Lapillus

Lapillus is the Latin singular form for a small piece of rock. In modern geological usage, the term lapilli (the plural) denotes tephra fragments between 2 and 64 millimeters in diameter that originate from explosive volcanic activity. This size range places lapilli between finer volcanic ash and coarser blocks or bombs in tephra classifications.

Etymology and usage: Lapillus comes from Latin, meaning pebble or small stone. In English-language geology, lapilli is the standard term for the tephra fragments of the specified size range, while lapillus is encountered mainly in Latin-based nomenclature or historical texts describing such fragments.

Characteristics and types: Lapilli can be composed of juvenile volcanic glass or crystals, as well as lithic fragments derived from pre-existing rock. They may be vesicular or dense and can occur in various shapes, including rounded to elongated forms, depending on transport and deposition. Subtypes commonly recognized include juvenile lapilli (originating directly from magma) and lithic lapilli (fragments of surrounding rock incorporated during eruption). Some lapilli are scoriaceous, reflecting rapid gas loss and vesiculation during eruption.

Depositional context and significance: Lapilli are a characteristic component of tephra falls and some pyroclastic flows. In the stratigraphic record, clusters of lapilli form tephra beds or lapilli tuffs, which help volcanologists interpret eruption styles, column heights, and distances from vents. The distribution and physical properties of lapilli provide clues about eruption energy, magma composition, and transport processes.

In practice, lapilli is the more common term in contemporary volcanology; lapillus appears primarily in Latin-based or historical contexts.