Accretionary lapilli, also termed pisolites, are small spheres of extremely fine ash, which may reach more than centimeter in diameter. They are commonly found in tephra deposits formed by eruptions where water has been involved -- hydrovolcanic or phreatomagmatic eruptions. Often, they consist of an outermost shell of harder ash surrounding a more friable core; sometimes they have a concentric, onion-like structure. Although it is clear that accretionary lapilli are associated with 'wet' eruptions, it is not clear exactly how they form. It used to be thought that they resulted from raindrops flushing through dense tephra clouds, fine ash accreting around rain drops. While this is one possible mechanism, it is clear that there are others, since lapilli are found in a range of deposits, not only air-falls. They are common, for example in base surge and other pyroclastic flow deposits. These examples are in the 400 million year old (Ordovician) Borrowdale Volcanic Series of the English Lake district, and illustrate how well subtle volcanic features can be preserved in ancient rocks. (Fig. 9.31).

Part 1       Part 2       Part 3       Part 4       Next Slide


 To VolcanoWorld