Oira Bay from the sea.  From Mawai Bay to Oira Bay the outcrop is largely a very thick unsorted, massive coarse pumice unit at least 100 m thick. Houghton et al, 1992 described this as the Oira Pumice Cone. As can be seen from the images below, the pumice is uniform in size and a highly vesicular. It is regarded to have formed by Strombolian style activity about 40,000 yrs B.P., with the vent free from any external water.

Back to Map

The tranquility of Oiora Bay during southeasterly winds makes a safe anchorage for ocean dwellers. Very frustating though as it takes about an hour to walk the wonderfully private track to this bay from home at Opo Bay, only to be "overcrowded" by these boats. One "nudist couple" did not expect me to arrive and observe and log the outcrop for the rest of the day. But they didn't care. Walking the track back at dusk can be a little scary as it is the time that Captain Cookers (wild pigs) roam. One day when I was late leaving, it was almost dark, and I encountered two piglets on the track. They squealed when they saw me, and mother was all too close. Fastest trip back I ever made.

Back to Map

The northern end of Oira Beach. Here the units below the Pumice Cone can be seen. The most conspicous feature is that the cone pumice lies on top of a very well developed palaeosol on top of a small lava flow.

Back to Map

The nature of the pumice in the Oira Pumice Cone. Tightly packed and massive, with no obvious bedding or sorting.

Back to Map

At the base of the Pumice Cone the pumices are even more tightly packed and they are fused together. Indicating the rapid accummulation of the pumice from the Stombolian style eruptions, and burial so quick that heat contained within the  basal section was sufficient to fuse the clasts together.

Back to Map

At the very base of the Oira Pumice Cone, the nature of the eruption is better understood. Here the pumice is second to lithic (lava) fragments. This indicates that this basal layer of the deposit is a product of the initial vent clearing.

Back to Map

This is the palaeosol at the base of the Oira Pumice Cone. The biggest problem with fossil soils is that its very hard to know how long in time it took for such palaesols to develop. Its resonable to assume that 100's if not 1000's of years soil development is here as the soil profile is deep and the A & B horizons are very clay-rich.

Back to Map

In my thesis I called this feature the remains of a fumerole vent. However, Houghton et al, 1992 describe these are tree moulds, formed by burning trees. I am yet to be convinced of this as the degree of fusing of the pumice to glass at the margin is thick and there is no tree texture evident on the surface of the glass, nor charcoal. Plus it is not a vertical or straight feature. It is inclined and changes inclination. I also consider the heat from the erupted coarse pumice would be insuffient to generate such welding. This feature is on Rua Mata Point at the south end of Oira Bay.

Back to Map