scoria - black basalt scoria from the 2018 lava flow that buried Kapoho, Hawaii - spectacular display specimen
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.
Near the top of a flow, gas bubbles form because of the reduction in pressure. These become frozen in place to form scoria.
Deeper in the flow the basalt is more dense, with smaller bubbles, or even none. As an aa flow progresses, chunks of scoria and blocky lava tumble down the advancing front and are either bulldozed ahead by the advancing flow or are covered.
This scoria was collected from an aa field at the edge of the 2018 Kilauea flow channel that almost engulfed the Puna geothermal plant, that buried the community of Kapoho and that eventually filled in Kapoho Bay completely.
Impressive specimen, both for size, light weight, and for the size of the vesicles. The pencil is 5 1/2" long, for scale.
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