Friday Rock Blogging: Gabbro

I know it’s getting rather late in the Friday for some of you; sorry about that. I was thinking that I’d finally throw up a few pictures of gabbro and explain this blog’s name, but when I went looking for pictures, well, I remembered why I’ve been putting it off. Then I was somehow distracted by the poor quality of Wikipedia’s tectonic plate entries – there’s some implicit libel of the Galapagos Rise going on there. Really, I should stop browsing the geology stubs, it wreaks absolute havoc with my anal retentive inner demons.

Anyway. Gabbro is a very unassuming sort of rock. It’s the intrusive equivalent to basalt – same stuff as here, but because it forms underground, it cools more slowly and forms visible crystals. It’s not green. Its primary components are plagioclase, amphibole, olivine, and pyroxene – white, dark, green, and dark, respectively. In general, if a gabbro contains so much olivine that the overall rock appears any greener than a dull green-gray, it gets classed as a peridotite.

But, I place way more value on alliteration than I do on truth, so there.


  1. Modulator wrote:

    Friday Ark
    Cats, Dogs, Spiders and ? every Friday. We’ll post links to sites that have Friday (plus or minus a few days) photos of their chosen animals as I see them (photoshops at our discretion and humans only in supporting roles). Leave a comment or trackback…

  2. Joe wrote:

    Gabbro is one of my favorites. There’s a really awesome gabbro complex in Canada along the North SHore of Lake Superior that I remember going to in college.
    Damn, right now I can’t remember where it is, or what its called…but to this day I can remember those HUGE awesome pyroxene’s. It was one of the first rocks I really Cracked with my new (at the time) rock hammer.
    Quite honestly, I don’t think I’ve been to another gabbro since then…I just don’t live in the right area.

  3. yami wrote:

    It’s not a continuation of the Duluth complex, is it? So many maps have an unfortunate tendency to stop at the border.
    We went to a nice gabbro-with-multiple-diking-events roadcut in Baja my sophomore year, grain size wasn’t so impressive though. I know there’s some entertaining stuff in Death Valley but haven’t ever visited it. I don’t even own a rock hammer, sadly enough.
    Are there ANY good igneous outcrops in Mississippi?

  4. Joe wrote:

    There are no good rock exposures in mississippi. Nothing “hard rock”. just a buncha boring old chalk and tertiary fluvial sediments. All the rocks here are Cretaceous and younger. none of which are well exposed.
    Thankfully, I’ll be leaving this shithole forever in about 1 week.

  5. yami wrote:

    I’ve developed a grudging respect for fluvial sediments… but Tertiary is too old to be really interesting. And I’m still pissed off that blackboard chalk is made of gypsum, and not really chalk. The world owes me one for that.
    Where are you off to?

  6. Bethany wrote:

    I have to write an obituary for a gabbro rock. I picked this rock because it interests me. We have to come up with a name and the name I chose was Gabby Gabbro!!! I’m in 8th grade and do many projects like this. We’re learning about and identifing different rocks.

Post a Comment

Your email is never published nor shared. Required fields are marked *