Mauna Loa Volcano Research Articles

Geology and geochemistry of basaltic lava flows and dikes from the Trans-Koolau tunnel, Oahu, Hawaii

A 200-m section of Koolau basalt was sampled in the 1.6-km Trans-Koolau (T–K) tunnel. The section includes 126 aa and pahoehoe lava flows, five dikes and ten thin ash units. This volcanic section and the physical characteristics of the lava flows indicate derivation from the nearby northwest rift zone of the Koolau shield. The top of the section is inferred to be 500–600 m below the pre-erosional surface of the Koolau shield. Therefore, compared with previously studied Koolau lavas, this section provides a deeper, presumably older, sampling of the shield. Shield lavas from Koolau Volcano define a geochemical end-member for Hawaiian shields. Most of the tunnel lavas have the distinctive major and trace element abundance features (e.g. relatively high SiO2 content and Zr/Nb abundance ratio) that characterize Koolau lavas. In addition, relative to the recent shield lavas erupted at Kilauea and Mauna Loa volcanoes, most Koolau lavas have lower abundances of Sc, Y and Yb at a given MgO content; this result is consistent with a more important role for residual garnet during the partial melting processes that created Koolau shield lavas. Koolau lavas with the strongest residual garnet signature have relatively high 87Sr/86Sr, 187Os/188Os, 18O/16O, and low 143Nd/144Nd. These isotopic characteristics have been previously interpreted to reflect a source component of recycled oceanic crust that was recrystallized to garnet pyroxenite. This component also has high La/Nb and relatively low 206Pb/204Pb, geochemical characteristics which are attributed to ancient pelagic sediment in the recycled crust. Although most Koolau lavas define a geochemical endmember for Hawaiian shield lavas, there is considerable intrashield geochemical variability that is inferred to reflect source characteristics. The oldest T–K tunnel lava flow is an example. It has the lowest 87Sr/86Sr, Zr/Nb and La/Nb, and the highest 143Nd/144Nd ratio found in Koolau lavas. In most respects it is similar to lavas from Kilauea Volcano. Therefore, the geochemical characteristics of the Koolau shield, which define an end member for Hawaiian shields, reflect an important role for recycled oceanic crust, but the proportion of this crust in the source varied during growth of the Koolau shield.

Absolute paleointensity from Hawaiian lavas younger than 35 ka

Paleointensity studies have been conducted in air and in argon atmosphere on nine lava flows with radiocarbon ages distributed between 3.3 and 28.2 ka from the Mauna Loa volcano in the big island of Hawaii. Determinations of paleointensity obtained at eight sites depict the same overall pattern as the previous results for the same period in Hawaii, although the overall average field intensity appears to be lower. Since the present results were determined at higher temperatures than in the previous studies, this discrepancy raises questions regarding the selection of low versus high-temperature segments that are usually made for absolute paleointensity. The virtual dipole moments are similar to those displayed by the worldwide data set obtained from dated lava flows. When averaged within finite time intervals, the worldwide values match nicely the variations of the Sint-200 synthetic record of relative paleointensity and confirm the overall decrease of the dipole field intensity during most of this period. The convergence between the existing records at Hawaii and the rest of the world does not favour the presence of persistent strong non-dipole components beneath Hawaii for this period.

Ecosystem development on Hawaiian lava flows: biomass and species composition

Abstract. The strong environmental gradients and ‘natural experimental design’ of Mauna Loa volcano, Hawaii, provide an outstanding opportunity to study controls on ecosystem development. We measured above‐ground vascular plant biomass and species composition on 42 sites on which precipitation, temperature, substrate texture, and substrate age varied substantially and largely independently. Biomass and species richness of live plants were strongly correlated with precipitation and lava flow age, but not with temperature or lava flow texture. Species composition, as measured by correspondence analysis, was likewise correlated with precipitation and flow age, but composition was also strongly influenced by temperature. Lava texture had a complex effect on vegetation, with ‘a’ a lava favoring vegetation development on wet sites and pāhoehoe favoring development on dry sites. Many locations remain virtually free of invasion by alien species; aliens appear where disturbance has facilitated invasion, either from stand‐level dieback in rainforest or a grass‐fire cycle on the dry, leeward side of the mountain. All four of the environmental factors studied here (precipitation, temperature, substrate texture, and substrate age) exert significant and independent control over vegetation biomass and/or species composition on Mauna Loa.

Trace element abundances of high-MgO glasses from Kilauea, Mauna Loa and Haleakala volcanoes, Hawaii

We performed an ion-microprobe study of eleven high-MgO (6.7–14.8 wt%) tholeiite glasses from the Hawaiian volcanoes Kilauea, Mauna Loa and Haleakala. We determined the rare earth (RE), high field strength, and other selected trace element abundances of these glasses, and used the data to establish their relationship to typical Hawaiian shield tholeiite and to infer characteristics of their source. The glasses have trace element abundance characteristics generally similar to those of typical shield tholeiites, e.g. L(light)REE/H(heavy)REEC1 < 1. The Kilauea and Mauna Loa glasses, however, display trace and major element characteristics that cross geochemical discriminants observed between Kilauea and Mauna Loa shield lavas. The glasses contain a blend of these discriminating chemical characteristics, and are not exactly like the typical shield lavas from either volcano. The production of these hybrid magmas likely requires a complexly zoned source, rather than two unique sources. When corrected for olivine fractionation, the glass data show correlations between CaO concentration and incompatible trace element abundances, indicating that CaO may behave incompatibly during melting of the tholeiite source. Furthermore, the tholeiite source must contain residual garnet and clinopyroxene to account for the variation in trace element abundances of the Kilauea glasses. Inversion modeling indicates that the Kilauea source is flat relative to C1 chondrites, and has a higher bulk distribution coefficient for the HREE than the LREE.

Melt migration and mantle chromatography, 2: a time-series Os isotope study of Mauna Loa volcano, Hawaii

We have determined the major element, trace element, and Os isotopic compositions of a stratigraphic suite of tholeiitic basalts spanning >30,000 years of the eruptive history of Mauna Loa volcano. Good correlations are observed between Os isotopes and the isotopes of Sr, Nd, Pb and He. In addition, the isotopes correlate with fractionation-corrected major element abundances within this single volcano, and provide a record of increased melting of mafic material with time at Mauna Loa. Chromatographic element fractionation during melt transport is shown to be negligible based on the good correlations of the isotopes of the compatible element Os with the other incompatible element tracers. The temporal variation at Mauna Loa is best described by the translation of the volcano over a Hawaiian plume which is radially zoned in composition. The radial zonation is a predicted consequence of thermal entrainment during flow in a mantle plume conduit.

Fluid flow and water‐rock interaction in the East Rift Zone of Kilauea Volcano, Hawaii

The East Rift Zone of Kilauea Volcano in Hawaii represents a major area of geothermal activity. Fluid inclusion and stable isotope analyses of secondary hydrothermal minerals in core samples from three scientific observation holes (SOH) drilled into the rift zone indicate that the geothermal system is dominated by meteoric waters to depths of as much as 1500 m below sea level. Calculated δ18O and δD values for fluids on the north side of the rift zone indicate that the deep meteoric fluids may be derived from precipitation on the upper slopes of Mauna Loa Volcano. In the interior of the rift zone, recharge is dominated by seawater mixed with local meteoric water. Water/rock ratios in the rift area are approximately 2, but strongly 18O‐enriched fluids in the deeper parts of the SOH‐2 and SOH‐4 drill holes (on the north side of the rift) indicate that the fluids underwent extensive interaction with rocks prior to reaching this part of the rift zone. Marine carbonates at the subaerial to submarine transition (between 1700 and 1780 m depth) in SOH‐4 have not fully equilibrated with the fluids, suggesting that the onset of hydrothermal activity in this area was relatively recent (&lt;2000 years). This may represent increased volcanic activity along the rift after the end of the Ai La'au phase of eruptive activity at the Kilauea summit approximately 1000 years ago, or it may reflect progressive evolution of the hydrothermal system in response to southward migration of intrusive activity within the rift.

Heavy Rain Events over the South-Facing Slopes of Hawaii: Attendant Conditions

Abstract Heavy rain events (>100 mm day−1) over the southeast flank of Mauna Loa volcano on the island of Hawaii are examined using surface, rawinsonde, rain gauge, and satellite data. The events occur in the presence of four types of synoptic-scale disturbances that include Kona storms, cold fronts, upper-tropospheric troughs, and tropical systems. The heaviest rainfall occurs at elevations above 0.5 km over the volcano slopes facing the prevailing low-level flow with the leeside slopes experiencing considerably smaller amounts. Rainfall duration and patterns demonstrate that the rains and flash floods are the result of more than one convective cell. Soundings prior to the heavy rain events show only modest instability, with only the K index serving as a useful predictor for heavy rain among the standard stability indices. There is an increase in both the moisture content of the midlevels (750–450 mb) and the onshore flow normal to the terrain gradient prior to the heavy rain. The lack of the trade winds...

Evidence of high deposition of ultrafine particles at mauna loa observatory

Evidence of high deposition of ultrafine particles at mauna loa observatory

A paleomagnetic study of the Pohue Bay flow and its associated coastal cones, Mauna Loa volcano, Hawaii: constraints on their origin and temporal relationships

Detailed paleomagnetic and rock-magnetic studies of the Pohue Bay flow and associated cones on its coastal flat were made to determine if the origin of the cones was due to primary (volcanic) or secondary (littoral) processes. We used paleomagnetism to determine the temporal relationships between the flow and cones. If the flows and cones are from the same eruption, the littoral origin of the cones is strongly favored. A total of 530 specimens from 232 core samples were collected and studied from the Pohue Bay flow and from lava ponded within the cones. Remanent magnetizations are very stable to stepwise alternating field demagnetization and show small angular dispersion and well-defined characteristic magnetizations. Magnetic carriers correspond to members of the titanomagnetite series with single or pseudo-single domain states. The overall mean directions for the Pohue Bay flow (Dec = 10.8°, Inc = 23.6°, k = 287.4 and α 95 = 2.3°) and the lavas ponded within the cones (Dec = 12.8°, Inc = 25.2°, k = 353 and α 95 = 4.9°) are statistically indistinguishable and have been drawn from a common Fisherian distribution, supporting a close age relationship. Additionally, the angular dispersion for the combination of the main flow and lavas ponded in cones is small, with a paleosecular variation (PSV) estimate of S f = 4.2°. This small PSV value supports the temporal association between the Pohue Bay flow and associated cones. They can thus be assigned to the same eruption timeframe. Because the source vent of the Pohue Bay flow is far upslope, the cones must therefore be littoral in origin, formed when the Pohue Bay flow entered the ocean. From the secular variation curve and comparison with age-dated flows with similar paleomagnetic directions, we estimate that the Pohue Bay flow was erupted approximately 1300 years ago. We were also able to distinguish both a possibly younger lava flow that later utilized the main tube of the Pohue Bay flow and an earlier sub-set of cones that were possibly formed before the Pohue Bay eruption.

Petrography and petrology of the Hawaii Scientific Drilling Project lavas: Inferences from olivine phenocryst abundances and compositions

The Mauna Loa (ML) and Mauna Kea (MK) lavas recovered by the Hawaii Scientific Drilling Project (HSDP) include aphyric to highly olivine‐phyric basalts. The average olivine phenocryst abundance in the reference suite of ML flows is 14.5 vol % (vesicle‐free and weighted by the flow thickness), while the average abundances of olivine in the reference suites of the MK alkalic and tholeiitic basalts are 1.1 and 14.0 vol %, respectively. Plagioclase and augite phenocrysts are rare in the ML and MK tholeiites, but the MK alkalic basalts can have up to 4 vol % plagioclase phenocrysts. Strained olivine grains, thought to represent disaggregated dunite xenoliths from the cumulate pile within the magma chamber(s), are ubiquitous in the drill core lavas. These deformed grains can comprise up to 50% of the modal olivine in a given rock. Olivine core compositions in the lavas span forsterite contents of 80.4–90.7 (median 88.8, ML tholeiites), 75.8–86.6 (median 85.8, MK alkalic basalts), and 76.3–90.5 (median 88.0 mol %, MK tholeiites). Olivines with core compositions in the range Fo89–90.5 are present in tholeiitic lavas with a wide range of whole‐rock MgO contents (9–30 wt %). Strained and unstrained olivines completely overlap in composition as do the compositions of spinels (100*Cr/(Cr + Al) ∼59–72; Mg# = 100*Mg/(Mg + Fe2+) ∼40–66) present as inclusions in the olivine phenocrysts. The presence of Fo90.5 olivine in the HSDP lavas requires magmas with ∼16 wt % MgO in the ML and MK plumbing systems. Rare dunite xenoliths are also present in the drill core lavas. While compositionally homogeneous within a given xenolith, the six xenoliths contain olivines that span a wide range of forsterite contents (78.3–89.2 mol %). Spinels in these xenoliths are chrome‐rich, have Mg# between 31 and 66, and define two populations on the basis of TiO2 contents. Whole‐rock compositions for the ML and MK tholeiites define olivine control lines on MgO‐oxide diagrams, and the relationship between whole‐rock MgO and olivine phenocryst abundance in these lavas suggests that the lavas with &gt;12 wt % MgO have accumulated olivine. Comparing the weighted bulk composition of all of the MK tholeiites in the drill core with a calculated parental magma suggests that, on average, the MK tholeiites entrained most of the olivine phenocrysts that crystallized from their parental liquids. Although deformed olivines in Hawaiian lavas are widely thought to represent disaggregated dunite xenoliths, none of the major‐ or minor‐element data on the strained or unstrained olivine phenocrysts suggest that the strained olivines in the HSDP lavas are exotic. We suggest that most of the olivine phenocrysts in a given flow, whether strained or unstrained, are closely related to the evolved liquid that now forms the groundmass. This is consistent with observed correlations between isotopic systems measured on olivine separates (e.g., O, He) and isotopic systems dominated by groundmass (e.g., Nd, Pb).

The formation of circular littoral cones from tube-fed p?hoehoe: Mauna Loa, Hawai'i

Pyroclastic cones along the southwest coast of Mauna Loa volcano, Hawai'i, have a common structure: (a) an early formed circular outer rim 200–400 m in diameter composed mostly of scoria and lapilli, and (b) one or more later-formed inner rims composed almost exclusively of dense spatter. The spatter activity locally fed short lava flows that ponded within the outer rims. Based on various lines of evidence, these cones are littoral in origin: relationships between the cones and associated flows; the degassed nature of the pyroclasts; and (although not unequivocal) the position of the cones relative to known eruptive vent locations on Mauna Loa. Additional support for the littoral interpretation comes from their similarity to (smaller) littoral cones that have been observed forming during the ongoing Kilauea eruption. The structure of these Mauna Loa cones, however, contrasts with that of “standard” Hawaiian littoral cones in that there is (or once was) a complete circle of pyroclastic deposits. Furthermore, they are large even though associated with tubefed pāhoehoe flows instead of 'a'ā. The following origin is proposed: An initial flow of tube-fed pāhoehoe into the ocean built a lava delta with a base of hyaloclastite. Collapse of an inland portion of the active tube into the underlying wet hyaloclastites or a water-filled void allowed sufficient mixing of water and liquid lava to generate strong explosions. These explosions broke through the top of the flow and built up the outer scoria/lapilli rims on the solid carapace of the lava delta. Eventually, the supply of water diminished, the explosions declined in intensity to spattering, and the initial rim was filled with spatter and lava.

The Uwekahuna Ash Member of the Puna Basalt: product of violent phreatomagmatic eruptions at Kilauea volcano, Hawaii, between 2800 and 2100 14C years ago

Kilauea volcano's reputation for relatively gentle effusive eruptions belies a violent geologic past, including several large phreatic and phreatomagmatic eruptions that are recorded by Holocene pyroclastic deposits which mantle Kilauea's summit area and the southeast flank of adjacent Mauna Loa volcano. The most widespread of these deposits whose original distribution can be reconstructed is the Uwekahuna Ash Member of the Puna Basalt, a basaltic surge and fall deposit emplaced during two or more eruptive episodes separated by a few decades to several centuries. The first episode occurred between 2770 ± 70 and 2265 ± 50 14C yr ago. It included two major pyroclastic surges, each preceded by unusually vigorous lava fountaining from a vent near the volcano's summit. Before the second eruptive episode, 2110 ± 120 14C yr ago, plants had re-colonized the rainforest environment northeast of the summit, and at least two lava flows from Mauna Loa had buried parts of the first-episode deposits. The second episode also began with vigorous lava fountaining, followed by widespread lithic ashfall, a third major surge and finally a fourth fountaining event. Before the final pumice deposit could be significantly reworked, it was partly buried by picritic basalt flows that are unusual in Kilauea's summit area. In proximal areas, the Uwekahuna Ash Member is more than 1 m thick (locally > 5 m) and includes lithic blocks up to 0.8 m in diameter. Coarse, primarily lithic debris was deposited around the vent by laterally expanding surges; fallout deposits accumulated preferentially to the northeast under the influence of high-altitude counter-tradewinds. The area devastated by surges and originally buried by at least 15 cm of the Uwekahuna was about 420 km 2. The bulk volume of the deposits was approximately 0.3 km 3, including less than 0.1 km 3 of juvenile material. Juvenile constituents are olivine-tholeiitic basalts similar in major-element composition to typical Kilauea summit lava flows, but variations in both major elements and trace elements suggest that the eruptions tapped more than a single, uniform source region. We infer that the eruptions which produced the Uwekahuna were driven by water interacting with a fluctuating magma column. Magma withdrawal episodes may have been accompanied by large-volume submarine effusive eruptions and by summit collapse. The volume, extent and character of the Uwekahuna deposits underscore the hazards posed by relatively infrequent but potentially devastating explosive eruptions at Kilauea, as well as at other basaltic volcanoes.

Litterfall and nutrient cycling in four Hawaiian montane rainforests

ABSTRACTThe mass of fine litterfall and nutrient circulation through litterfall were determined in four Melrosideros polymorpha/Cibotium spp.-dominated rainforests that differed in substrate age, parent material texture and annual precipitation on Kilauea and Mauna Loa volcanoes on the island of Hawaii. Three of the sites had rates of litterfall of 5.2 Mg ha−1 y−1; the fourth, which was on the most fertile soil, produced 7.0 Mg ha−1 y−1 of litterfall with higher concentrations of nitrogen and phosphorus. Tree ferns of the genus Cibotium cycled relatively large amounts of nitrogen, phosphorus and potassium through litterfall; their contribution to nutrient circulation was disproportionate to their mass in the forest, or in litterfall. The forest on the youngest substrate, which also had the lowest concentrations of nitrogen in litterfall, was fertilized with complete factorial combinations of nitrogen, phosphorus and a treatment consisting of all other plant nutrients. Additions of nitrogen increased the quantity and nitrogen concentration in litterfall during the second year following the initiation of fertilization, while no other treatment had a significant effect. Additions of nitrogen had no effect on litterfall mass or nutrient concentrations in the most nutrient-rich site.

Hydrogen and oxygen isotopic compositions of waters from fumaroles at Kilauea summit, Hawaii

Condensate samples were collected in 1992 from a high-temperature (300° C) fumarole on the floor of the Halemaumau Pit Crater at Kilauea. The emergence about two years earlier of such a hot fumarole was unprecedented at such a central location at Kilauea. The condensates have hydrogen and oxygen isotopic compositions which indicate that the waters emitted by the fumarole are composed largely of meteoric water, that any magmatic water component must be minor, and that the precipitation that was the original source to the fumarole fell on a recharge area on the slopes of Mauna Loa Volcano to the west. However, the fumarole has no tritium, indicating that it taps a source of water that has been isolated from atmospheric water for at least 40 years. It is noteworthy, considering the unstable tectonic environment and abundant local rainfall of the Kilauea and Mauna Loa regions, that waters which are sources to the hot fumarole remain uncontaminated from atmospheric sources over such long times and long transport distances. As for the common, boiling point fumaroles of the Kilauea summit region, their 18O, D and tritium concentrations indicate that they are dominated by recycling of present day meteoric water. Though the waters of both hot and boiling point fumaroles have dominantly meteoric sources, they seem to be from separate hydrological regimes. Large concentrations of halogens and sulfur species in the condensates, together with the location at the center of the Kilauea summit region and the high temperature, initially suggested that much of the total mass of the emissions of the hot fumarole, including the H2O, might have come directly from a magma body. The results of the present study indicate that it is unreliable to infer a magmatic origin of volcanic waters based solely on halogen or sulfur contents, or other aspects of chemical composition of total condensates.

Giant blocks in the South Kona landslide, Hawaii

A large field of blocky sea-floor hills, up to 10 km long and 500 m high, are gigantic slide blocks derived from the west flank of Mauna Loa volcano on the island of Hawaii. These megablocks are embedded in the toe of the South Kona landslide, which extends ∼80 km seaward from the present coastline to depths of nearly 5 km. A 10–15-km-wide belt of numerous, smaller, 1–3-km-long slide blocks separates the area of giant blocks from two submarine benches at depths of 2600 and 3700 m depth that terminate seaward 20 to 30 km from the shoreline. Similar giant blocks are found on several other major submarine Hawaiian landslides, including those north of Oahu and Molokai, but the South Kona blocks are the first to be examined in detail using high-resolution bathymetry, dredging, and submersible diving. Dredging of two of the giant blocks brought up pillowed tholeiitic lava. Observations from the U.S. Navy submersible Sea Cliff on the asymmetrically steep eastern flank of one block 10 km long and 300 m high revealed a succession of fractured massive basalt, laminar lava flows, hyaloclastite, and pillow lavas. Chemical analyses of dredged lava identified 19 units that overlap compositionally with lavas from the south rift-zone ridge of Mauna Loa. Sulfur content indicates that most of the lavas were erupted in subaerial and shallow submarine (

Basalt vesicularity as a measure of atmospheric pressure and palaeoelevation

THE pressure in, and thus the size of, a bubble in a lava flow is determined by the atmospheric pressure and the hydrostatic pressure of the overlying lava. If atmospheric sea-level pressure is known (or assumed), vesicle size distributions in basalt flows can thus be used as an indicator of the palaeoelevation of emplacement1. Here we show by analysis of the vesicle size distribution of basalt samples collected from the summit and base of Mauna Loa volcano in Hawaii that the technique provides estimates of ambient pressure that are accurate to within 0.1 bar, and thus estimates of elevation with a resolution of about 1,400m. This palaeoaltimetric technique should find useful application in palaeogeographical reconstruction2 for times when sea-level pressure was similar to the present value. (Conversely, if the elevation of emplacement is known, our technique should be able to provide a measure of sea-level palaeopressure.) Unlike palaeo-botanical methods for estimating elevations3–6, this method does not have to assume a relationship between temperature and altitude; on the other hand, its use is restricted to lava flows that have had a simple emplacement history.

Successional Development of a Hawaiian Montane Grassland

Substrate and elevation fundamentally affect the primary successional fate of plant communities on Mauna Loa Volcano, Hawai'i. Native perennial grasslands dominated by Descharnpsia nubigena form after initial forest development within well-defined elevational limits on pahoehoe lava flows. Woody vegetation continues to dominate at both higher and lower elevations. Woody vegetation is also dominant at all elevations of 'a'a flows. The exotic perennial grass, Anthoxanthum odoratum, has invaded sites where D. nubigena is a major component of the vegetation.

An Age--Altitude Matrix Analysis of Hawaiian Rain-Forest Succession

Species composition and biomass of vascular plants were determined in a matrix of 29 sites on the wet eastern slope of Mauna Loa volcano, Hawaii. Sites were sampled at five or six altitudes (914-2438 m a.s.l.) on each of five known-age lava flows (5, 47, 137, 300 and 3400 years BP). The resultant age-climate matrix was used to examine patterns of biomass accumulation, species distribution, and diversity during primary succession. Biomass increased with substrate age and ranged from 0 to over 36 000 g m −2 across the matrix. Generally, biomass increased as altitude decreased, although the maximum value was observed at intermediate altitude on the oldest flow (...)

Motion of Kilauea Volcano during sustained eruption from the Puu Oo and Kupaianaha Vents, 1983–1991

Kilauea erupted almost continuously from January 1983 through 1991. Although the summit began subsiding during the rift zone dike intrusion that initiated this eruption, remarkably steady ground surface motions began in late 1983 after a magnitude 6.6 earthquake beneath the slopes of nearby Mauna Loa volcano and continued until the onset of brief upper east rift zone earthquake swarms in late 1990. During these 7 years the summit and upper rift zones subsided up to 10–11 and 4–8 cm yr−1, respectively, and summit baselines contracted up to 6 cm yr−1. Baselines directed northward from the summit to stations on Mauna Loa extended at rates up to 7 cm yr−1, and a baseline from south of the summit to Mauna Loa extended 4 cm yr−1. Much of this extension is inconsistent with deformation caused solely by summit magma reservoir collapse and more likely reflects rifting as the south flank of the volcano moved seaward from the summit and rift zones. Farther from the summit, baselines crossing the south flank extended up to 2 cm yr−1, and a south flank tide gauge rose 2 cm yr−1; the lower east rift zone, 40–50 km from the summit, subsided about 2 cm yr−1. Motion on Kilauea, then, is broadly consistent with slip along low‐angle south flank faults, generating subsidence that is focused at the summit and along the rift system behind the faulting and uplift along the coastal south flank ahead of it. Dislocation models that combine these elements show that much of Kilauea's edifice migrated seaward, producing ground surface motions along the south flank of up to about 6 cm yr−1. The magnitude 6.1 earthquake of 1989 punctuated these motions along the eastern south flank, producing more than 25 cm of seaward displacement and, 15 km east of the epicenter, up to 24 cm of subsidence south of the lower east rift zone. Unlike the magnitude 7.2 south flank earthquake of 1975, the 1989 event was preceded neither by summit magma reservoir inflation nor by rift zone dike intrusions and accompanying compression of the south flank. Deformation was probably caused by the weight of the volcanic overburden and by ongoing dilation and slip within the rift system.

The hydrogeology of Kilauea volcano

The hydrogeology of Kilauea volcano and adjacent areas has been studied since the turn of this century. However, most studies to date have focused on the relatively shallow, low-salinity parts of the ground-water system, and the deeper hydrothermal system remains poorly understood. The rift zones of adjacent Mauna Loa volcano bound the regional ground-water flow system that includes Kilauea, and the area bounded by the rift zones of Kilauea and the ocean may comprise a partly isolated subsystem. Rates of ground-water recharge vary greatly over the area and discharge is difficult to measure, because streams are ephemeral and most ground-water discharges diffusely at or below sea level. Hydrothermal systems exist at depth in Kilauea's cast and southwest rift zone, as evidenced by thermal springs at the coast and wells in the lower east-rift zone. Available data suggest that dike-impounded, heated ground water occurs at relatively high elevations in the upper east-and southwest-rift zones of Kilauea, and that permeability at depth in the rift zones (probably < 10 −15 m 2 ) is much lower than that of unaltered basalt flows closer to the surface ( > 10 10 m 2 ). Substantial variations in permeability and the presence of magmatic heat sources influence the structure of the fresh water-salt water interface, so the Ghyben-Herzberg model will often fail to predict its position. Numerical modeling studies have considered only subsets of the hydrothermal system, because no existing computer code solves the coupled fluid-flow, heat- and solute-transport problem over the temperature and salinity range encountered at Kilauea.