Volcanic and sedimentary processes in phreatomagmatic volcanoes
Phreatomagmatic volcanoes form when ascending magma explosively interacts with surface or groundwater at shallow depths. Three types of phreatomagmatic activity are recognised- phreaticp phreatomagmatic (s. s. ) and surtseyan - based on the degree of involvement of magma with water and the depth of the interaction. Phreatic maars and phreatomagmatic tuff-rings are underlain by pipe-like diatremes but these structures are poorly developed or absent in surtseyan tuff-rings. Comparisons of phreatomagmatic volcanoes with their eroded diatreme equivalents, which contain subsided subaerially-deposited material, allow a model for activity of this type to be constructed. The Saefell tuff-ring SW Iceland is a surtseyantype structure whose crater remained open to the sea during most of its activity, allowing easy access of water to the magma. Base-surges sourced partly from directed blasts, formed large dunes with internal structures indicating deposition by density currents whose flowpower decreased with time and with distance from the vent. Syndepositional slumping and minor en masse collapse of crater deposits formed a pile of massive tuffs above which subsequent surge and airfall activity deposited a nestedg inner crater rim. The Medano tuff-ring Tenerife, is a phreatomagmatictype structure whose crater contains reworked tuffs deposited during subsidence into the underlying diatreme. Initial activity ejected much country rock material as magma contacted groundwater at depth but with time eruptions became more strombolian, as water was used up or failed to gain access to the vent. Surges were less common than in the Saefell eruption because the Medano water: magma ratio and explosion depth less often fulfilled the optimum conditions for surge production. The East Lothian diatremes in Scotland are subdivided into two groups on the basis of their infilling. The Red group diatremes contain high proportions of sediment and represent the subsided products of phreatic maars which erupted into a pile of water-rich poorly-consolidated alluvial plain sediments. The later Green group diatremes contain mainly juvenile basalt fragments and formed as phreatomagmatic or sometimes surtseyan tuff-rings, due to magma contacting water at shallow depths or in marginal lakes respectively. The Parade diatreme, Dunbar, contains over 300m of largely base-surge tuffs thought to represent the subsided inner flank deposits of a large maar. The Heads of Ayr and the East Fife diatremes expose different levels in subsided phreatomagmatic tuff-rings due to collapse-ahd erosion. Deep levels such as that exposed at Lundin Links, contain unbedded tuffs and abundant intrusive material. Shallower levels, such as at Elie Ness contain high proportions of bedded tuffs which are often centroclinally orientated. Base-surge, airfall, slumped and reworked tuffs in the Scottish diatremes are directly comparable to deposits in the modern tuff-rings studied proving their origin. A model for the formation of surtseyan tuff-rings is presented, with phreatomagmatic explosions resulting from steam expansion jets which disrupt an already vesiculating magma as it engulfes subsiding water-laden ash. A base surge model is also presented, involving deposition of tuffs with characteristic bedforms and structures by the head, body and tail of each surge analogous to turbidity currents. Cooling of hot, dry steam to cool moist steam towards the rear of surge pulses leads to lag breccias and progressive dune deposits being succeeded by regressive dunes and plastering structures with time. Juvenile sideromelane fragments erupted by phreatomagmatic volcanoes rapidly alter to palagonite as heated pore-waters circulate through the newly-deposited tuffs. Palagonitization results in cation mobility within unstable glass and precipitation of authigenic minerals in voids. Non-equilibrium growth of such minerals results in variable compositions and crystal forms. Subsequent alteration occurs slowly as a weathering process whose rate is greatly reduced as authigenic precipitation closes pore spaces within the tuffs. On diagenesis, unstable alteration products are commonly replaced by chlorite, calcite and clay. Reddening of some tuffs occurs by in situ breakdown of ironbearing minerals and release of Fe to solution although groundwater exchange with red country rock sediments may also occur. Unless present in diatremes phreatomagmatic products have a low preservation potential due to :- extreme alteration, rapid syn- and post-volcanic reworking, low ejecta volumes and breaching and burial beneath later lavas. In contrast the sedimentary structures petrography, morphology and grain size characteristics of diatreme tuffs are shown to be often sufficiently well preserved to permit the identification of their original surface volcanoes and their eruptive histories.