Formation of phreatomagmatic maar-diatreme volcanoes and its relevance to kimberlite diatremes

摘要: Abstract Studies of maars and diatremes suggest a specific process in their formation. Magma rises along fissure contacts ground- or surface derived water. The resulting phreatomagmatic eruptions give rise to base surge air-fall deposits consisting juvenile wall-rock material. Spalling the wall-rocks enlarges into an embryonic vent. At critical diameter vent large-scale spalling at depth slumping near gives ring-fault large subsidence enclosed overlying pyroclastic debris. This leads maar crater surface. Fluidization processes are active narrow fractures faults inside surrounding structure which subsides en masse . Various features kimberlite seem be consistent with this model. They extend fissures hot magma rose. diatremes, however, indicate emplacement by cool gas phase, probably steam. Indicators for ring-faults may slickensides on walls, saucer-shaped structures, subsided “floating reefs”, concentration xenoliths from horizons within certain areas, zoning diatreme rocks. It is suggested that formation have been influenced non-juvenile