Quote: (02-15-2017 09:03 AM)komatiite Wrote:
Quote: (02-15-2017 02:06 AM)weambulance Wrote:
Looking at that bedrock with my geologist vision without actually getting anywhere near it, it looks like rotten shit. First glances can be misleading, though.
That's what I thought too -- this paper calls it the Smartville Ophiolite (Jurassic)
http://www.water.ca.gov/orovillerelicens...rt%201.pdf
I would envision that the gabbroic base of an ophiolite suite would be very competent, with the overlying pillow basalt, dikes and sills and marine sediments much weaker. but since it was exposed from sea floor to surface pretty rapidly in the young geological history of California (uplift of Sierra Nevadas) it was pretty strongly foliated and structurally deformed -- plus clearly exposed to serious alteration as you point out. I'm no metamorphic petrologist with a civil engineering passion though, and don't really know the rock-mechanics of this stuff, but I think it's safe to say extreme alteration during metamorphism and uplift definitely did a number on the rock competence. It almost looks serpentinized?
Interesting link, thanks. I didn't try very hard but I wasn't having much luck figuring out what kind of rock it was.
It's hard to say much without being there, because so much depends on orientation and degree of weathering. But it's certainly no better than middling-strong bedrock at best. If it's heavily weathered, it might be no better than shmoo. All the heavily serpentinized mafic/ultramafic rock I've seen was weak as hell; I could basically tear it apart with a rock hammer.
As I recall, mafic/ultramafic rocks are quite prone to rapid weathering because some of the common minerals like olivine are not very stable at surface conditions. I don't remember most of the details off the top of my head... I haven't had to use this knowledge since I was a TA teaching geology noobs.
But if roberto's right and they were just ripping it up with a bulldozer, it's nasty weak stuff. Even a low grade crummy schist is way too sturdy to attack with a ripper. What they were ripping might have been just the surface layers that were halfway to soil already, though.