Lapilli-stone

Lapilli-stone is a pyroclastic deposit, dominated by lapilli-sized (2–64 mm) fragments, which becomes solidified by secondary processes after deposition. Pyroclastic deposits dominated by lapilli are usually produced by small eruptions in a wet environment, such as in a shallow lake or in a swamp. The lava shreds ejected during the eruption are quenched on contact with water, and due to rapid cooling contraction they shatter into smaller fragments. The rapid cooling also inhibits crystallization of minerals and formation of gas-bubbles (vesicles). The lapilli accumulated and formed the slopes of the small volcano. The lappili deposit was subsequently cemented together by calcite (calcium carbonate), which precipitated from circulating water solution. As a result, the deposit gained character of compact black-and-white marbled rock – lapilli-stone.

Rock fragments of different colour and texture can be seen in the lapilli-stone during close-up observations. These alien stones, called xenoliths, represent fragments of underlying granites and claystones originating from the beneath the volcano that were ejected by the eruption .

Besides Krásný vrch Hill, lapilli-stones can also be found in other places in Doupovské hory Mts., e.g. near Valeč, Dětaň, Brodce or in Rokle near Kadaň.

Sample donated by the company Sedlecký Kaolin, corp.