Lateral variations in strata competence and alternating eruption styles in time and space as morphological controls on a maar crater in the Lunar Crater Volcanic Field, Nevada, USA.

摘要

Bea's Crater is two coalesced maar craters with diameters of ∼440 m and ∼1050 m, combined with a co-eruptive scoria cone that straddles the northeast rim of the larger crater, and protrudes onto the crater floor, with the protruding section having a much lower elevation than the rest of the cone. The smaller maar crater is located on the southwest sector of the larger crater, such that the two craters and the protruding cone form an alignment that parallels many local and regional structures, and is interpreted to represent the orientation of the feeder dyke near the surface. The maar formed amongst a dense cluster of scoria cones in the LCVF, such that the deposits of older cones are exposed in parts of the crater walls. The proximity of these cones results in a crater rim that has a variable elevation. These older cones are composed of variably welded spatter, such that poorly welded layers are less competent than strongly welded layers. Thus, the internal architecture of the proximal cones seems to have had a control on the shape of the larger maar crater, which grew preferentially towards less competent rocks. Deposits on the northeast cone, on the large crater floor, and in the tephra ring, record variations in the eruption style in both space and time. These deposits are variably resistant to erosion, and have added further complexity to the morphology of the feature. Many identified maar craters around the world have a simple circular to elliptical shape in plan view, however it is apparent that maar craters can have quite complicated shapes too. Identification of previously unrecognized maars can improve hazard assessments in volcanic fields.