spherules

Spherules are small, nearly spherical particles that range in size from tens of micrometers to a few millimeters. They occur in rocks, sediments, ice, and extraterrestrial soils and are typically glassy or metallic in composition. The defining feature is their rounded shape produced by rapid melting and quenching of material in high-energy environments.

Cosmic spherules form when micrometeoroids or dust grains melt and ablate as they pass through an atmosphere. Molten droplets are ejected, cool rapidly, and solidify into spheres, with compositions ranging from iron-nickel-rich to silicate-rich. In terrestrial settings, spherules accompany impact events, arising from vapors and molten droplets in ejecta plumes and condensing into glassy beads. Microtektites are related glassy spherules. Volcanic eruptions can also produce spherules when droplets of lava fuse and quench during explosive fragmentation.

Spherules are preserved in deep-sea cores, glacial ice, sediment layers, and lunar regolith, where they serve as indicators of past meteorite flux, atmospheric entry conditions, or explosive volcanism. Their chemical and isotopic compositions help distinguish origin and reconstruct environmental histories.

Classification of spherules is by origin and composition. The general term includes cosmic spherules, microtektites (glass spherules from impact plumes), and volcanic glass spherules.