Tuesday, July 4

Maungatea's mysterious basalt boulder

Hmmm. Loooooong time since I added anything to RaroLens.
That's because most of my videos these days are of various travel destinations so I've decided to put them on a new blog:
But today I have a Rarotonga-based video so here it is.
A group of friends recently hiked up to Maungatea Bluff, heading into the hills on the northern side of Rarotonga.
We were going to see a strange basalt boulder that Gerald McCormack, the director of the Cook Islands Natural Heritage Trust, found a while ago.
This boulder had tumbled from the bluff down the side of the hill and come to rest a short way off the side of the track so wasn't particularly easy to see if you didn't know it was there.
There are lots of basalt boulders up there but most of them are breccias, conglomerates made of basalt rubble held together by molten lava pouring over them.
The mystery boulder has a series of parallel grooves with rounded bottoms along its surface and on Rarotonga there's nothing else like it that anyone knows about.
Gerald wanted to re-visit the boulder to take more photographs and we all tagged along.
He originally thought the grooves might be man-made but after a lot of online research he found similar rocks in New Zealand, in particular at the Wairere Boulder Nature Park in Northland.
Geologists say these were formed when rain dripped from trees onto humus on the ground. The humus acidified the water which then formed streamlets that ran down the face of the rocks. Over millions of years the water dissolved the rock to form grooves (often called flutes).
There may be other fluted boulders somewhere on the hillside or possibly even signs of flutes up on the bluff but the ground isn't easy to walk over. We were on a track and it was quite a scramble in a lot of places. It's not surprising, then, that nobody has reported seeing anything similar on Rarotonga.
It was lovely being up in the bush but not particularly peaceful. The squawking sound you can hear on the video is the sound of birds, I think they are petrels, flying back from feeding at sea which they apparently do around midday.
Flying is clearly the best way to get to and from the bluff.
We had to walk, make that scramble, back down.