On the formation of maars

Abstract

The Pleistocene maars in the Eifel region of Germany, and Massif Central in France, formed when fissures opened at the bottom of older valleys allowing stream water to pour down them and come into contact with rising magma. The resulting phreato-magmatic eruptions gave rise to both base surge and air-fall deposits. Spalling of wall rock at depth enlarged the fissure into an eruption chamber. Subsidence along a ring fault into the eruption chamber accounts for the larger crater cut into the country rocks. The volume relationship between the crater excavated, the ejected pyroclastic debris of the rim and the volume below the floor of the crater, indicates that the volume of the maar ejecta is always larger than the volume of the crater. The relationships between maars and tuff-rings are described; the distinctive features of the two depend on density differences between the pyroclastic debris and country rocks, on the distribution ratio between ejected material and debris remaining within the underlying diatreme, and most importantly on the total amount of juvenile material produced. Larger contents of juvenile material result in the formation of tuff-rings instead of maars, and in most cases also indicate a shallower eruption source of the former. As a result of these many variables, large diatremes, which display subsidence structures bounded by ring-faults, may produce either maars or tuff-rings at the surface.