Breccia

breccia

Here is a nice example from the Oakland hills of breccia, a kind of rock that consists of broken pieces of rock. There are several different ways to make a breccia, and not having a lab and a petrographic microscope I can’t specify what made this. But considering that the hills have been raised by vigorous tectonic action accompanied by earthquakes, it’s easy to imagine some seismic event opening a fracture, shattering the rock, and allowing mineral-charged fluids into the space. Other ways involve dissolving the rock until it collapses, slumping while the rock is young and poorly consolidated, and simply cementing together a pile of landslide debris or volcanic deposits.

Because breccia really signifies an activity rather than a material, geologists would rather think about brecciation when they see a breccia.

UPDATE: I should mention that I have lots more breccia pictures on About.com: start here and try a search on “breccia” for more.

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3 Responses to “Breccia”

  1. Callan Bentley Says:

    Is the “host” rock a basalt? I think I might be seeing amygdules in there, and even in one of the larger fragments in the brecciated zone.

  2. Holly Says:

    Is the primary rock serpentinite? It’s got that slippery green color.

  3. Andrew Says:

    I was staring at the brecciation more than the host rock. It may well be basalt, and welcome from the other side of the continent, Callan. Holly it may well be chloritized, or it may just be skylight. The rock was wet with dew and surrounded by greenery.

    Darn, now I have to go back.

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