10 October 2016

Oddball Icelandic rocks, part II: granite!?!?

Posted by Callan Bentley

Around the corner from the Hvalnes Lighthouse in eastern Iceland is the second non-basaltic igneous rock I saw in Iceland.


I couldn’t drive past something like that and not stop. Seriously – this was the only high-albedo rock I saw on the entire island!


(Note the lens cap for scale, as in all the photos in this post.)

Yes, that’s what it appears to be – here on the flanks of the Mid-Atlantic rift, partial melting of the mantle yields basalt and partial melting of the basalt has apparently generated a granite.


This was unexpected (to me), as I associate granites with more evolved crust – like that of the continents, where the material has been cycled through multiple rounds of “distillation” to pull out the most easily-melted components. (Felsic minerals melt at lower temperatures than mafic ones, so if you partially melt a given rock, it’ll segregate into a more-felsic liquid and a more-mafic solid residue.) Iceland’s apparently got a sort of “hot spot” underneath it (like Hawaii) as well as being situated on a divergent boundary (the Mid-Atlantic Ridge), so I would have expected nothing but basalt. (And indeed, that’s mostly what Iceland is made of!)

I wouldn’t have guessed any of the rocks here would have had the opportunity to get the granite sweated out of them. But: apparently it has happened, at least once.

The granite contains xenoliths of more mafic compositions:



And now for a few GigaPan views of this place:

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Link GigaPan by Callan Bentley

Finally, a GIGAmacro view of a sample I collected at the site. You can explore its phaneritic (coarse-grained) texture and see its quartz content readily with this image:
Link GIGAmacro by Robin Rohrback

A granite in Iceland! Who’d have thunk it?