Gravity and magnetic investigation of maar volcanoes, Auckland volcanic field, New Zealand

Abstract

Detailed gravity and aeromagnetic data over maars in the Auckland volcanic field reveal contrasting anomalies, even where surface geology is similar. Pukaki and Pukekiwiriki, almost identical maars marked by sediment-filled craters and tuff rings, have gravity and magnetic anomalies of − 6 g.u. and 20 nT, and 8 g.u. and 160 nT, respectively. The Domain and Waitomokia maars, with similar tuff rings but each with a small central scoria cone, have gravity and magnetic anomalies of 32 g.u. and 300 nT, and 21 g.u. and 310 nT, respectively. These differences in geophysical expression are attributed to varying volumes of dense, magnetic basalt in the form of shallow bowl-shaped bodies up to several hundreds of metres in diameter and up to 140 m thick beneath the maar centres. These bodies are interpreted as solidified magma that ponded into early-formed phreatomagmatic explosion craters. Where magma supply was limited relative to groundwater availability, no residual subsurface basalt occurs (as at Pukaki); continued magma supply, but limited groundwater, resulted in ponding (e.g. at Pukekiwiriki) and eventually the building of a scoria cone (as at Domain and Waitomokia). There is no evidence in these geophysical data for diatreme structures below the maars or for shallow and/or extensive feeder dykes associated with these maars. If diatreme structures do occur, their lack of geophysical signature must be a consequence of either their small geophysical contrast with host Miocene sediments and/or masking by the stronger anomalies associated with the subsurface basalt. In addition, any magma conduits appear to be confined centrally beneath the maars, at least to shallow depths (upper 100 m).