Volcanic rock find central otago

Hi Guys i thought this may be of interest. Me and dad were out detecting (goldbug) before christmas in Cromwell area and got a signal, dug it up and it turned out it was this rock, it was buried in a bank, puzzeld by it dad wrote/sent pics to
Dave Craw
Geology Department
University of Otago
Dunedin, NZ
Here is his reply.

I’m pretty sure that rock is a volcanic basanite, like I mentioned in the paper article. That is the right density. The most distinctive feature, apart from being black and fine-grained inside, is the white weathering coating. That comes from weathering of the mineral nepheline, which is abundant in the rock. Also, the magnetic signal will be from magnetite, black iron oxide, which is also abundant in the basanites. An iron meteorite would weather to brown iron oxide.
The magnetic signal from these volcanic rocks can be seen even from the air when an aeromagnetic survey is done, as has been done over much of Otago. That has shown up some previously-unknown volcano sites in Maniototo.
Unfortunately, there is a lot of magnetite in the green schists in the Wanaka area; I’m sure you have seen those too. And that is the magnetite that makes the black sand in some of the Clutha and Kawarau gravels. There are lots of magnetic signals in Wanaka-Hawea area, but the aerial surveys cannot distinguish between schist magnetite and the volcanic magnetite.
I hope that helps


Dave Craw

And here is a recent article from odt he wrote on it too. In search of Upper Clutha’s 25m-year-old hidden volcano | Otago Daily Times Online News


Does it stick to a magnet & does it feel heavy for its size?

JW :cowboy_hat_face:

Hi,There was a crack in it so we gave it a whack and were surprised to
see how black the interior is. The rock is quite heavy (~1.5kg) for its
size (~500cc displacement), density is ~3g/cc, (which is I understand less
than iron rich meteorites) and a magnet responds to it.:+1:t3:

Gday, there are plenty of extinct volcanoes here in the maniototo. The volcano field moved east, you can see the extinct ones around the Palmerton area. So as the crow flies, not that far for a projectile to fly?

1 Like

You might find this interesting.

JW :cowboy_hat_face:

1 Like

I worked around the crater on taieri ridge near middlemarch some years back. The dark rock around the edge contained tiny pieces of green glass looking stone. Pretty cool place.