I wonder if these small pyrite crystals would show some kind of zoning if they were sliced and polished. Zoning might reflect variations in acidity or some of the other factors you mention.
I guess it might be surprising if there were NO variations in the pyrite as the crystals grew. Whether or not such possible zoning was visible short of microprobe levels might be another story :)
But if they grew fairly quickly, especially geologically speaking, there would likely not be lots of changes taking place to make for zoning, would there?
That sounds reasonable too, but I guess I was thinking of the likely variability in the solutions that precipitated the pyrite, that might vary enough on even say a seasonal basis to have measurable differences (zoning) at least at the microprobe level. Not sure I would expect any visible zoning. But for sure, if the crystals grew really quickly (a few years vs 100 years) then maybe such zoning would not happen. Maybe the pyrite precipitated in one year or less - I don't know if that's even possible in a natural system (well, a natural system so screwed up by man that such an event might actually be possible).
Very cool Richard. Funny, just today (and other days) I was walking a gravel road away from the beach, and the gravel road is lined with cobbles and boulders here on Palawan. Not sure why, since the road itself was cobbles and larger rocks, rough and uneven. But it made me wonder how long have these rocks, on this level area, been here? Hundreds of years, if not thousands for the tougher ones? This is not on the 'ring of fire', so it's "boring" here no dramatic events to alter rocks. It doesn't flood with strong current, there's only weathering. So you mention you figure the creek (er, crik(!) was there 12K years. (IF this island moves 4cm/yr, guessing an average drift, in that time it's moved 1500+ feet. But not much else has changed.) They are mostly all well rounded from slow weathering, and these rocks are tough, not like the crumbling hillsides of serpentine around here. (So after all that rambling,) my question: Could it be calculated how long these rocks have been here?
I don't need an answer, a ballpark figure or even some ideas would suffice.
I’m not enough of a sedimentologist to really say anything definitive, but I learned (from sedimentologists) that cobbles and boulders are the first and fastest of clasts to round, so rounding isn’t a valid measure of distance from source once you get above maybe coarse sand sizes. It makes sense to me that big clasts would round relatively quickly, even in short episodic events like flash floods, or in rare movements in debris flows, or even during quiescent weathering, since they have so much more exposure to be affected, and because many or most would be polymineralic, so (theoretically) easier to break up than say coarse, pure quartz on its way to becoming fine sand.
As far as quantifying the time, I wouldn’t be surprised if someone has tried to investigate that, but I don’t know of any studies – sorry. Certainly a good question.
I wonder if in the case of your road, the edges might represent a selective removal from the road bed to the sides? Or some other human intervention? There’s a trail where I ride my bike that is in coarse alluvium or glacial outwash, but the trail bed was produced using brought-in material (probably from a quarry in the same outwash a few miles away), and the largest (20-30 cm) boulders are always on the edge of the bed – I think selectively moved there during the remediation and construction of the paved trail.
I wonder if these small pyrite crystals would show some kind of zoning if they were sliced and polished. Zoning might reflect variations in acidity or some of the other factors you mention.
Cheers!
I guess it might be surprising if there were NO variations in the pyrite as the crystals grew. Whether or not such possible zoning was visible short of microprobe levels might be another story :)
But if they grew fairly quickly, especially geologically speaking, there would likely not be lots of changes taking place to make for zoning, would there?
That sounds reasonable too, but I guess I was thinking of the likely variability in the solutions that precipitated the pyrite, that might vary enough on even say a seasonal basis to have measurable differences (zoning) at least at the microprobe level. Not sure I would expect any visible zoning. But for sure, if the crystals grew really quickly (a few years vs 100 years) then maybe such zoning would not happen. Maybe the pyrite precipitated in one year or less - I don't know if that's even possible in a natural system (well, a natural system so screwed up by man that such an event might actually be possible).
Very cool Richard. Funny, just today (and other days) I was walking a gravel road away from the beach, and the gravel road is lined with cobbles and boulders here on Palawan. Not sure why, since the road itself was cobbles and larger rocks, rough and uneven. But it made me wonder how long have these rocks, on this level area, been here? Hundreds of years, if not thousands for the tougher ones? This is not on the 'ring of fire', so it's "boring" here no dramatic events to alter rocks. It doesn't flood with strong current, there's only weathering. So you mention you figure the creek (er, crik(!) was there 12K years. (IF this island moves 4cm/yr, guessing an average drift, in that time it's moved 1500+ feet. But not much else has changed.) They are mostly all well rounded from slow weathering, and these rocks are tough, not like the crumbling hillsides of serpentine around here. (So after all that rambling,) my question: Could it be calculated how long these rocks have been here?
I don't need an answer, a ballpark figure or even some ideas would suffice.
Any thoughts?
Thanks.
Mike
I’m not enough of a sedimentologist to really say anything definitive, but I learned (from sedimentologists) that cobbles and boulders are the first and fastest of clasts to round, so rounding isn’t a valid measure of distance from source once you get above maybe coarse sand sizes. It makes sense to me that big clasts would round relatively quickly, even in short episodic events like flash floods, or in rare movements in debris flows, or even during quiescent weathering, since they have so much more exposure to be affected, and because many or most would be polymineralic, so (theoretically) easier to break up than say coarse, pure quartz on its way to becoming fine sand.
As far as quantifying the time, I wouldn’t be surprised if someone has tried to investigate that, but I don’t know of any studies – sorry. Certainly a good question.
I wonder if in the case of your road, the edges might represent a selective removal from the road bed to the sides? Or some other human intervention? There’s a trail where I ride my bike that is in coarse alluvium or glacial outwash, but the trail bed was produced using brought-in material (probably from a quarry in the same outwash a few miles away), and the largest (20-30 cm) boulders are always on the edge of the bed – I think selectively moved there during the remediation and construction of the paved trail.