#IGotaRockSince it first premiered 48 years ago, the TV show “It’s the Great Pumpkin Cha

#IGotaRockSince it first premiered 48 years ago, the TV show “It’s the Great Pumpkin Charlie Brown” has become a modern-day classic. The show features the characters from Charles Schulz’s Peanuts comic strip in a Halloween-themed setting, including a night of trick-or-treating.Charlie Brown goes out trick or treating with a group of kids, all of who get candy other than him. After every house, he looks into his bag and says the words we all want to hear as geologists: I GOT A ROCK!!!! For some reason he never sounds as excited as I would, but that doesn’t diminish the honor.I’ve picked 3 rocks that I’d love to get in a Halloween bag, one from each of the 3 big rock types. This is my igneous rock, from Obsidian Dome in California – shoe and hand lens for scale. To the right, you see true obsidian – a layer of perfectly black volcanic glass. In the center of the same rock, you see thin patches of obsidian stretched out in the same direction, but surrounded by a pink matrix.The compositions of these 2 layers are nearly identical. They’re part of the same dome, a big blob of thick, rhyolitic magma that pushed its way up through the crust. But, something amazing happened as this magma was moving; some parts started to crystallize, some parts didn’t.When magma cools off quickly enough, it can turn into obsidian – a black, glassy volcanic rock that also had a starring role alongside Morgan Freeman in the movie “The Shawshank Redemption”. If it doesn’t cool fast enough or something makes it start crystallizing, it will turn into a layer like the pink – a fine-grained rhyolite. In the same rock, we have parts that turned to glass and parts that didn’t. This could be a result of the shearing force on the magma as it was being emplaced, or a subtle difference in where the magma was held in the magma chamber, or even a difference in how much gas was retained by each part of the magma prior to quenching. The glassy bits in the middle of the rhyolite layer testify to the stretching and flowing of the magma as it was emplaced, and they could even represent former bubbles (vesicles) sheared out as the magma moved.It’s a bit big, but man this would be an exciting rock to find in a Halloween sack.-JBBImage credit: JBB. -- source link
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