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.