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What are these cut marks? glaciers/man made

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  • #16
    Thank you . I live on a volcanic island and we have two types but it has to do with the flow by name not composition . I appreciate the answer as it clears up a few rocks I have collected .
    understanding the formation of the earth is one thing but the results of cooling by Mother Nature and what she has left us from gem stones to raw pieces such as this I can’t get enough of .

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    • #17
      I presume you mean Hawaii and are referring to the lava types known as “Pāhoehoe” and “ʻAʻā” (although you have a common third type known as “pillow” lava that forms under water). They are indeed characterised by the way they flow which in turn relates to their temperature and viscosity rather than their composition.

      ʻAʻā can have a ‘bubbly’ surface but, again, these aren’t vesicular structures. They’re derived from broken pieces of previously solidified lava carried as ‘clinker’ with the molten flow and usually end up as a distinct layer at the top and bottom of ʻAʻā flows. This type of flow also often contains (sometimes very large) lava balls, but these are solid accretionary structures, not hollow vesicles. It’s also possible for these accretions to be very iron-rich as a result of temperature-related separation of iron oxides.

      Regarding volcanic gem stones, you might be interested in this (from Mexico). It’s a chunk of andesitic volcanic magma that was on its way to becoming pumice. Under normal circumstances, water turning to steam and escaping would have created the typical round empty vesicles that characterise a pumice. But, in this case, the magma was under such extreme pressure deep underground such that the vesicle formation was constrained and the steam didn’t escape. The extreme temperature and pressure caused the trapped water to hydrate the silica, leaving cavities filled with a type of opal known as ‘fire opal’.

      Click image for larger version

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      I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.

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      • Tam
        Tam commented
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        I just love this pic . It tells a story for me .

    • #18
      I only know of the 2 not the pillow .
      I find this pumice type only at one natural cove and nowhere else . It has a obvious green hue to it and stands out next to any black lava .
      I don’t believe it’s the gem from the Big island those are shiny and a gem stone . Maybe the beginning of it .

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      • #19
        Kind of highjacked this thread but who doesn’t love to learn from painshill

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        • nwark
          nwark commented
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          I love it, it makes me learn even more. Thanks everyone,

      • #20
        Yes, we're a bit off-piste, but what the heck.

        You have a lot of pillow lava in the Hawaiian islands, but most of it is on the sea bed around the coastal areas, It's formed when magma is extruded from below the earth directly into water. It then cools extremely quickly and forms rounded pillow-like lobes (again, not hollow). This is off the Hawaiian coast:

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        The greenish hue often seen in lava is from olivine. Sometimes it will crystallise out as a distinct mineral, and especially so in magma that has been forcefully ejected as 'bombs'. I have one from Hawaii that has been sectioned to show the olivine crystals:

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        When olivine crystals are large enough and pure enough to be regarded as gem quality and suitable for cutting, we call the mineral "peridot". Like this (also from Hawaii):

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        I keep six honest serving-men (they taught me all I knew); Their names are What and Why and When and How and Where and Who.

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        • #21
          I have been to beaches where the peridot is all over .
          ok I am going with just olivine at an early stage on these .
          Thank you was wondering as I knew it was not the grade of the peridot .
          boy it stands out in the water .

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          • #22
            This is a very interesting thread. I learn something new everyday.
            South Carolina

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