Accessibility: EASY

Walk 400 m on the sand alongside the Waitakere River estuary to Te Henga/Bethells Beach, turn left (south) and walk a further 1200 m to the rocky cliffs at the south end of the beach. Not accessible at high tide. Best accessed at mid to low tide levels. You can see most of the interesting geology from standing on the sand at the base of the cliffs but if you wish you can clamber on to the high tide platform and walk along that out to the point, but not beyond.

Features
Sedimentary Volcanic Landform
Geological Age
Early Miocene, about 18-19 million years ago
Links
See Hayward, B.W., 2017. Out of the Ocean into the Fire. History in the rocks, fossils and landforms of Auckland, Northland and Coromandel. Geoscience Society of New Zealand Miscellaneous Publication 146, p. 304, site 2.Fig. 6.13.