Hydrovolcanic Cycles and Geothermal Energy

Hydrovolcanic phenomena occur in regular patterns at some volcanoes and thus can assist in defining cycles that in turn are useful in both predictions of future activity and estimates of subsurface hydrological conditions. The eruptive cycles portrayed in Fig. 1.25, for example, show the changing availability of groundwater during periods of activity at several volcanoes. Cycles can be documented by careful field and laboratory analyses of volcanic products in which the abundance of erupted steam and its temperature are constrained by textural indicators of grain cohesion, deposit mobility as a function of moisture abundance, and degree of clast alteration. Cycles are characterized as "wet" when the volcanic products indicate an increase of water during the eruptions; "dry" cycles produce tephra that indicate decreasing water abundance throughout the eruption. The nature of these water indicators also demonstrates whether the erupted steam is saturated (wet) or super-heated (dry). As a general rule, locations that show wet cycles might be better candidates for geothermal exploration because

Fig. 1.22
Relationship of eruptive phenomena, deposit type, and landform to water-to-magma interaction ratio.
(Adapted from Sheridan and Wohletz, 1983a.)

Fig. 1.23
Correlation of deposit texture and grain size to water-to-magma ratio.
(Adapted from Frazzetta et al., 1983 and Sheridan and Wohletz, 1983a.)

Fig. 1.24
Sketches of pyroclast textures resulting from hydrovolcanism. These textures include
(a) a characteristic blocky and equant glass shard, (b) a vesicular grain shard with cleaved vesicle
surfaces, (c) a platy shard, (d) a drop-like or fused shard, (e) a blocky crystal with conchoidal fracture
surfaces, and (f) a perfect crystal with layer of vesicular glass.
(Adapted from Sheridan and Wohletz, 1983a.)

they prove that water is sufficiently abundant in the volcanic system to quench the magma to water-vaporization temperatures. When estimating the volume of erupted hydroclastic products, this general rule constrains the volume of water involved in the eruptions and thus provides a measure of water abundance in the volcanic system.

Funiciello et al . (1976) pointed out the correlation between geothermal localities and phreatomagmatic volcanoes in Italy, especially those showing wet cycles. In addition, these authors demonstrated how the study of phreatomagmatic products helps locate and characterize a geothermal reservoir with respect to its lithology and fracture permeability, topics that Heiken et al . (1988) discussed in further detail. These studies provide an excellent background for our discussion of hydrovolcanism in Chapter 2.

Fig. 1.25
Various cycles of hydrovolcanism displayed by several type of studied volcanoes. Temporal
variations of water-to-magma mass ratios are shown for (1) Crater Elegante, Mexico;
(2) Kilbourne Hole, New Mexico; (3) Peridot Mesa, Arizona; (4) Taal volcano, Philippines;
(5) Ubehebe crater, California; (6) Zuni Salt Lake, New Mexico; (7) Cerro Colorado, Mexico;
(8) Diamond Head, Hawaii; (9) Koko Crater, Hawaii; (10) Pavant Butte, Utah; and (11) Surtsey,
Iceland. These cycles illustrate general trends (see Sheridan and Wohletz, 1983a, Fig. 5), including
wet to dry (well demonstrated by Vulcano in the Aeolian Islands, Italy) and dry to wet
(activity characteristic of Vesuvius). Some volcanoes show reversals in cyclic activity
(7, 9, and 10 here are tuff cone structures), whereas repeated cycles between dry (Strombolian)
and wet (Surtseyan) occur at others (5).
(Adapted from Wohletz and Sheridan, 1983a.)