Nature of Hydrovolcanic Phenomena

The physical phenomena of hydrovolcanism belong to a class of well-studied physical processes termed fuel-coolant interactions (FCI). Fig. 1.20 depicts a hypothetical geologic

Fig. 1.19
illustration of the calculated pyroclastic surge
generated during initial moments of Plinian
eruption. (a)An overpressured burst propagates
a bow shock into the atmosphere ahead of
lithic-rich ash as a rarefaction wave recedes
into the magma reservoir. (b and c) Reflected
rarefactions from the reservoir and flow margins
form weak shocks that accelerate ash in surges.
Together, these phenomena constitute a blast
wave that precedes (d) the flow of
juvenile ash from the vent.
(Adapted from Wohletz et al ., 1984).

system in which magma (fuel) explosively interacts with water-saturated sediments (coolant). This process occurs in stages of (a) initial contact and steam-film development, (b) coarse mixing of magma and water or water-rich rock, (c) vapor expansion and flow, and finally (d) explosion and fine fragmentation of the magma. The process does not necessarily evolve through all these stages and may be arrested, for instance, before mixing or explosion.

Fig. 1.20
Hypothetical setting of subsurface hydrovolcanic activity, showing (a) initial contact of magma with
water-saturated sediments, (b) vapor film growth, (c) mixing of magma with the sediments, and
(d) expansion of the high-pressure steam in an explosion.
(Adapted from Sheridan and Wohletz, 1983a.)

Much of our theoretical-understanding of hydrovolcanism has developed from laboratory experiments (for example, Wohletz and McQueen, 1984). This approach has made it possible to quantify some controlling parameters by using field and laboratory measurements of hydrovolcanic products. Figure 1.21 shows results from early experiments (Sheridan and Wohletz, 1983a), in which the mass ratio of water and magma (thermite analog) interacted and confining pressure controlled the explosive efficiency of the system.

The thermodynamics of heat transfer is also a significant aspect of hydrovolcanic systems and their physical and chemical effects. The mechanical work produced by interaction of magma with external water is partitioned into many possible modes, including fragmentation of the magma and country rock; excavation of a crater; dispersal of tephra; seismic and acoustic perturbations; and chemical processes such as solution and precipitation, mass diffusion, and magma quenching and crystallization.

Fig. 1.21
Some results of experimental studies (indicated by number) by Wohletz and McQueen (1984)
showing explosive efficiency as a function of water-to-magma mass ratio and confining pressure.
Note the apparent maximum near 0.5 and the increased explosivity of high-confinement
pressure tests. Explosive interaction occurred at 5200 psi (~350 bars) confining pressure,
which is well above the previously assumed limit at water's critical pressure.

This work, D W_sys , is the sum of changes in kinetic energy (D E_k ), potential energy (D E_p ), and volume-change work (pD V_sys ), which is given by

There are several ways by which to evaluate the above expression, but one of the most direct methods is to estimate the change in the internal energy of the water/magma mixture (D U_mix ); by definition D W_sysº -D U_mix , where

and m = mass, U = internal energy, and subscripts w and m denote water and magma, respectively. Wohletz (1986) demonstrated a method for evaluating Eq. (1-23) that requires data from extended steam tables (Burnham et al ., 1969; Haar et al ., 1984). Further consideration of the mixing and explosion stage (Wohletz, 1986) yields information on particle velocities and sizes. Heiken et al . (1988) extended these calculations to explain country rock fracture by hydrovolcanic processes.

Experimental investigations of water/magma interactions have displayed a variety of explosive and nonexplosive behaviors that are analogous to natural volcanic activity. These results support observations of hydrovolcanic eruptions in which a wide variety of classical eruption types (for example, Strombolian,

Surtseyan, Vulcanian, and Plinian) have involved external water. There is additional evidence of hydrovolcanism in a variety of landforms that range from small maar/tuffring craters to some large caldera outflow sheets of tephra. Such features as peperites, mud volcanoes, hydrothermal explosion pits, pillow lavas and breccias, and parts of composite cones can also be attributed to hydrovolcanic activity. These eruptive behaviors and resulting landforms are, in a general fashion, related to the degree of water interaction, as is shown in Fig. 1.22.