Felsic and mafic rock

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This is a transcript of a QuickTime movie (14.3 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, discusses felsic and mafic rock.


Is it unusual to find felsic rock right up against mafic rock?


At the same time?

No, in the Pilbara it’s very, very common. What’s important about that observation is it means it’s not ocean crust, because ocean crust is all mafic rock. These rocks were all built like Kerguelen or Ontong, they’re all a big volcanic plateau. And yet big magma chambers that resided, in crustal magma chambers, and they fractionated, differentiated through time. And the first phase that came out were the basalts and pillows and then as fractionation increased you get more and more felsic compositions. And you see that in, I’ll show you a stratigraphic column, but you get mafic to felsic, mafic to felsic, mafic to felsic and at each transition, chert. And chert is the hydrothermal waning phase of that volcanic cycle, again, again, again, again.

So if it’s continental, why is it under water? Or it’s probably not under sea water?

Well, think about the hill. It probably is. I mean not all continents are exposed all the time. All the middle of Australia, you know, is these, like, cretaceous basins and stuff like that so continents go up and down depending on sea level rise and depending on how high they build. And if you think of Hawaii, Hawaii’s a really big island that’s got an exposed volcanic carapace but ninety percent of its history is built near water so it has to get to a certain level where it reaches this surface so …




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