On the formation of deep-seated subterranean peperite-like magma–sediment mixtures

In addition to large amounts of fragmented country rocks, maar–diatreme volcanoes commonly eject juvenile lapilli and cauliflower bombs containing many dispersed xenoliths and/or xenocrysts derived from the country rocks surrounding the diatremes. Formation of these lapilli and bombs is attributed to thermohydraulic explosions in the root zones of the diatremes and their consequences. The thermohydraulic explosions release shock waves which are capable of intensely fragmenting the surrounding country rocks. Vaporisation of water superheated during thermohydraulic explosions and its expansion to lower pressure causes ejection of major parts of the fragmented country rocks. Surviving sections of fragmented country rocks may collapse or slide into the partially evacuated root zone and thus form a mass-flow deposit subsequent to each explosion. When, prior to a new explosion, magma rises from the underlying feeder dyke into the root zone, it intrudes this clastic debris, inflates it and may mingle with the debris to form a peperite. Subsequent explosions can fragment and eject fragments of this peperite in the form of juvenile ash grains, juvenile lapilli and bombs containing xenoliths or xenocrysts from the country-rock debris. Lack of further explosions, but continued rise and intrusion of magma will cause emplacement of such inflated peperite masses as plugs.