Kilauea Volcano and Kapoho Geothermal Area of Hawaii

The Hawaiian-Emperor Island chain consists of ~107 volcanoes that range in age from 80 Ma at the northwestern end to currently active volcanoes at the southwestern end (Decker et al ., 1987). The islands are remnants of these volcanoes, which rise between 5,000 and 10,000 m above the floor of the Pacific Ocean. The chain was formed as the Pacific plate moved over the Hawaiian hot spot at 9.2 cm/yr (Hawaiian Chain) and 8.6 cm/yr (Emperor Chain) (Clague and Dalrymple, 1987). Subaerial portions of these islands consist of thousands of thin basaltic lava flows and minor deposits of pyroclastic rocks and differentiated lavas. Hawaii, the youngest island in the chain, is made up of five overlapping shield volcanoes, two of which remain active (Fig. 6.6).

The still-active shield volcanoes of Kilauea and Mauna Loa are believed to be made up of a below-sea-level mass of submarine pillow basalt that is interbedded with and overlain by hyaloclastite deposits and subaerial basalt flows. Hill and Zucca (1987) report that under the Kilauea and Mauna Loa shields, the Mohorovicic

Fig. 6.4
Map and cross-section of Kilauea Iki Crater before and after the 1959 eruption. The deep lava lake
has been drilled many times for research purposes and has been the testbed for the U.S.
Department of Energy's magma energy concepts.
(Adapted from Richter and Moore, 1966.)

Fig. 6.5
Models and observations of the thermal history of the Kilauea Iki lava lake. (a) Model of cooling
above a downward-moving thermal front at 2000, 4000, and 6000 days. (b) An early model proposed
to explain the drastic temperature change in the one-phase advective (dry-out) zone. (c) This
summary of the major physical characteristics of the lava lake in 1980 (20 years after the eruption)
shows depth to melt, changes in resistivity, temperature, percent of glass,
and depth (120 m) to the preflow surface (lake bottom).
(Adapted from Hermance and Colp, 1980.)

discontinuity increases from ~10 km (for normal oceanic crust) to 13 and 18 km, respectively. Fast P-wave travel times through rift zones and summit calderas as well as positive gravity anomalies over them indicate that intrusive cores form a significant fraction of these volcanic edifices (Fig. 6.6).

Eight of the youngest and largest islands in the chain make up the State of Hawaii, which is an area of intense urban and agricultural development. A comprehensive evaluation of Hawaii's geothermal resources, conducted by university and federal scientists (Thomas et al ., 1979, 1983), concentrated on calderas and associated rift zones. Much of the work focused on the Island of Hawaii and drew upon the vast research base built by the State of Hawaii, the Hawaiian Volcano Observatory, research drilling programs, and commercial geothermal drilling projects. The report cites an output of 3MW_e at the one operating hydrothermal electrical generating plant, which is located at the Kapoho geothermal site on Kilauea Volcano's east rift zone. At this time, a well field is being drilled and a 25-MW_e (net) power plant is being constructed in the Kapoho area (Clark and Stewart, 1991).

Migration of Magma and Evaluation of Thermal Sources

As is the case for all hydrothermal systems, geothermal system development in Hawaii depends on an understanding of volcanic heat sources and groundwater. In this area, where there is little surface water and a constant volcanic hazard, geoscientists have achieved a good understanding of both of these crucial aspects. Most of the geothermal exploration on the Island of Hawaii is concentrated on Kilauea Volcano, where the storage, migration, and eruption of basaltic magmas is monitored by the Hawaiian Volcano Observatory's geophysical network.

In this area, magma rises buoyantly along a very irregular network of vertical, roughly cylindrical conduits that are marked at the surface by a caldera. Kilauea Caldera, located at the summit of the volcano, consists of concentric collapse craters, the widest of which is 4.5 by 3 km. Helz (1987) has determined that the picritic (~20% olivine phenocrysts) basalt of the 1959 Kilauea eruption originated at depths of 45 to 60 km and rose to the surface at velocities of 0.58 to 0.77 cm/s; which is fast enough to carry along olivine xenocrysts and aggregates.

Eruptions in calderas and along rifts are spectacular—with lava fountains, fast-moving lava flows, and occasional phreatomagmatic activity—however, most of the magma never reaches the surface but comes to rest in a shallow crustal environment. These magma bodies, located within the uppermost 7 km of the crust, are responsible for the heat that drives geothermal systems on Kilauea. Ryan (1987a,b) addressed the reason most magma comes to rest at shallow levels in the crust in his elegant study on the regions of neutral buoyancy. Ryan integrated seismic and surface deformation data with measurements of the physical properties of rock and magmas under pressure to provide information on the variation of crustal rock densities with depth. Below 9 km, all macrofractures, microfractures, vesicles, and joints are eliminated by bulk compression [Fig. 6.7(a)]. Below 7 km, magma is transported buoyantly within the central conduit and the intensity of hydraulic fracturing is high. Between 7 and 2 km, the in-situ densities of the fractured crustal rock and the magma are similar, and a magma

Fig. 6.6
(a) Map of the island of Hawaii shows the major rift-zones of the five overlapping shield volcanoes.
Elevation and depth contours are in 500-m intervals on land and 1,000-m intervals offshore.
(b) This schematic cross-section of the central and western Mauna Loa shield is based on the
P-wave velocity and density models of Hill and Zucca (1987).

body is formed at a point where buoyant rise is no longer possible; the fact that this is an increasingly aseismic region indicates a higher fluid/rock ratio (Fig. 6.7). The center of neutral buoyancy is located at depths of 2.5 to 4.5 km, suggesting a correspondence between the depths at which magma is in mechanical equilibrium with the surrounding rock and the depth of Kilauea's subcaldera magma reservoir. From depths of 0 to 2 km, the deformed and fractured crust allows vesiculating magmas to pass through dikes. As the volcano grows, the summit (subcaldera) magma reservoir and associated rift system is elevated and achieves mechanical equilibrium within the lava shield (Ryan, 1987a,b). The entire igneous system rises with both time and a continuing supply of magma, but it retains the same depths below the volcano's surface. This is an important concept to keep in mind when evaluating thermal sources within shield volcanoes similar to those at Kilauea and Mauna Loa.

Ryan (1987a,b) determined that most magma movement into rift zones from the summit reservoir is at a depth of 3 km—a level of neutral buoyancy. Magma moves rapidly along the rift until the fluid pressure falls below that of the tensile strength of the host rock. Ryan (1987a,b) cites three types of dike formation.

(1) Slow movement of magma, with a gradually enlarging fracture front. The dike top rises toward the surface and the bottom descends at a similar rate.

(2) Rapid movement of magma, during which the dike top rises rapidly and, simultaneously, the base sinks. Subsequent pressure reduction narrows the dike and restricts it to the neutral buoyancy zone.

(3) Pressure differentials within the growing dike form an intrusion shaped like a doubly serrated knife. The "serrations" have amplitudes of 2 to 6 km. Where a rising dike intersects the surface, cracks open and a rift eruption begins, as is depicted in Fig. 6.8.

These models have been developed from data collected over the last 20 or 30 years, when activity at Kilauea volcano has been mostly along the rifts.

A well-exposed analog to Kilauea volcano is the Koolau volcano on Oahu, for which Walker (1986; 1987) described the internal structure. The shield of Koolau was constructed by small but frequent eruptions of basaltic magma. Erosion has eroded the 57-km-long volcano to a depth of ~1 km and exposed the plumbing within its shield. Kailua caldera, located at the southeast end of the Koolau volcano, is analogous to Kilauea Caldera and consists of mainly thick, massive lava flows. It lacks the thin pahoehoe lava flows that characterize the Kilauea shield volcano. Breccias within the 4-km-diameter Kailua caldera may represent either periods of collapse or phreatic blasts. A rift extending northwest from the caldera is composed of ~5100 dikes over its 3.3-km width, as is illustrated in Fig. 6.9. The rift

Fig. 6.7
Earthquake abundance and in-situ density with depth beneath Kilauea volcano.
(a) Distribution of earthquakes beneath Kilauea's summit region, to a depth of 20 km.
The dark pattern refers to the volume beneath the whole Kilauea caldera and the light
pattern refers to the volume beneath only Halemaumau crater. The aseismic
region from 2 to 7 km is believed to have high magma:rock ratios.
(Adapted from Ryan, 1987a.)
(b) In-situ  densities of olivine tholeiitic basalt near its liquidus, volcanic shields, and the upper
mantle below Hawaii. The depth region of density crossover coincides with the subcaldera region of
magma storage below Kilauea; based on data from Salisbury and Christensen, 1976; Fuji and
Kushiro, 1976; and Zucca, Hill, and Kovach, 1982.
(Adapted from Ryan, 1987a.)

Fig. 6.7
(c) Schematic diagrams show the evolution of oceanic shield volcanoes such as Hawaii, progressing
from Mokuaweoweo caldera of Mauna Loa volcano (oldest) to the Loihi seamount (youngest).
Low r  = low density; V_p  = P-wave velocity; and V_s  = shear-wave velocity. As the volcano grows,
it carries with it its contractancy profile and regions of fracturing; the region of neutral buoyancy
rises from below sea level to well above sea level.
(Adapted from Ryan, 1987b.)

follows listric faults that dip outward on either side of the Koolau shield. Dikes range in width from <5 to 670 cm, with a median of 53 cm, and typically have glassy, chilled margins and sheetlike cooling joints that are perpendicular to dike margins. These dikes sometimes occur in clusters (or swarms) as wide as 20 m, in which successive members were injected either along the margins or inside preceding dikes. Within such clusters, dikes may make up 100% of the rock.

Most of Hawaii's recent exploration and drilling for hydrothermal development has been along the east rift zone of Kilauea Volcano. The subaerial part of Kilauea is 80 km long and 20 km wide and still growing. [For the most recent compilation of geologic maps, refer to Holcomb (1987)]. Kilauea rises to an elevation of 1240 m above sea level and serves as a topographic barricade to the trade winds and rainfall; as a consequence, the eastern slopes of the volcano are covered with dense vegetation and the southwestern slopes are a desert— an important observation to consider in planning geothermal development. Kilauea is young and very active: 90% of the surface is younger than 1000 years (Holcomb, 1987). Approximately 50% of the volcano surface is covered with lavas that overflowed the summit caldera, but such an event has not occurred for 200 years; 81% of the lava flows are pahoehoe , and aa flows make up a smaller volume. During the last 500 years, periods of sustained summit activity have included only minimal flank (rift) activity. However, when there is little activity at the summit, other than caldera collapse, the flanks are more active.

Kilauea's caldera is actually a collection of nested (but not concentric) calderas, in which the outermost visible caldera is 7.5 by 5.1 km and the innermost (Halemaumau Crater) is 0.9 km in diameter. The subaerial portion of Kilauea's east rift zone is 50 km long and 2 to 3 km wide; the actual rift extends about 50 km further below sea level. Figure 6.10 illustrates the surface manifestations of rift tectonic and volcanic activity: normal faults, open fissures, and pit craters; cinder cones, spatter ramparts, tuff cones, and steaming ground are associated with these features, but are not shown on this map.

Hydrothermal Systems at Kilauea Caldera and Along Its East Rift Zone

A 1.2-km-deep research drillhole, located on the southwest margin of Kilauea caldera, was drilled to a depth just below sea level to test hypotheses concerning the thermal state and groundwater movement near a magma body (Keller et al ., 1979). The drilling operation encountered lava flows as well as a few sills and ash layers. Porosity decreases with depth, which is commonly the result of pore spaces filling with secondary minerals. Permeabilities above the water table are 100 mD to 1 D; below the water table they are <100µD (Zablocki et al ., 1974).

Within the Kilauea well, temperatures of 20 to 30°C are maintained with increasing depth until, at 480 m, the water table is reached. At that depth, temperatures rise rapidly in lavas, which are saturated with brackish water below the water table; the temperatures begin to decrease at a depth of 725 m (Fig. 6.11). Below 725 m, the thermal gradient is conductive and reaches a temperature of 137°C at the bottom of the drill-hole. Zablocki et al . (1974) concluded that if this gradient persists, magmatic temperatures should be found at a depth of 4 km.

Figure 6.12 shows the locations of five deep (>2000 m) and four shallow exploration wells drilled in the Puna District along Kilauea's east rift zone in the 1960s, 1970s, and early 1980s. The first two wells were drilled adjacent to vents of the 1955 eruption, where there is steaming ground. These wells, drilled to depths of 54 and 167 m where the maximum temperatures were 54 and 102°C, respectively, were sited in the unsaturated zone and no geothermal resource was found (Thomas et al ., 1983).

Fig. 6.8
Three-dimensional model of the internal structure of Kilauea volcano. The medium stippled pattern
denotes the structure of the southwest and eastern rift zones that extend outward from the summit
magma reservoir, as well as the main conduit that rises from a depth of 40 km. The reservoir is a lightly
shaded region at a depth of 2 to 7 km. Periodic high-level injection of magma into the rift zones occurs
along the horizon of neutral buoyancy (arrowed pathways) and is associated with the lateral
formation of dikes at 3 km below the volcano's surface.
(Adapted from Ryan, 1987b.)

Fig. 6.9
Dike swarms of the ancient Koolau dike complex and Kailua caldera. (the inset shows dike intensity
as well as the number of dikes per cluster and site). This ancient shield caldera and rift zone is
analogous to the modern Kilauea volcano and is used to
interpret the younger volcano's structural framework.
(Adapted from Walker, 1987.)

Fig. 6.10
Geology of Kilauea volcano. (a) Structural map depicting Kilauea caldera, pit craters, normal faults,
fissures and eruption fissures. HGP-A is the location of the Hawaii Geothermal Project well in
the Puna District. The Kilauea research drill hole is denoted GW. (b) Map of Kilauea volcano
showing lava flows classified by eruption type.
(Adapted from Holcomb, 1987.)

Fig. 6.11
(a) Cross-section of Kilauea caldera including the Kilauea Research drill hole (GW). (b) Temperature
logs from the Kilauea drill hole. There is no increase in temperature until the water table is reached
at a depth of 480 m. Below 725 m, the gradient is conductive
it reaches a maximum of 137°C at a depth of 1250 m.
(Adapted from Zablocki et al ., 1974.)

Fig. 6.12
(a) The lower east rift of Kilauea volcano indicating the limits of the surface expression of the rift
and the location of groundwater and geothermal wells. Contour intervals are 200 ft. The inset shows
topography, the apparent water table, and self-potential for profile A—A'.
(Adapted from Thomas, 1987.)

In 1976, a government-sponsored well (HGP-A) was drilled near the center of the east rift zone at an elevation of 200 m and ~300 m east of a spatter rampart formed during the 1955 eruption. This well is located near the intersection of the north-east-trending rift and a northwest-trending, right lateral fault mapped by Holcomb (1987). The drillhole is also 28 km west of and 1043 m lower than Kilauea's summit caldera. It was drilled to a depth of 1968 m and has a bottom-hole temperature of 358°C (Fig. 6.13). The reservoir is located in lava flows and dikes, where Stone and Fan (1978) reported three zones of hydrothermal alteration: (a) 675 to 1894 m—montmorillonite, (b) 1350 to 1894 m—chlorite, and (c) 1894 to 1962 m—actinolite and calcite. These authors concluded that the present-day thermal regime is related to recent

Fig. 6.13
Temperature profiles measured in the Hawaii Geothermal Project well HGP-A. The inset is a
conceptual cross section of the lower east rift of Kilauea that depicts fluid circulation across
the rift zone. Thermal waters leaving the rift to the southeast (downslope) form a plume overlying
the colder fluids below. Drilling on the downslope side of the rift would initially encounter hot
fluids but at greater depths would enter cool aquifers.

magma injection. The fluids are slightly saline but have high concentrations of silica and sulfide (Kroopnick et al ., 1978). Thomas et al ., (1983) calculated that the flowing well produces 50,000 kg/hr: 50% liquid (mixed seawater and meteoric water) and 50% steam. An electricity generating plant has been producing 3 MW_e . Because the possibility of an eruption close to the plant was considered in its design, most of the equipment is on skids and can be removed if necessary.

At the time of this writing (1991), five production wells have been drilled near HGP-A. Most of these wells are located within the northern edge of the rift, in line

with fractures and a spatter rampart formed during the 1955 eruption and on the slopes of Puu Honuaula, a cinder cone believed to be between 1500 and 10,000 years old. These were drilled to depths of over 2000 m and have bottom-hole-temperatures of 312 to 334°C. The reservoir begins at a depth of ~1200 m and extends to a depth of at least 2250 m; it consists of a basaltic dike complex composed of near-vertical dike swarms, each separated from the other by brecciated wall rock (Clark and Stewart, 1991). The reservoir is overlain by ~750 m of submarine basalt flows with low permeability and by 450 m of subaerial basalt flows, which are permeable and contain an unconfined aquifer (Clark and Stewart, 1991).

The dikes and fractures of the east rift act as guides for fluid flow, allowing hot fluids to rise parallel to the dikes. The rift also dams water from upslope; the apparent water table rises to the near-surface on the north-west side and plunges to a depth of 400 m on the southeast side (Jackson and Kauahikaua, 1987; Thomas, 1987). Much of the potential sea water inflow is also blocked by rift dikes. Within the rift, where there is adequate fracture permeability and depth (~2000 m), the hydrothermal plume can be drilled and developed. Beyond the southern edge of the rift, one well penetrated thermal fluids at shallow depths but reentered cold sea water below the outflow plume from the rift, as is shown in Fig. 6.13 (Thomas, 1987).

Exploration and production drilling has taken place only along the east rift of Kilauea Volcano; this area is accessible and is located on private land (except for the research well that is sited in the National Park). The southwest rift zone of Kilauea and all rift zones of Mauna Loa, Mauna Kea, and Hualalai have not yet been drilled. These resources have been examined only at the surface during the Thomas et al . (1979, 1983) evaluation of the geothermal resources of Hawaii.