Hydrothermal Systems Associated with Domes

Role of Water in Dome Eruptions

In the above descriptions of dome-related tephra, water from either meteoric or magmatic sources is the primary volatile phase that leads to pyroclast formation. Magmatic water is widely recognized as a primary phase that contributes to the formation of pumice and promotes Plinian tephra emissions. The same water has a profound effect on the development of dome lava textures such as flow foliation, crystallization, vesiculation, and fumarolic (vapor-phase) alteration. The latter effect is especially important in weakening the dome and making it vulnerable to future phreatic or Merapian bursts that produce tephra. On the other hand, meteoric waters also contribute to fumarolic and phreatic activity, and it may be difficult to distinguish its effects from those of magmatic water.

Because water is of such consequence in hydrothermal systems, it is important to identify any evidence of its presence in the dome. The best evidence is hydrovolcanic activity, which is common in the pyroclastic activity that precedes dome extrusion. However, meteoric water is also vital for Vulcanian activity during dome growth. The presence of lahars on dome flanks is most reliable in identifying the presence of meteoric water, whether from rainfall or from saturated conditions of fractured, porous lavas beneath the dome. In the case of either magmatic or hydrovolcanic explosive activity, tephra layers provide avenues for water circulation in hydrothermal systems that are driven by residual magma heat below the dome.

Heiken and Wohletz (1987) discussed the migration of magmatic and meteoric water in dome magma systems as a diffusion process and emphasized that this process can proceed at many different rates, often

Fig. 5.8
Schematic illustration of Vulcanian tephra
associated with dome destruction in
(a) preeruptive and (b) eruptive stages.
(Adapted from Heiken and Wohletz, 1987.)

well above those established by laboratory measurements (Shaw, 1972). High effective diffusion rates are expected for hydrovolcanic situations in which meteoric water diffuses from host rocks into the rising magma to cause magma/water interactions such as explosive phenomena and the pervasive chemical alteration of tephra. The development of lithophysae and vaporphase alteration and crystallization, which are generally attributed to magmatic water diffusion in extruded lava, can proceed at rates higher than those expected for slow molecular diffusion because of fracture flow of vapors produced by degassing and devitrification of glass to nonhydrous phases. We mention these processes to emphasize the importance of recognition of the presence of water in silicic dome systems.

Fig. 5.9
Schematic illustration of Peléean and Merapian
dome destruction. (a) An initial failure of the
dome leads to a landslide followed by
decompression of trapped vapors, which
produces a rapidly moving pyroclastic avalanche
and surge. (b) This scenario, described by
Fisher and Heiken (1982), can be promoted by
intrusion of volatile-rich magma, which may vent
after dome failure (c). (d) Destruction of lava
flow fronts that are weakened by fumarolic
activity also produces block and ash flows,
as proposed by Rose et al . (1976).
(Adapted from Heiken and Wohletz, 1987.)

Geothermal System Models

The following discussion of geothermal systems associated with domes presents some possible models for hydrothermal circulation that reflect conditions observed in the field, some of which have contributed to exploitable geothermal systems. These models include systems associated with

Fig. 5.10
Schematic illustration of phreatic tephra
associated with silicic domes. (a) A passive
fumarolic stage might be enhanced by a rapid
change in hydrothermal flux so that (b) vigorous
vapor emission opens a vent and discharges
lithic ash and lahars (Heiken  et al ., 1980)
(Adapted from Heiken and Wohletz, 1987.)

· tephra aprons surrounding domes,

· cratered domes,

· faulted domes,

· dome complexes,

· caldera ring fault domes, and

· caldera resurgent extrusive domes.

The models highlight the importance of pyroclastic materials in locating hydrothermal systems. Each of these dome types may be modeled by the heat flow program discussed in Chapter 2.

A review of pyroclastic rock physical properties indicates that pumiceous materials range in bulk density from 200 to 1200 kg/m³ , and

bedded ash—often less vesicular because of its hydrovolcanic origin—may range from 1000 to 1500 kg/m³ . If a particle density of 2300 kg/m³ is assumed, pumice has a void space between 50 and 90%; bedded ash varies from 35 to 60% void space. The primary permeability of pyroclastic rocks is provided by both vesicles and intergranular spaces. Whitham and Sparks (1986) showed that at temperatures >150°C, the pumice vesicles are effectively interconnected and readily allow absorption and movement of water. However, the primary permeability of pyroclastic materials can rapidly decay during hydrothermal circulation because circulation promotes the solution of glass and the redeposition of silica and secondary minerals that effectively seal the tephra.

Tephra Aprons

Figure 5.11 depicts a hypothetical dome extruded over a tephra collar that resulted from initial explosive eruptions. The collar extends down into the vent area and defines the crater and tuff ring apron onto which lava was extruded. Porous tephra allows hydrothermal fluids to rise convectively. The heat source is a magma conduit below the dome; water in the country rock below the dome promotes heat convection upward into the tephra. In this situation, fumarolic activity at the base of the dome demonstrates the existence of the hydrothermal system, which is locally capped by dome lavas. The dome lavas have a lower permeability and do not permit much heat transfer.

Cratered Domes

Figure 5.12 depicts a second hypothetical case: a dome with a crater formed by either Vulcanian or phreatic activity that was driven by intrusion of magma below the dome. Below the crater, a region of strongly fractured lava and breccia provides a pathway for the convective rise of hydrothermal fluids from the cooling magma at depth. Here again, the water supply is from country

Fig. 5.11
Schematic illustration of geothermal system in
which a vapor-dominated zone is concentrated
in the porous tephra apron. This very
hypothetical system is based on heat
transported convectively from a magma
chamber at depth below the dome upwards
along the vent conduit.

rock into which the magma has intruded. In such a situation, the dome will have a mantle of Vulcanian or phreatic tephra that provides evidence of the explosive origin of the crater. By examining the isotopic and chemical nature of fumarolic gases and rock alteration in the crater, it is possible to determine the origin of the vapors, whether solely magmatic (and hence limited) or meteoric and of potential economic significance.

Faulted Domes

Figure 5.13 presents a dome cut by a fault that has triggered collapse of the dome— perhaps in a Merapian fashion. The fault has fractured lavas and basement rock sufficiently to allow strong convection of hydrothermal fluids into the dome. The fumarolic alteration of fault breccias and related pyroclastic breccias is evidence of the convective heat source. In this case, the tectonic activity has altered heat flow from depth and, where residual magmatic heat exists below the dome, the faulting provides a new circulation pathway.

Dome Complexes

Where numerous dome extrusions have occurred in a volcanic field (for example, along the ring fault system of a caldera or within a graben), lavas and dome-related tephra layers overlap. The lavas act as caprocks and the pyroclastic layers serve as geothermal reservoirs. Figure 5.14 illustrates such a situation in which heat from the youngest eruptive activity drives hydrothermal circulation below: steam reservoirs exist at some depth below the dome and in older tephra layers. In areas consisting of many overlapping dome extrusions, high conductive heat flow at the surface may indicate a convective system at depth.

Caldera Domes

Two cases of caldera-related dome extrusions are depicted in Fig. 5.15. In one case, domes along the ring faults manifest magma intrusions at depth that control the

Fig. 5.12
A cratered dome may have a prolonged
fumarolic stage during which a convective
system develops within the crater conduit in
response to a new magma body
intruded at depth.

overall heat flow toward the margins of the caldera. In the other case, resurgence of the caldera creates a structural or extrusive dome that may produce hydrothermal convection towards the center of the caldera. In the first instance, recharge of the hydrothermal system is strongly controlled by the hydrology of the down-thrown region of the caldera, which acts as a ground-water concentrator or trap. In the instance of the resurgent caldera, the hydrothermal system is recharged from higher topographic regions surrounding the caldera. Both of these models are simplified, but they demonstrate the way convective heat flow in calderas can develop in diverse manners. There is no simple rule-of-thumb that is adequate for determining where to explore in a caldera. The existence and locations of young silicic domes are helpful in predicting recharge and outflow areas of possible hydrothermal systems.

Fig. 5.13
Tectonic activity at a relatively young dome
produces fracture pathways along newly
activated faults; this activity allows deep
circulation of meteoric water downwards to a
still-hot magma body at depth. Such a faulted
dome may develop vigorous fumarolic activity
as a surface manifestation of a subsurface
hydrothermal system, but gradual sealing of
 the fractures by alteration minerals and silica
 will eventually slow the fluid convection.

Fig. 5.14
Dome complexes form when lavas overlap their related pyroclastic sheets. The stratigraphy of a dome
field is characterized by various potential hydrothermal reservoirs in porous and formation-permeable
pyroclastic strata that are capped by one or more impervious lavas. Most recent dome eruptions
result from renewed thermal infusion from related magma bodies at depth. Vapor-dominated reservoirs
characterize upper pyroclastic horizons, whereas deeper ones may be brine-filled.

Fig. 5.15
Caldera domes form both (a) along ring fracture zones of calderas and (b) within the resurgent cores.
The character of caldera-fill materials and their formation permeability works in conjunction with
caldera faults to allow deep circulation of meteoric water. The magma conduits below such zones
probably occur directly above the caldera magma chamber, where it is closest to the surface.
Because silicic caldera magma bodies are relatively large, hydrothermal systems developing below
related dome structures may have prolonged activity and high heat flux.

Structural Influences

In addition to the localized stratigraphic and volcanic structure that influences the development of geothermal systems associated with domes, regional structure and tectonics can also strongly affect these hydrothermal systems. Six structural settings for domes and their effect on the development of hydrothermal circulation are (1) caldera faults; (2) extensional block faulting; (3) tectonic plate convergence; (4) basin fill; (5) intrusive deformation, sector grabens, and radial faults around volcanoes; and (6) volcanic fracture systems. These features, four of which we briefly discuss here, are of more regional significance than the models discussed above.

Caldera Faults

Figure 5.16 shows the pattern created by the intersection of regional linear faults with the ring faults of a caldera. In this example drawn from the Qualibou caldera on the island of St. Lucia in the West Indies (Wohletz et al ., 1986), the youngest domes are expressions of caldera resurgence. The regional and still active fault system controls the location of fumarolic activity

Fig. 5.16
Structural control of domes associated with
calderas is generally related to regional fault
trends and caldera ring fractures. Because
regional faults may remain active long after
caldera tectonics have declined, the most
recent dome activity will be found
along such faults.

near the young extrusions. Young faults are important because they maintain a fracture permeability in rocks that otherwise are sealed by secondary minerals and silica. Where regional faults intersect caldera ring faults, greater fracture permeability can also be expected. Although the presence of young dome lavas and pyroclastic materials is useful for determining a potential geothermal area, it is the structural elements that are the most significant when actually locating a hydrothermal system.

Block Faulting and Grabens

Strongly developed block faults can localize extrusive features such as domes. Figure 5.17 depicts a hypothetical dome that is emplaced along the margins of a graben deeply filled with several kilometers of sediments. The hydrology of the graben is influenced by both the topographically high recharge areas in surrounding horst blocks and the deep, permeable graben fill. Such a situation promotes not only deep circulation of meteoric waters but also the development of extensive hydrothermal convection along margins of the intrusive body below the dome. In such cases, it is difficult to characterize the geothermal potential as either volcanic or tectonic. Block faulting in extensional areas may result in a thinner crust and greater heat flow so that deep circulation of meteoric water alone can develop a geothermal system. However, the same tectonic regime is typical of volcanic systems that develop in rifts.

Intrusive Deformation

Silicic domes indicate the existence of intrusive bodies at depth. Figure 5.18 illustrates a situation in which a laccolith is associated with a dome in a compressed tectonic region. The laccolithic intrusion has domed and created concentric and radial faults in the country rocks above it. An intrusion such as a laccolith or a buried

dike can produce small phreatic craters (Miller, 1985; Fink, 1985) of structural features of localized extensional surface deformation (Mastin and Pollard, 1990).

Henry and Price (1990) described laccolithic doming associated with caldera formation in the Christmas Mountains of Trans-Pecos, Texas. Most of the magmatism in this area is expressed as small (1- to 4-km-diameter) laccolithic intrusions that commonly involve high-silica, peralkaline rhyolites and quartz trachyte. These intrusions have formed structural domes of steeply tilted sedimentary strata. Field relations show that these domes were emplaced along ring fractures during the evolution of larger caldera structures. Numerous sills occurring around the flanks of the laccolithic domes are cut by radial faults that probably formed in response to the deformation associated with dome intrusion.

Basin Fill

A basin filled with lahars shed from a dome on the basin margin is shown in Fig. 5.19. Lahars can be impervious to fluid flow because of the cementation mechanisms promoted by ash carried in lahars. In the model shown, the lahars have capped pyroclastic materials erupted earlier from the dome. Hydrothermal circulation below the dome is confined within the pyroclastic strata, and hot water flows from the dome into the basin below the laharic apron. In such cases, lahars mask the geothermal system at depth and geophysical or geochemical methods are needed to confirm the existence of the system in the basin.

Fig. 5.17
The block faults that bound basins in extensional terranes frequently have long histories and deep
extensions. This structure may be accompanied by sufficient crustal heat flow to cause crustal
melting (for example, by basalt intrusion) and silicic dome emplacement. Deep circulation of meteoric
water from the horst highlands through the graben-filling sediments can promote hydrothermal
activity within graben-bounding fault breccias and along intrusions (hachured) below domes
(v pattern) that were extruded along these faults.

Fig. 5.18
Intrusive deformation associated with domes is
manifested by uparching and fracturing of rocks
above a laccolith. Hydrothermal circulation
can then develop around and above
the laccolith as it cools.