Maar-diatreme volcanoes: A review

Abstract

Maar-diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock. A maar is the crater cut into the ground and surrounded by an ejecta ring, while the diatreme structure continues downward and encloses diatreme and root zone deposits. Here we attempt an evenhanded review of maardiatreme volcanology that extends from mafic to kimberlitic varieties, and from historical maar eruptions to deeply eroded or mined diatreme structures. We conclude that maar-diatreme eruptions are episodic. Ejecta rings provide invaluable insight into eruption processes and sequence, but are incomplete records of diatreme formation. Deposits within the diatreme structure include, in varying proportions, lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially, and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere. A basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock; root zones are irregular in form, and the clastic deposits are typically intruded by contorted dikes. Irregular root zone-like chaotic breccias cut by contorted dikes are also present within diatreme deposits, where they represent intra-diatreme fragmentation zones and record changes in the location of the explosion locus during eruption.

A tuff cone erupted under frozen-bed ice (northern Victoria Land, Antarctica): linking glaciovolcanic and cosmogenic nuclide data for ice sheet reconstructions

“…Smellie et al 2016). Similar bedded pyroclastic megablocks commonly occur within non-bedded volcanic hosts in the lower, unbedded parts of diatremes together with ill-defined diffuse decimetre-scale patches or domains of better-sorted lapilli-rich rock within ash-rich lapilli tuff (White and Ross 2011;and personal communication from P.-S. Ross, 2017).…”

“…The conditions are most similar to those that construct tuff cones, with their abundant water sourced from surface water or a free-flowing aquifer, rather than either tuff rings or maars, with their less efficient delivery of groundwater-sourced water (cf. Wohletz and Sheridan 1983;Verwoerd and Chevalier 1987;Sohn 1996;White and Ross 2011).…”

“…The source of the water is therefore likely to be groundwater or else ponded meltwater created during subglacial eruptions. However, eruptions involving interaction with groundwater create deposits characteristically rich in basement lithics, although the proportion can be variable (Wohletz and Sheridan 1983;Sohn 1996;White and Ross 2011). The volume of basement-derived lithics is very low in the Harrow Peaks deposits (c. 1 vol%), apart possibly from the lowest (earliest-formed) beds.…”

Three-dimensional potential field modelling of the subsurface morphology of complex maar volcanoes - Examples from the Newer Volcanics Province, Western Victoria

“…Maar volcanoes are commonly formed in monogenetic volcanic fields, and are characterised by a near-circular crater cut below the pre-existing surface and surrounded by a low, shallowly dipping tephra ring (Lorenz 1975(Lorenz , 1986(Lorenz , 2003White & Ross 2011;Valentine & White 2012). Maar volcanoes form as ascending magma comes into contact with ground water and interacts explosively.…”

“…A maar-diatreme has a high petrophysical contrast (lower density and higher magnetic susceptibility) with the surrounding host rock, making maar volcanoes ideal for gravity and magnetic modelling (Cassidy et al 2007;Blaikie et al 2012). Maar volcanoes are also considered as an analogue for some kimberlite pipes (Lorenz 1975;White & Ross 2011), therefore better understanding their subsurface structures and improving geophysical modelling techniques has important economic implications.…”