Polygenetic Volcanoes and Calderas

The phenomenon of hydrovolcanism is not associated solely with eruptions at small, monogenetic volcanoes. The following descriptions illustrate the significance of hydrovolcanic processes in (a) wide-spread tephra deposits from silicic calderas, (b) the development of wet and dry cycles at composite cones, (c) the evolution of calderas, and (d) pyroclastic episodes during the eruption of domes of intermediate to silicic composition (see Chapter 5).

Taupo

The Taupo volcanic zone of New Zealand's North Island is one of the best studied examples of silicic volcanism. An important hydrovolcanic feature of this volcanic field is the extremely widespread, fine-grained silicic tephra deposits, especially those from the Taupo volcanic center (Healy, 1962; 1964).

Self (1983) presented an extensively documented account of the Wairakei eruption (20,000 ka), which produced the Oruanui Pumice Formation (Vucetich and Pullar, 1964) and the Wairakei Breccia, both of which are part of the Wairakei Formation. Self addressed the exceptionally fine grain size, wide dispersal, high content of accretionary lapilli (up to 33 wt%), and irregular thickness distribution—features that Self and Sparks (1978) noted as indicators of silicic, phreatomagmatic (Phreatoplinian ) volcanism. Figure 2.13 illustrates the stratigraphy of the Wairakei Formation, which consists of interbedded, fine-grained pyroclastic fall and flow deposits as well as two main phreatoplinian phases that were followed by ignimbritic phases. Member 1 has a median diameter of 4.0 f (0.064 mm) even near the source and is representative of the typical phreatomagmatic materials shown in Fig. 2.14.

Heiken and Wohletz (1985) described volcanic ash samples and their phreatomagmatic textures from this section. Through interpretation of tephra deposits, Self (1983) illustrated the eruption sequence and phreatomagmatic factors of the Taupo eruption (Fig. 2.15). More detailed descriptions of geothermal studies in the Taupo region are given in Chapter 4.

Vulcano

The Island of Vulcano in the Aeolian archipelago of Italy is a classic example of hydrovolcanic activity. The Fossa cone of Vulcano has been historically active and poses an ongoing hazard (Keller, 1980). Mercalli and Silvestri (1891) observed the most recent eruptive episode and described the eruption phenomena now termed Vulcanian . Frazzetta et al . (1983) built on the work of Sheridan et al . (1981) to interpret the detailed stratigraphy of the cone and show how hydrovolcanism contributed to the five most recent episodes of volcanism; their summary of the Fossa tephra stratigraphy is illustrated in Fig. 2.16. These authors further proposed that all five episodes of volcanism were characterized by a cyclic eruption pattern that consists of the four stages shown in Fig. 2.17.

(1) Initial quiet, fumarolic activity was stimulated by heat transfer from (possibly) two magmas of differing compositions that rose below the volcano.

(2) A triggering event initiated a mixing of the two magmas, which was followed by further rise of the mixed magma to the surface where it contacted ground-water. The resulting hydrovolcanic eruptions of pyroclastic surges

comprised chilled, nonvesiculated tephra that progressed from wet to dry.

(3) As the groundwater source was separated from the magma by a steam envelope, the eruptions became magmatic, expelling vesiculated tephra interspersed with the chilled tephra.

(4) The cycle's final stage is marked by eruption from the pumiceous cap of the magma and, later, extrusion of an obsidian-cored lava flow. Frazzetta et al . (1983) interpreted the products of the most recent eruptive cycle with respect to water:magma ratios, as shown in Fig. 1.22.

Fig. 2.12
An idealized hydrovolcanic deposit section
illustrating typical bedding textures and
bedforms and their inferred water:magma mass
 ratios (R_m ). Initial eruptions, represented by
the  basal pumice fall, involved little or no
external water; however, in later eruptions,
the stratigraphic section records bedforms
that indicate increasing water:magma ratios.
For ratios >1.0, caused by eruptions into a
standing body of water, pillow lavas/breccias
and peperites are usual, as are lahars, which
commonly occur in eruptions of high
water:magma ratios on land.

Vesuvius

The ejecta deposits of another long-active and much-studied volcano, Vesuvius, indicate that hydrovolcanic activity is significant during its eruptive cycles (Barberi et al., 1981; Rosi and Santacroce, 1983). The AD 79 eruptions of Vesuvius are among its best documented in terms of actual observations (Pliny the Younger, 1763; Radice, 1972), deposit descriptions (Sigurdsson et al ., 1985), and interpretations of eruption mechanisms (Sheridan et al ., 1981). Figure 2.18 shows representative tephra stratigraphic sections from archaeological excavations at three Roman sites that were devastated by

Fig. 2.13
Wairakei Formation tephra stratigraphy for
locations within 20 km of the vent in Lake
Taupo, New Zealand. Members 4 and 6 (m4,m6)
were previously named the Oruanui Pumice
Breccia and Wairakei Breccia, respectively.
(Adapted from Self, 1983.)

Fig. 2.14
Grain-size characteristics of the Wairakei Formation (Self, 1983). (a)Median diameter (Md_{f )} ) vs sorting
coefficient (s_f ) is shown for the various members (m1,m2,..) illustrated in Fig. 2.13; also noted are
textural types, including accretionary lapilli (crushed), lithic/pumice-rich ignimbrite, and base surge.
The dashed field represents pyroclastic flows. (Adapted from Wright  et al ., 1981.) (b) Grain-size
fractions for m4 and m6: the dashed field represents pyroclastic flows; coarse variant shown by
symbols. (Adapted from Walker et al., 1980.) (c) Grain diameter frequency curves for fallout products
of m2 and m6 show gradual loss of coarse products with increasing distances from the source
(curve numbers in kilometers). (d) Cumulative probability distribution of size fractions (f ) for Plinian
and phreatoplinian deposits is compared to the distribution of a representative m3 sample.
(Adapted from Carey and Sigurdsson, 1982, and Walker, 1981.)

the eruptions. Fallout deposits of white and gray pumice from early magmatic eruptions were followed by hydromagmatic products emplaced as surges, pyroclastic flows, and lahars—all containing abundant lithic ejecta derived from carbonate aquifer rocks that underline the Somma Vesuvius at a >2-km depth. Figure 2.19, the model presented by Sheridan et al . (1981), interprets the stratigraphy and illustrates the effects of hydrovolcanic activity during the devastating phases of the eruption. Accretionary lapilli, abundant in the upper portions of the tephra stratigraphy, were studied in detail by

Fig. 2.15
Wairakei eruption model showing the sequential stages or eruption of the various
members and periods of lake water/magma interaction.
(Adapted from Self, 1983.)

Sheridan and Wohletz (1983b), who described possible mechanisms for their formation in wet eruption plumes.

Contrasting hydrovolcanic behavior is evident in the early-stage interaction with water at Vulcano and the late-stage interaction shown at Vesuvius. One general explanation for this contrast is the overall hydrologic setting of these volcanoes: Vulcano is an island edifice characterized by abundant near-surface groundwater, whereas Vesuvius is built on a sedimentary platform with a deep aquifer system. Access of water to the vent system at Vulcano gradually decreases during eruptive episodes as magma congeals along vent walls where water initially infiltrates. At Vesuvius, access of groundwater to the magma chamber and vent conduit is initially limited by thermal metamorphic rocks that have sealed fractures. However, as the conduit and chamber wall rocks are fractured by expansion of magmatic gases early in Plinian eruptive episodes, groundwater gains access to the magma, especially after overpressures in the magma body and conduit have fallen below the local thermally perturbed hydrostatic

Fig. 2.16
Composite stratigraphic section illustrates
the hydrovolcanic cycles of the Fossa volcano
at Vulcano, Italy. The Pietre Nere, Palizzi,
Commenda, and Pietre Cotte cycles all show
a progression from hydrovolcanic eruptions to
emplacement of lava flows.
(Adapted from Frazzetta et al ., 1983.)

pressure. The behaviors exhibited at Vulcano and Vesuvius are generally termed shallow and deep hydrovolcanic eruptions, respectively; the former becomes dryer and the latter becomes wetter as eruptions progress.

Throughout the entire Latium volcanic province of Italy, hydrovolcanism has been a vital component in the development of caldera complexes such as those of Vulsini, Vico, Sabatini, Albani, and the Phlegraen Fields. Broad, low-profile calderas with widespread, fine-grained silicic tephra characterize these volcanic areas. Because of their geothermal importance, we will describe them in detail in Chapter 4.