Pillow basalts

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This is a transcript of a QuickTime movie (29.8 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, discusses the formation of pillow basalts.


These are basaltic lavas or metamorphosed basaltic lavas because their composition has been changed. They’ve been recrystallised a bit. They’ve been squashed and deformed. But right here is a really nice one and as you go over the platform you’ll see lots of examples but when basaltic magma is erupted under water it instantly freezes on the outside because the temperature difference goes from 800 degrees to 30 or 60 or whatever it was. But you know you’ve got a 750, 800 degree temperature difference so the skin instantly freezes but the middle stays warm of course so it’s a freeze thaw thing. And magma passes through that skin and goes out as these blobs. You may have seen the films, you know, in Hawaii and the magma comes and breaks through that skin and goes out as a blob but it freezes and then it moves somewhere else. It follows gravity, of course. And once you create a tube of lava that’s moving through the older rock you get parts where there’s cavities remaining and more magma’s moved through that and then it drains and then it leaves open space. And as that magma quenches against the sea it gets a glassy rind on the outside, and that glassy rind, because of movement of basalt over itself, starts to break off and it fills up the spaces between the pillows. But the pillows have a very characteristic geometry. They go into a fee surface, a water free surface so it’s a perfectly smooth contact, right. See that you’ve got beautiful smooth top of this pillow structure. And the way that we identify it as a pillow is that the middle is quite homogeneous except for these little veins that go through. But then surrounding it is a glassy rind. It’s a slightly different colour and then all this fragmental material, including beer bottle fragments but that’s more recent. But that fragmental material in between the pillows is just the glassy rind and as it gets expanded by later magma coming in falls off and little pieces of that glass go down just like that beer bottle does. If you break a beer bottle on here all the fragments will go into all the little spaces between the cracks, right? So that’s exactly what this is. This is called a hyaloclastite breccia. It’s just glassy shards of basalt that fall into open space. But the bottom of this pillow is fairly regular but it does have a little bit of wobbles in it if you look at it and stuff. And those wobbles are filling in little valleys just like we see on this surface now, right? So the bottom will be filling in the empty spaces and the top will be nice and smooth. And then as the lava cools it crystallises slowly and it changes from glass to a very small crystalline texture and there are point sources in there where crystallites nucleate around and you get these little spherical dots everywhere, right? These are called ocelli or some people call them varioles. In fact we’ve got two pillows here actually. But the middle of it actually devitrifies around these little nucleation points. You get these spherulitic devitrification textures, so taking glass out. And you can see by Stephen’s foot all these little pea shaped things that are weathering out. And pea shaped things turn out to be important for a variety of reasons. But one example is, you know, they found these blueberries up on Mars and so when they found these blueberries they had to ask all the questions. Well, what kind of blueberries were they? Were they pea shaped things weathering out of devitrified volcanic rock? Were they concretions in sediment? Are they x, y z? And then you do a battery of tests on those peas to see what they are.

This is a good one here. You’ve got this nice (slope) down below. You can see the glassy rind. You can see it’s all very parallel finely fractured. That’s the chill zone of the pillow and then it gets a little bit coarser here. But there’s the round top and the convoluted base so top was that way. That was the up surface of the old sea floor. And then you get these little patches of quartz like this. And they’re filling hole spaces where the magma has transported through the pillow and left behind a gape as the magma cooled. These are called pillow shelves and we’ll see them on top of Marble Bar chert. They’re really nice. But you see them through the section, so.

Why isn’t this just a much later event?

No, it’s not because when the pillows form lava buds out from it and it passes through the pillow. And it’s got a glassy rind around it but the inside is still, was where liquid rock passed through but that eventually dries up and it leaves behind a hole. And then fluids, sea water circulates through the hot basalts and leaves behind silica. So the thing about silica that you see here, there and everywhere is that silica is the most mobile element at relatively low temperatures. So silica gets stripped by sea water flowing through basalt and it gets reprecipitated as the temperature drops down a little bit more and so you always get silica. Sometimes you get other elements in there but usually low temperature stuff is silica. And if it cools slowly you get quartz and if it cools pretty quickly you get chert. We’ve got mostly chert through here.

But you’ve got nice pillow shapes you can see all the way through here. So they’ve got very classic chill margins, that’s where it hit the sea water and froze, little glassy shards all the way around it and then a more solid base. But you see it draping on the old pillows then a big blobby one there. So you can really see the process of these things despite the fact that they’ve been turned over so they’re all dipping down like this. Everybody happy with that?




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