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.