Te Henga Pillow lavas
The pillow lavas at Te Henga, part 1. Landside.
It began with a submarine landslide about 15 million years ago when a huge lahar cascaded down the slopes of an ancient volcano. The volcanic slurry of pebbles and boulders came to rest close to the lifeguard station at the north end of the beach.
The local geography has changed quite a bit. The Hikurangi subduction zone originally lay off the west coast of Aotearoa. It slipped relentlessly through the Miocene to its new position on google maps off the east coast.
Te Henga Beach, the story part 2. The Pillow Lavas
Walking south, the remains of the ancient marine landslide are on your left. Rounded boulders still break loose and travel down the slope under gravity. I gave one a kick, moving the geological time clock forward slightly.
The wedge of submarine debris sloping gently to the right is the foundation for the second part of the story.
Soon after the landslide came to a halt, a large volume of molten andesitic lava spewed into the ocean from a reservoir deep within the mantle.
Squeezed like toothpaste from multiple tubes, a pile of pillow lava lobes covered the avalanche debris 20 metres deep.
Andersitic lava at 1000 degrees Celsius and cold sea water form an explosive mixture on contact. Large volumes of glass fragments were formed around the flowing lava, producing a distinctive orange yellow colour in the cliffs at the far end of the beach.
Not all cooling in contact with cold sea water is explosive. Light coloured, chilled envelopes highlight individual pillows within the massive dump.
When the outer layers cooled, the surface area contracted, and radial cooling cracks formed.
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Cross section through a pillow lava……
………..and perhaps a pillow lobe tumbling and running over other pillows. Squirted squeezed and squashed.
What happened here?
Magma squeezed under immense pressure found a line of weekness in the cold pillow lavas.
Cooling joints run at right angles to the cold pillow lava surface.
Te Henga Beach the story part 3. Raised from the deep.
Sediments, hundreds of metres thick, buried the pillow lavas and other rocks in the region. Uplift and erosion have since removed this cover leaving the pillow lavas beside the lifeguard station at Te Henga.