Basalt at Ti Point?
17 December 2004 - 8:02am
I saw in the write-up about Ti Point that it was "basalt columns jutting directly out of the pacific ocean." I'm not an expert but I thought it was a kind of calcareous sandstone out there. Can somebody verify? Thanks.
17 December 2004 - 9:39am
#2
definitely basalt. actually it's about the same age as the stuff that makes the boulders on Waiheke - about 10 million.
17 December 2004 - 11:58am
#6
http://data.gns.cri.nz/geoatlas/text.jsp?Page=17
The sandy feeling you talk about is probably the crusted salt from the ocean sitting on the rock. Also bearing in mind that it is very exposed there, so is highly weathered by the ocean, wind etc which creates both the unique shapes of the rock there and also the lighter colour (basalt is normally quite dark if I remember correctly)
17 December 2004 - 1:58pm
#8
dude it's DEFINITELY basalt. no 2 ways about it. (trust me i'm a geologist) Those cracks are columnar joints, just like at the quarry. The reason it looks different to the quarry is that it's 10 million years old (the quarry is 10 thousand) and has been weathered therefore it's a lighter colour and looks crappier.
All those boulders that you walk over from mathesons bay are basalt eroded from the lava flow that forms Ti Point. Mikal has the link i was going to point you to.
17 December 2004 - 4:09pm
#9
Tastes like basalt.
Seriously it completely is. Craig has already stamped his authority but thought I better say something in my defense seeing as it was my article/information that you were disputing.
18 December 2004 - 8:05am
#11
Mainly different physical conditions. The whakamaru ignimbrite eruption (which forms Waipapa and also the Bay) was huge (1000 cubic km), or about 10 times the size of the last taupo eruption. The Waipapa area was close to the caldera center, somewhere north of the present day lake so the pyroclastic flow was a lot hotter there (compared to the bay) and hence the ignimbrite is better welded together. The closeness to the source also meant thicker deposts at Waipapa which combines with the hotter temperatures to produce a very dense welded ignimbrite (from high temperatures and compaction under it's own weight). The Bay being further away from the blast has more variablitiy in the welding (as the ignimbrite cooled as it spread out), it was also less compacted (because of a smaller volume of material further away from the source) hence gas pockets could be preserved and not totally compacted like at Waipapa. Thus pockets at the Bay and not at Waipapa. This all happened 340,000 years ago.
here endth the lesson.
27 July 2010 - 9:47pm
#13
Ti Point is olivine vesicular basalt, the climbing rock is the remainder of a volcano that was seaward and has been eroded, and what is left is the westward lava dykes that extruded lava out the flanks. Argon dated to 8.3 million years old.
Ti Point stream is waitemata grit and yes its carboniferous shale that I was looking at(if one breaks open the softer boulders) but even the geologists cant work out what the probable plant was that makes up the black flat layers vic 3 million years old.
Data from nz geologic journal 1958 and 1998.
SO THE CLIMBING ROCK AT TI POINT IS BASALT.
Well - unless my old geography teacher was wrong, its basalt.
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