Friday, January 23, 2009

Non-basaltic rocks seen in Hawaii

We all know that the Hawaiian Islands are supposed to be huge blobs of Basalt piled up in the middle of the Pacific plate. So I was moderately surprised to find other rock types. Here is what I saw:

K-spar porphyry. Fine-grained matrix hard to ID, but presumably some sort of Monzonite. A late differentiate? On the East shore of Oahu.

Sandstone. Lithified beach sand. Cement type not obvious, but I’d guess some sort of silcrete. Possible silica precipitation due to pH change from groundwater to seawater? I honestly don’t know. Oahu and Kauai.

Dunite. Xenoliths in basalt. Kauai.

Felsic Gneiss. K-spar, plag, quartz, biotite folded foliated granite. Possibly an artifact.
Hotel countertop in Waikiki.


Chris Phoenix said...

If the beach sand was ground-up coral (from parrot fish eating it), then it should be fairly soluble anyway (calcium carbonate). Could it just have welded itself together?

How else would (white) beach sand have gotten to Hawaii?

Mel said...

I agree Chris. They are probably calcarenites (carbonate dunes). I remember seeing quite a few of those when I was on Oahu.

Chuck said...

Fresh roundwater carrying dissolved silica ought to precipitate quartz / opal/ something else when it reaches high pH seawater, yes?

jude said...

has anyone dated the porphyry? Because then you could see where it sits in the geological record with respect to the eruptions.

Wayfarer Scientista said...

laugh! counter tops! By the way, how is your knowledge of karsts?

Alison said...

Late comment, I know, but could the lithfied sandstone be some sort of beach rock (like those that can be seen on coral cays of the Great Barrier Reef? (e.g. Heron Island)). On the coral cays, it is made of carbonate clasts bound by carbonate cement, but for somewhere like Hawaii, there could be some siliceous components to it? Couple of ideas on the cement are that its pure precip from the sea water, or that it is to do with the interaction of sea and rain water (as the rock lies in the intertidal zone).