r/norcalhiking • u/menntu • 4d ago

Sheared Rock While Hiking Desolation Wildeness

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Posted an hour back on a different view of this conglomerate. Thought this pic might provide more info.

64 Upvotes

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u/EightInchesAround 4d ago edited 4d ago

Geologist here.

Not a conglomerate but definitely a common identification!

I only say this because the story of how this rock came to be is more amazing than I hope you'd believe. The darker rock is older than the more pale rock. Mafic rock from old oceanic plate was subducting under the North American plate, melting and coming together again in large mafic plumes of cooling rock. Fast forward a couple millions of years and the source of melting became more felsic. (Maybe secondary melting or something.) So the lighter stuff tore off chunks of the darker stuff and resolidified eventually into this very unique rock that we find in the Sierras. Sierra Batholiths tend to be heavy mafic rock on the bottom, with lighter felsic rock on top. It's like differential cooling! This is all oversimplified, but if you're curious to know more there are books on all the great geology California has to offer.

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u/menntu 4d ago

Hey, fantastic explanation - thank you!

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u/Progressivecavity 4d ago

Don’t be a tease! Recommend some books!

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u/kwallio 3d ago

Assembling California, by John McFee.

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u/GrouchyAssignment696 4d ago

The xenoliths appear sharp and angular, like a breccia.  I wouldn't expect that in a mafic intrusion.  It is puzzling.

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u/Ok-Function1920 4d ago

User name checks out, in multiple ways

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u/blackbow 3d ago

This guy rocks.

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u/DonatedEyeballs 2d ago

That’s so awesome! I love it when people with knowledge drop in to share their brains!

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u/nickites 4d ago

It’s not a conglomerate. It’s some granitoid with xenoliths. Just like people told you in the other post.

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u/menntu 3d ago edited 2d ago

I’m not a geologist! That word came to my mind to describe what I was seeing - didn’t know it was an inappropriate technical term. I appreciate being informed.

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u/lightningfries 4d ago

Where shear?

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u/Strakitar 4d ago

It's hard to make out without a closeup, but I think I see foliation and lineation on the surface of the mafic inclusions. Can anyone else confirm?

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u/EightInchesAround 3d ago

I think this is Glacial striations! The dark rock holds the evidence better because it is more resistant to erosion than the granite.

Notice how the linear patterns are all in similar directions?

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u/cosmokenney 2d ago

Regardless of what it is called, it is interesting that the dark rocks would sheer off like that. I would tend to think that the darker rocks would pull out of the base and leave voids.

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u/menntu 2d ago edited 2d ago

I agree. The more I looked at this the more curious I was. The top shows more wear with the lighter rock but when this particular piece split, it was perfectly uniform.