SULPHUR

How do you get volcanic ash and tuff in intrusive igneous areas?

ps418

You don't, unless intrusive igneous rock is exhumed by erosion and a tuff is deposited on top of the erosion surface. The salient point is that intrusives cool below ground, while extrusives cool above ground. I'm not sure if that answers your question. Also, there is a geographic and temporal association of intrusive and extrusive igneous rocks. This is simply because extrusives form when magma reaches the surface, and this will only happen where there is subsurface magma. For instance, when the Sierras formed, massive granitic intrusives were emplaced, and at the same time there was massive extrusive volcanism at the surface.

Patrick

SULPHUR

The kimberlite pipes are the remains of a volcano and as you said erosion may result in extrusive material being part of the diamond mining.I think we are beginning to split hairs. Itis 5.47am and I think I needasleep. I look forward to further discussions

tom

ps418

Aha! I see where the confusion is. The kimberlite pipes may indeed have fed a surface volcano, but the contents of the pipe as we find them today never reached the surface, they cooled below surface, and therefore are intrusive. And the diamonds themselves are zenoliths that cooled below surface but were transported upwards with the magma. Even bona fide extrusive rocks may contain intrusive rocks as zenoliths.

Patrick

ps418

Oops. Make that xenoliths, not zenoliths. Its an exotic rock, not a philospher turned to stone. . .

Patrick

SULPHUR

Quite common in the permian in van diemansland dropped by glacial icebergs into the sediment. We must have a talk about the comparative geology of our two areas and the difference in nomenclature

Nowhere357

Talk about make-work!