Pillow Lava (North Otago) EarthCache
Pillow Lava (North Otago)
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This feature is located at Boatman’s Harbour on the Graves
Walkway from the Oamaru Harbour.
At the foot of the cliffs at an inlet called Boatman’s
Harbour on Graves track, are some very good examples of unusual volcanic structures known as "pillow lavas". The lava is in the form of spheroid and pillow form masses from 0.2 to 4 m in diameter. These hard black “pillows” are formed from basalt lava which flowed as a red-hot molten stream over the sea bed from a volcano situated near Oamaru some 40 to 50 million years ago. The volcano and its surrounds was then up to 60 metres below the sea surface along with much of New Zealand’s coastline. Under the sea the viscous lava flowed slowly over the sea floor.
Some 30 metres in thickness, it flowed as hundreds of elongated interconnecting lobes or fingers. Seen now in cross section in the cliff, the tangled mass of lava lobes resemble separate sac like pillows or varying diameters. As the hot lava met the water the lava surface cooled very quickly to form a solid glassy “shell”. Inside the molten lava continued to flow as it slowly cooled before solidifying. As the mass cooled it contracted and this resulted in radial fractures in the lava lobes. These are obvious where a cross-section of lava lobes is visible as here at Boatman’s Harbour. In the succeeding millions of years sediment or light coloured calcareous fossil bearing limestone was deposited over the pillows and with uplift the Oamaru region was raised above sea-level. Subsequent erosion has formed the cliffs which dominate the walkway and expose the pillow lava.
The walkway sidles around a headland from the Blue Eye Penguin colony to Boatman's Harbour where spotted shags nest on the cliffs beneath the path. Across the tiny harbour pillow lava is visible at the bottom of the cliff. A path leads down to the beach The area is best visited visible at low tide although at other times the formations are still viewable. The main access to this track is through the penguin viewing area. Whilst there is sign stating the track is closed you can walk it at OK. It's open from 8.30am to about an hour before sunset. There is another access to the track from the top at lookout point, but will this turn a 15min cache into about 1 hour.
To Claim this Earth cache, please complete the following requirements.
(1) Take a picture of the Pillow lava with your GPS visible in the picture.
(2) Take a picture of one other spectacular rock formation in the harbour.
(3) Email the cache owner the name of the volcano or volcanic group which formed these pillow lava formations.
(4) Once the owner has confirmed (3) above you can post both photos with your log.
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Treasures
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