Volcanic and sedimentary processes in phreatomagmatic volcanoes

Phreatomagmatic volcanoes form when ascending magma \nexplosively interacts with surface or groundwater at \nshallow depths. Three types of phreatomagmatic activity \nare recognised- phreaticp phreatomagmatic (s. s. ) and \nsurtseyan - based on the degree of involvement of magma \nwith water and the depth of the interaction. Phreatic \nmaars and phreatomagmatic tuff-rings are underlain by \npipe-like diatremes but these structures are poorly developed or absent in surtseyan tuff-rings. Comparisons \nof phreatomagmatic volcanoes with their eroded diatreme \nequivalents, which contain subsided subaerially-deposited \nmaterial, allow a model for activity of this type to be \nconstructed. \n \nThe Saefell tuff-ring SW Iceland is a surtseyantype \nstructure whose crater remained open to the sea \nduring most of its activity, allowing easy access of water \nto the magma. Base-surges sourced partly from directed \nblasts, formed large dunes with internal structures \nindicating deposition by density currents whose flowpower \ndecreased with time and with distance from the vent. \nSyndepositional slumping and minor en masse collapse of \ncrater deposits formed a pile of massive tuffs above which \nsubsequent surge and airfall activity deposited a nestedg \ninner crater rim. \n \nThe Medano tuff-ring Tenerife, is a phreatomagmatictype \nstructure whose crater contains reworked tuffs \ndeposited during subsidence into the underlying diatreme. \nInitial activity ejected much country rock material as \nmagma contacted groundwater at depth but with time eruptions \nbecame more strombolian, as water was used up or failed to \ngain access to the vent. Surges were less common than in \nthe Saefell eruption because the Medano water: magma ratio \nand explosion depth less often fulfilled the optimum \nconditions for surge production. \n \nThe East Lothian diatremes in Scotland are subdivided \ninto two groups on the basis of their infilling. The Red \ngroup diatremes contain high proportions of sediment and \nrepresent the subsided products of phreatic maars which \nerupted into a pile of water-rich poorly-consolidated \nalluvial plain sediments. The later Green group diatremes \ncontain mainly juvenile basalt fragments and formed as \nphreatomagmatic or sometimes surtseyan tuff-rings, due to \nmagma contacting water at shallow depths or in marginal \nlakes respectively. The Parade diatreme, Dunbar, contains \nover 300m of largely base-surge tuffs thought to represent \nthe subsided inner flank deposits of a large maar. \n \nThe Heads of Ayr and the East Fife diatremes expose \ndifferent levels in subsided phreatomagmatic tuff-rings \ndue to collapse-ahd erosion. Deep levels such as that \nexposed at Lundin Links, contain unbedded tuffs and \nabundant intrusive material. Shallower levels, such as at \nElie Ness contain high proportions of bedded tuffs which \nare often centroclinally orientated. Base-surge, airfall, \nslumped and reworked tuffs in the Scottish diatremes are \ndirectly comparable to deposits in the modern tuff-rings \nstudied proving their origin. \n \nA model for the formation of surtseyan tuff-rings is \npresented, with phreatomagmatic explosions resulting from \nsteam expansion jets which disrupt an already vesiculating \nmagma as it engulfes subsiding water-laden ash. A base surge \nmodel is also presented, involving deposition of \ntuffs with characteristic bedforms and structures by the \nhead, body and tail of each surge analogous to turbidity \ncurrents. Cooling of hot, dry steam to cool moist steam \ntowards the rear of surge pulses leads to lag breccias and \nprogressive dune deposits being succeeded by regressive \ndunes and plastering structures with time. \n \nJuvenile 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 \nvariable compositions and crystal forms. Subsequent \nalteration occurs slowly as a weathering process whose rate \nis greatly reduced as authigenic precipitation closes pore \nspaces within the tuffs. On diagenesis, unstable alteration \nproducts 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. \n \nUnless present in diatremes phreatomagmatic products have a low preservation potential due to :- extreme \nalteration, rapid syn- and post-volcanic reworking, low \nejecta volumes and breaching and burial beneath later lavas. \nIn contrast the sedimentary structures petrography, \nmorphology and grain size characteristics of diatreme tuffs \nare shown to be often sufficiently well preserved to permit \nthe identification of their original surface volcanoes and \ntheir eruptive histories. \n