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Pumice vs Scoria: Density, Colour and Vesicles

Pumice and scoria are both highly vesicular volcanic rocks formed when gas bubbles are trapped as erupting material cools. Pumice is typically pale, very low density, glassy, and associated with silica-rich explosive eruptions. Scoria is typically dark red, brown, or black, denser, and associated with mafic to intermediate magma. Their boundary is partly descriptive, so unusual specimens can overlap. Compare the material between the holes. Pumice usually has a frothy network of very thin glass walls and feels exceptionally light. Scoria usually has thicker, darker, stonier walls and feels denser. Floating supports pumice but is not decisive because connected pores can fill with water and some unusual scoria is extremely light. Confirm that the holes are true vesicles distributed through volcanic material rather than weathering pits or manufactured bubbles. Inspect a clean, dry surface. Pale frothy glass favours pumice; dark red, brown, or black stony material favours scoria. Use a hand lens to compare the walls between pores. Very thin, crowded glass walls support pumice; thicker and more irregular walls support scoria. Compare dry heft with specimens of similar size. Exceptionally low bulk density supports pumice, but density ranges overlap. Use a float test only as a secondary observation on a clean expendable piece. Record whether it floats initially and whether it later waterlogs, then confirm with texture and geologic context. Floating is not a definition. Some pumice sinks after connected pores fill with water, and very frothy mafic material can be unusually light. Weathering, oxidation, and mineral coatings can change colour. Use a clean surface and do not classify from colour alone. Fresh volcanic glass and dust can irritate or cut. Do not crush the specimen, avoid inhaling dust, and follow collecting rules and volcanic-hazard closures.