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Vesicular Rocks: Basalt, Scoria and Pumice

Vesicles are cavities left by gas bubbles in cooling magma. Dark, denser scoria and vesicular basalt differ from pale, highly porous pumice, but the names depend on texture and composition—not whether a single specimen floats. Estimate the proportion, size, and shape of vesicles and whether they are stretched by lava flow. Record overall colour and density; mafic material is commonly darker and denser than silicic pumice. Check whether cavities contain secondary minerals; filled vesicles are called amygdales. Describe the surrounding groundmass and field relationship to lava flows, dikes, or pyroclastic deposits. Industrial slag and furnace products can be vesicular and are frequently mistaken for volcanic rock or meteorites. Floating is not a universal test: some pumice sinks after pores fill with water, and other porous materials may float. A weathered surface can exaggerate pores, so examine a safe fresh surface when appropriate.

  • Pumice vs Scoria — Compare vesicle walls, density, colour, composition, and floating limits.
  • Meteorite vs Slag — Use bubbles, context, streak, magnetism, and interior texture safely.

Verification resources

Cross-check terminology, classification, methods, and safety with these authoritative external resources.

  • Rocks — U.S. National Park Service. Authoritative overview of rock groups, the rock cycle, and geologic processes.
  • Can you identify my rock or mineral? — U.S. Geological Survey. Identification limitations and routes to in-person professional verification.

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