scoria - black basalt scoria from the 2018 lava flow that buried Kapoho, Hawaii - hand 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 in water.

Near the top of an andesitic or basaltic flow, gas that was dissolved in the magma comes out of solution to form bubbles 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.

Very light weight, but with a specific gravity greater than 1, so it will not float in water, unless just momentarily.

The pencil is 5.5" long, for scale.

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