A new style of layered tuff deposit in the SW of Oldoinyo Lengai’s active crater consists of bedded spherical lapilli 8–12 mm in diameter with hollow cores and vesicular rims. Lapilli of this type have not previously been reported from extrusive carbonatites and similar deposits are apparently unreported amongst siliceous pyroclastics. Some lapilli are solid and are either vesicular throughout or contain vitreous crystal cores. A mechanism for the eruption and a sequential model for the formation of hollow lapilli are suggested.

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