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.

When dealing with ancient subalkaline volcanic rocks, the alkali-total iron-magnesium (AFM) diagram is of limited use in assigning a tholeiitic vs. calc-alkaline affinity because these elements are often mobile during alteration and metamorphism. Classification diagrams using immobile trace elements are preferable, but need to be tested and optimized on unaltered rocks.To this end, a geochemical database containing over a thousand presumed unaltered subalkaline volcanic samples from young oceanic arcs was assembled. The data were classified using both

Basaltic maar-diatreme volcanoes, which have craters cut into preeruption landscapes (maars) underlain by downward-tapering bodies of fragmental material commonly cut by hypabyssal intrusions (diatremes), are produced by multiple subsurface phreatomagmatic explosions. Although many maardiatremes have been studied, the link between explosion dynamics and the resulting deposit architecture is still poorly understood. Scaled experiments employed multiple buried explosions of known energies and depths within layered aggregates in order to assess the effects of explosion depth, and the morphology and compaction of the host on the distribution of host materials in resulting ejecta, the development of subcrater structures and deposits, and the relationships between them. Experimental craters were 1-2 m wide. Analysis of high-speed video shows that explosion jets had heights and shapes that were strongly influenced by scaled depth (physical depth scaled against explosion energy) and by the presence or absence of a crater. Jet properties in turn controlled the distribution of ejecta deposits outside the craters, and we infer that this is also reflected in the diverse range of deposit types at natural maars. Ejecta were dominated by material that originated above the explosion site, and the shallowest material was dispersed the farthest. Subcrater deposits illustrate progressive vertical mixing of host materials through successive explosions. We conclude that the progressive appearance of deeper-seated material stratigraphically upward in deposits of natural maars probably records the length and time scale for upward mixing through multiple explosions with ejection by shallow blasts, rather than progressive deepening of explosion sites in response to draw down of aquifers.

The Ferrar large igneous province of Antarctica contains significant mafic volcaniclastic deposits, some of which are interpreted to fill large vent complexes. Such a complex was re-examined at Coombs Hills to map individual steepsided cross-cutting bodies in detail, and we found several contrasting types, two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing, non-consolidated debris. These transient conduits were opened because of the propagation of debris jets -upward-moving streams of volcaniclastic debris, steam, magmatic gases +/-liquid water droplets -following explosive magma-aquifer interaction. Some debris jets probably remained wholly subterranean, whereas other made it to the surface, but the studied outcrops do not allow us to differentiate between these cases. The pipes filled with country rock-rich lapilli-tuff or tuff-breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex, causing fragmentation of both magma and abundant country rock material. In contrast, some of the cross-cutting zones filled with basalt-rich tuff-breccia or lapilli-tuff could have been generated following explosions taking place within preexisting basalt-bearing debris, well away from the complex walls or floor. We infer that once focused jets were formed, they did not incorporate significant amounts of existing Mawson debris while travelling through them; instead, incorporation of fragments from the granular host took place near explosion sites. Other basalt-rich tuff-breccia zones, accompanied by domains of in situ peperite and coherent basalt pods, are inferred to have originated by less violent processes.

Flood volcanic provinces are assumed generally to consist exclusively of thick lavas and shallow intrusive rocks (mostly sills), with any pyroclastic rocks limited to silicic compositions. However mafic volcaniclastic deposits (MVDs) exist in many provinces, and the eruptions that formed such deposits are potentially meaningful in terms of potential atmospheric impacts and links with mass extinctions. The province where MVDs are the most voluminousthe Siberian Traps -is also the one temporally associated with the greatest Phanerozoic mass extinction. A lot remains to be learned about these deposits and eruptions before a convincing genetic link can be established, but as a first step, this contribution reviews in some detail the current knowledge on MVDs for the provinces in which they are better known, i.e. the North Atlantic Igneous Province (including Greenland, the Faeroe Islands, the British Isles, and tephra layers in the North Sea basin and vicinity), the Ontong Java plateau, the Ferrar, and the Karoo. We also provide a brief overview of what is known about MVDs in other provinces such as the Columbia River Basalts, the Afro-Arabian province, the Deccan Traps, the Siberian Traps, the Emeishan, and an Archean example from Australia. The thickest accumulations of MVDs occur in flood basalt provinces where they underlie the lava pile (Faeroes: >1 km, Ferrar province: ≥400 m, Siberian Traps: 700 m). In the Faeroes case, the great thickness of MVDs can be attributed to accumulation in a local sedimentary basin, but in the Ferrar and Siberian provinces the deposits are widespread (>3 x 10 5 km 2 for the latter). On the Ontong Java plateau over 300 m of MVDs occur in one drill hole without any overlying lavas. Where the volcaniclastic deposits are sandwiched between lavas, their thickness is much less. In most of the cases reviewed, primary MVDs are predominantly of phreatomagmatic origin, as indicated by the clast assemblage generally consisting of basaltic clasts of variable vesicularity (dominantly non-to poorly-vesicular) mixed with abundant country rock debris. The accidental lithic components often include loose quartz particles derived from poorly consolidated sandstones in underlying sedimentary basins (East Greenland, Ferrar, Karoo). These underlying sediments or sedimentary rocks were not only a source for debris but also aquifers that supplied water to fuel phreatomagmatic activity. In the Paraná-Etendeka, by contrast, the climate was apparently very dry when the lavas were emplaced (aeolian sand dunes) and no MVDs are reported. Volcanic vents filled with mafic volcaniclastic material, a few tens of metres to about 5 km across, are documented in several provinces (Deccan, North Atlantic, Ferrar, Karoo); they are though to have been excavated in relatively soft country rocks (rarely in flood lavas) by phreatomagmatic activity in a manner analogous to diatreme formation.