Boatmans Harbour Pillow Lavas

BY JULIAN THOMSON (GEOLOGIST)
Accessibility: EASY
Chilled margins of pillow lavas. J.Thomson / GNS Science
These exquisite pillow lavas from an undersea eruption are seen in cross section, showing their internal structure in detail.
Boatman's Harbour pillow lavas. J.Thomson / GNS Science
30 to 35 million years ago, the Oamaru area was underwater, and part of a continental shelf environment. Intraplate volcanoes (i.e. volcanoes not located along a plate boundary) in the area erupted in shallow water of less than 100 metres depth. The mix of hot magma and sea water produced very violent eruptions. The volcanic rocks from this time are now seen along the coast from Oamaru to Moeraki, and cover an area of about 60km by 30km. They include lavas and layers of fragmented and chilled tephra (ejected material from explosive eruptions) and interbedded layers of sedimentary rocks (sandstone and limestone containing marine fossils) that accumulated between eruptions.
Pillow lavas are erupted underwater and here at Boatman's Harbour are some examples that are beautifully exposed. The pillows are formed when hot lava spills out into cold sea water and is rapidly chilled to form a lobe shaped blob, with a hard crust on the outside, and molten lava in the middle. The pillow then splits open, allowing the molten material to emerge and form a new pillow next to it, so that eventually a whole mass of interlocking pillows piles up over a wide area. Any sediment caught up within the lava gets baked by the heat.
Boatman's Harbour, Photo J.Thomson / GNS Science
Take a close look at some of these pillows. You can see that they are appropriately named. The rock is basalt but there is variation in the colour and texture within each pillow. The outside edge of each one was rapidly chilled (quenched) by the sea water and is therefore glassy, whereas in the middle they have a coarser crystalline texture. Usually the upper surface is convex and the lower part is concave, fitting snugly over the underlying pillows.
In between the pillows you can see patches of pale coloured limestone. This is baked carbonate rich mud that was part of the sea floor sediment.
Directions/Advisory

Access is via Test Street or the lookout at Selwyn Street and descend via the Wanbrow track.

Note that the path from the blue penguin colony has eroded in recent years and has been closed. A better approach is to access the rocks on Second Beach (where you can also see the pillow lavas at the north end of the beach) via the Cape Wanbrow walking track.

Google Directions

Click here for Google driving directions

Accessibility: EASY

From the car park, follow the Graves track for 10 mins, descending down a steep path to the rocky bay with the pillow lavas.

Features
Volcanic
Geological Age
Waiareka Volcanics, Late Eocene to Early Oligocene, 30 to 35 million years old.
Zealandia Evolution Sequence
Waka Supergroup (Flooding): 35 – 25 million years ago
Links
Check out this video about the volcanic rocks at Oamaru (3m 49s): https://youtu.be/yoS3AIn82Iw The Qmap for this area can be purchased at http://shop.gns.cri.nz/geology-of-the-waitaki-area-scale-1-250-000/