scoria - hand/display specimen of maroon scoria from a satellite cone on Mauna Loa
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Scoria is the silica poor equivalent of pumice. Lava with a low silica content is runny, so gas bubbles tend to coalesce to form larger bubbles in basalt. In a silica rich granitic magma, high viscosity prevents bubbles from coalescing, so there are many more, but smaller, gas bubbles in pumice, one reason it floats longer in water.
This scoria is surprisingly light and is amazingly red from the oxidation of hematite. It was collected at an elevation of 10,500 feet on the north slope of Mauna Loa, from a mass of blocks, scoria and cinder associated with a satellite cone that was quarried for road material. An interesting example from a famous volcano.
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