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.