The origin of Mauna Loa’s Nīnole Hills: Evidenceof rift zone reorganizationJeffrey Zurek1, Glyn Williams-Jones1, Frank Trusdell2, and Simon Martin3

1Department of Earth Sciences, Simon Fraser University, Burnaby, British Columbia, Canada, 2Hawaiian VolcanoObservatory, U.S. Geological Survey, Hawai’i National Park, Hawaii, USA, 3School of Environmental Sciences, University ofLiverpool, Liverpool, UK

Abstract In order to identify the origin of Mauna Loa volcano’s Nīnole Hills, Bouguer gravity was used todelineate density contrasts within the edifice. Our survey identified two residual anomalies beneath theSouthwest Rift Zone (SWRZ) and the Nīnole Hills. The Nīnole Hills anomaly is elongated, striking northeast,and in inversions both anomalies merge at approximately �7 km above sea level. The positive anomaly,modeled as a rock volume of ~1200 km3 beneath the Nīnole Hills, is associated with old eruptive vents. Basedon the geologic and geophysical data, we propose that the gravity anomaly under the Nīnole Hills records anearly SWRZ orientation, now abandoned due to geologically rapid rift-zone reorganization. Catastrophic submarinelandslides from Mauna Loa’s western flank are the most likely cause for the concurrent abandonmentof the Nīnole Hills section of the SWRZ. Rift zone reorganization induced by mass wasting is likely morecommon than currently recognized.

1. Introduction

Mauna Loa volcano is the largest active volcano on Earth and has erupted 15 times since 1900. Its voluminouslava flows have repaved 90% of its surface in the past 4000 years [Lockwood and Lipman, 1987], obscuring itsinternal structures and earlier evolution. To open a window into Mauna Loa’s history and internal structure,geophysical techniques such as Bouguer gravity are needed to image concealed structures and intrusions.Our study focuses on an anomalous surface feature, the Nīnole Hills, which forms topographic highs onthe southeast flank of Mauna Loa and represents the oldest exposed subaerial rocks on the edifice datedat 100–200 ka [Lipman et al., 1990; Jicha et al., 2012].

Several theories have been proposed to explain the formation of the Nīnole Hills [Hitchcock, 1906; Stearns andClark, 1930; Lipman, 1980; Lipman et al., 1990; Kauahikaua et al., 2000; Morgan et al., 2010]. Hitchcock [1906]proposed that the hills were the remnants of an older summit of Mauna Loa or a different Hawaiian volcano(Mohokea proto-volcano). Geochemical characterization of the lava flows comprising the hills showed no appre-ciable difference, beyond supergene alteration, between the Nīnole Hills and HoloceneMauna Loa lava flows; thisled Lipman et al. [1990] to propose faulting and landslides as the formation mechanism. Several studies havesuggested similar hypotheses revolving around the idea that the Nīnole Hills were once the site of an active riftzone. Lipman [1980] and Kauahikaua et al. [2000] proposed the existence of a previously unrecognized orienta-tion of the Southwest Rift Zone (SWRZ) or a proto-rift zone south of Mauna Loa’s summit to explain an elongatedgravitational high. More recently, seismic velocity modeling suggested that the Nīnole Hills may be a failedrift zone stretching from Mauna Loa’s summit through the Nīnole Hills and out past the coastline (Figure 1)[Park et al., 2007;Morgan et al., 2010]. Each of these mechanisms will leave a distinct geophysical signaturedue to differences in subsurface modification. This study presents Bouguer gravity results to infer subsur-face density structures and distinguish between formational mechanisms for the origin of the Nīnole Hills.

2. Data Reduction and Residual Gravity Field

Bouguer gravity measurements were collected in April and May 2013 at 367 locations (using a Sintrex CG-5gravimeter; Figure 2) covering an area of ~600 km2 following standard survey procedures [e.g., Rymer andBrown, 1986; Berrino et al., 1992; Battaglia et al., 2008]. Daily closures were on average 80μGal, and elevationcontrol was obtained via kinematic GPS at each location. The GPS data were corrected using a nearby contin-uous GPS base station (PIIK; Figure 1) providing a vertical positional error <10 cm. The largest daily closures

ZUREK ET AL. NĪNOLE HILLS: RIFT ZONE REORGANIZATION 1

PUBLICATIONSGeophysical Research Letters

RESEARCH LETTER10.1002/2015GL065863

Key Points:• 100–200 ka ago Mauna Loa’s SWRZran through the Ninole Hills

• Series of mass wasting eventscaused a geologically instantaneousreorganization of Mauna Loa’s SWRZ

• Reorganization and rapid movementof rift zones is not uncommon;although generally unrecognized

Supporting Information:• Figure S1• Figure S2• Figure S3• Figure S4• Text S1 and Figures S1–S4

Correspondence to:J. Zurek,jmz3@sfu.ca

Citation:Zurek, J., G. Williams-Jones, F. Trusdell,and S. Martin (2015), The origin ofMauna Loa’s Nīnole Hills: Evidence of riftzone reorganization, Geophys. Res. Lett.,42, doi:10.1002/2015GL065863.

Received 20 AUG 2015Accepted 2 OCT 2015Accepted article online 5 OCT 2015

©2015. American Geophysical Union.All Rights Reserved.

occurred when surveying over jungle paths, dirt roads, and large changes in elevation where the gravimeter wassubject to vibration and opportunities to repeat base station measurements were limited. The travel-induceddata tares are the most probable cause for the large closure errors observed [e.g., Crider et al., 2008; Zureket al., 2012]. Data reduction included terrain corrections, bathymetry, free air correction, removal of solid Earthtides, Earth curvature, and removal of the regional field. Terrain corrections were made using a clinometer toestimate the average slope of the terrain within 20m of the measurement location and a prism method usinga digital elevation model for distances greater than 20m [Hammer, 1939]. Both sets of terrain corrections usean assumed density of 2330 kgm�3 for topography, based on previous gravity studies on the Island of Hawai’i[Kauahikaua et al., 2000]. Bathymetry was corrected using a density of 1028 kgm�3 and a 50m resolutionbathymetric elevation model (www.soest.hawaii.edu/HMRG). To correct for elevation, a free air gradient of�322±6μGalm�1measured at the survey base stationwas used; this is similar to thosemeasured at the summitof Kīlauea (�327.3μGalm�1 [Johnson, 1992], �330.25μGalm�1 [Kauahikaua and Miklius, 2003]).

Removal of the regional field was accomplished with a two-step inversion process using GRAV3D [GRAV3D,2007]. This first involved the creation of a density contrast model through the inversion of a regional data set[Kauahikaua et al., 2000]. The density contrast model was then forward modeled, with each block in the surveyarea (down to 15 km depth) set to a 0 kgm�3 density contrast, effectively removing the influence of any densityvariations within the survey area. The resulting forward model represents the survey area’s regional field. Theapplication of the corrections described above and the removal of the regional field from the measured gravitydata give the residual Bouguer anomaly field (Figure 2). The advantage in using a two-step inversion process toremove the regional field is the reduction in edge effects compared to removal through polynomial regression orderivatives. This study primarily uses a different inversion suite, GROWTH2.0 [Camacho et al., 2011], to invert thecorrected data set. However, GRAV3D was chosen for removal of the regional field due to its flexibility andease of use manipulating large density models (see Figure S1 in the supporting information).

The residual Bouguer map (Figure 2) shows anomalies in two locations, one in the region of the Nīnole Hills(center of the figure) and the other under Mauna Loa’s SWRZ (lower left potion of the figure). No significant(>12mGal) positive anomalies are present off the strike of the SWRZ or the Nīnole Hills. The lowest relative

Figure 1. Map of the Island of Hawai’i. Gray lines delineate approximate boundaries of each volcano, solid black lines representapproximate locations of the current rift zones of Mauna Loa, and green oval shows the location of the Nīnole Hills.Orange lines represent a suggested previous rift zone configuration [Park et al., 2007;Morgan et al., 2010], and blue lines/arrows show one possibility for the migration of the SWRZ [Lipman, 1980]. Dashed box approximates the area coveredduring the Bouguer gravity survey. Yellow star is the location of the continuous GPS station (PIIK) used for elevationcontrol. Contours at 200 m.

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gravitational field (0mGal; Figure 2) is south of the Nīnole Hills, on the edge of the survey area, and locatedaway from obvious topographic features. Although on the edge of the survey area, the gravity low is unlikelyto be composed of only edge effects or error in data reduction as it is present in the data prior to terrain cor-rections and regional field removal. Furthermore, Moore and Chadwick [1995] identified slump and landslidedeposits just off shore (Figure 1), and the Ka’ōiki-Honu’apo fault system is also in this region, both of whichwould be consistent with low gravitational field.

Due to difficult terrain and limited access, gravity measurements were concentrated near trails and four-wheel drive-accessible roads. This led to an unequal spatial distribution of measurements, and thus, the reso-lution varies depending on survey point density. Individual roads and paths have station spacing of 250m,500m, or 1000m, and therefore have minimum resolvable wavelengths of 500m, 1000m, and 2000m,respectively. Due to the lack of station coverage, survey points to the NE, it is not possible to determinethe extent of the anomaly beneath the Nīnole Hills to the northeast. However, given that the anomaly is over5 km in length, it is likely that it does not abruptly end and continues beyond the edge of the survey area.Likewise, it is not possible to determine conclusively if the SWRZ and the anomaly beneath the NīnoleHills connect, although they likely do.

To place the positive anomaly beneath the Nīnole Hills in context, it is important to compare it to other posi-tive anomalies that have been described on the Island of Hawai’i [Kauahikaua et al., 2000]. Due to differencesin data reduction, the size of different volcanic edifices, and their cumulate cores, comparing the maximumslope of anomalies rather than their magnitude is a meaningful approach. The positive anomaly associatedwith the Nīnole Hills has a maximum seaward slope of 3mGal km�1 (averaged over 10 km). The perpendicularseaward slope of positive anomalies associated with rift zones are 3 to 4.5mGal km�1 (averaged over 10 km).Positive anomalies associated with the summits of volcanoes, with steeper slopes, Kohala, Hualālai, andMauna Kea on the Island of Hawai’i have on average between 4.5 and 5mGal km�1 (averaged over 10 km).Kīlauea and Mauna Loa summit anomalies are outliers with 3.5 and 6.5mGal km�1 (averaged over 10 km),respectively. This comparison suggests that the positive Bouguer gravity anomaly beneath the Nīnole Hillsis more similar to those at rift zones then anomalies found at the summits.

3. 3-D Inversions

Two gravity inversion suites, GRAV3D [GRAV3D, 2007] and GROWTH2.0 [Camacho et al., 2011], were used toinvert the Bouguer gravity data in order to obtain density contrast models beneath the survey area. Only

Figure 2. Residual Bouguer anomaly map contoured using kriging. Black dots represent gravity survey locations, and thegray diamond represents the location of the gravity base station. Map area is approximately the same as the dashed box inFigure 1. Elevation contour interval of 100m.

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results from GROWTH2.0 are presented due to its greater ability to handle data sets with irregular station spa-cing; a detailed discussion of the two inversion suites and inversion sensitivity can be found in Figure S1. Thesize of each cell (height 520m to 1500m and areal extent of 0.1 to 6 km2) in the inversion models were deter-mined by GROWTH2.0 based on distance from the edge of the data set and data resolution. GROWTH2.0 wasable to invert the data with density contrast end-members from ±185 kgm�3 to ±900 kgm�3, each inversionresulted in positive linear anomalies beneath the SWRZ and the Nīnole Hills. Density bounds outside of thesetwo end-members resulted in a failure to converge on an inversion solution. The resulting inversion fromdensity bounds of ±185 kgm�3 containedmany short wavelength perturbations forcing a fit to much coarserdata and also produced the largest volume, ~3200 km3 (Figure S3), of positive density contrast from beneaththe Nīnole Hills to the bottom of the model. In contrast, inversions using the maximum density bounds,±900kgm�3, showed only sparse bodies and produced the lowest anomalous volume of ~500km3 (Figure S4).In the optimized inversion model (based on the stability of inversion structures and approximately halfwaybetween end-member bounds; Figure 3; ±400 kgm�3), the two distinct density anomalies beneath theSWRZ and the Nīnole Hills begin at 0m above sea level (asl) and nearly merge at �7 km asl with a totalpositive volume of ~1220 km3. The merging of these two anomalies suggests that both the SWRZ andthe Nīnole Hills anomalies share the same root. The linearity of the model density contrasts beneath theNīnole Hills is persistent in every inversion regardless of the starting density bounds suggesting that alinear structure runs under the Nīnole Hills at depth.

4. Anomalous Mass, Density Contrast, and Intrusion Size

The calculated anomalous mass in each inversion model is ~1 × 1015 kg with each differing by less than 2%from each other. To calculate the size of any intrusions suggested by the inversion models requires assump-tions of the average density with depth. Due to the lack of density information within the Nīnole Hills, theonly constraints available are from drill holes [e.g., Keller et al., 1979; Moore, 2001]. From these studies weassume that subaerially erupted basalts have an average density of 2300 kgm�3 (for dry basalt) to2500 kgm�3 (basalt below the water table) and submarine basalt from 2700 kgm�3 to 2900 kgm�3.Intrusive complexes are then assumed to have densities in excess of 2900 kgm�3. Without making assump-tions regarding subsidence due to volcanic loading near the Nīnole Hills, we apply both ranges to the opti-mized inversion model. Furthermore, due to data resolution, intrusive volume calculations begin at�1000m asl. Based on these assumptions, the density anomalies map out the 2900 kgm�3 or 3100 kgm�3

isosurfaces and contain ~1200 km3, which represent a significant additional mass, requiring a mechanism toconcentrate it beneath the Nīnole Hills.

5. Formational Mechanisms

It has been suggested that the Nīnole Hills were created by a number of mechanisms including: the eruptionsof a protovolcano [Hitchcock, 1906], landslide/fault blocks [Lipman et al., 1990], and a separate Nīnole Hills riftzone [Park et al., 2007; Morgan et al., 2010]. Each mechanism has a different expected gravitational field;therefore, the processed gravity data should enable us to discriminate between these hypotheses. If theNīnole Hills were the location of a centralized proto-volcano summit, the expected gravity signal would becircular and centered above a dense intrusive complex [e.g., Zurek and Williams-Jones, 2013]. The measuredgravity field cannot exclude a protovolcano summit as it appears circular; however, the Bouguer gravityanomaly centered on the Nīnole Hills is at least 5 km in length (Figure 2), may extend further to the northeast,and 3-D inversions allude to a connection with the SWRZ at depth at its SW extent, suggesting a more elon-gate structure. Geochemical work by Lipman et al. [1990] shows that the chemistry of the hills are not appre-ciably different from other Mauna Loa lavas and therefore rule out the possibility that the Nīnole Hills was aseparate and different volcano. Furthermore, seismic data shows a P wave velocity anomaly stretching fromMauna Loa’s summit to the Nīnole Hills and a gravity anomaly [Kauahikaua et al., 2000] striking NS from thesummit (Figure 4). Taking all the data together suggests that the Nīnole Hills was not the site ofa protovolcano.

The Nīnole Hills rift zone was first proposed by Park et al. [2007] to explain seismic tomography data.However, if the proposed rift zone existed, residual gravity measurements should increase toward the centerof the structure due to the likelihood of denser intrusive material [e.g., Broyles et al., 1979; Kauahikaua et al., 2000].

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Two parallel gravity profiles, along roads (Figure 2) that would have approximately traversed the proposedrift zone perpendicularly, did not detect an anomalous density; therefore, the gravity data do not supportthe existence of a separate failed rift zone beneath the Nīnole Hills (Figures 3 and 4). A recent review on thegrowth of Hawaiian volcanoes suggests that the original interpretation by Park et al. [2007] and Morganet al. [2010] tied the high-velocity zone beneath the Nīnole Hills to the continuation of Kīlauea’s SWRZ[Lipman and Calvert, 2013]. Furthermore, Jicha et al. [2012] provides age constraints for the submarineextension of the SWRZ that suggests that it has been active for more than 470 ka, including the time whenthe Nīnole Hills formed. In order for both the proposed Nīnole Rift zone and the SWRZ to be activesimultaneously, a junction angle between them approaching 120° would be expected [Oertel, 1965;Reches and Dieterich, 1983]. However, the approximate orientation of the two rift zones would have beenbetween 45 and 90°, which has not been documented in any geologic setting.

Lastly, if the Nīnole Hills represent slump blocks and landslide scarps, characteristically basaltic flows andfaults are low density and should generate a gravitational low. Although there is a negative anomaly in the

Figure 3. Smoothed density contrast depth slices (starting at�1000m asl) from the optimized GROWTH2.0 inversion withdensity bound of ±400 kgm�3 and residuals for each station. Positive density variations shown in red correspond withinterpreted intrusive material based on a zero density contrast obtained from drill holes [e.g., Keller et al., 1979; Moore,2001]. Elevation contour interval of 100m.

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southern portion of the survey area (Figure 2), it is not situated beneath the Nīnole Hills. A large dense bodybeneath the Nīnole Hills is not consistent with the topographic feature being created by faulting andlandslides but is consistent with intrusive material. While the Bouguer gravity data suggest that faultingand landslide blocks are not the primary cause of the Nīnole Hills, the hills would almost certainly have beensubsequently modified by the widespread mass wasting documented across the southeast flank of MaunaLoa [e.g., Lipman et al., 1990].

6. Rift Zone Migration or Reorganization

The substantial Bouguer gravity anomaly beneath the Nīnole Hills, elongated SW-NE, is consistent with alarge intrusive complex that likely connects with the SWRZ. Furthermore, previously documented northtrending dikes exposed on the south flank of one of the Nīnole Hills [Lipman et al., 1990] together with recentgeologic mapping by Trusdell and Lockwood [2015] are consistent with the former presence of a rift zonesince the number of dikes decreases rapidly away from rift zones [Walker, 1987].

The data presented here do not support the previous suggestions for the creation of the Nīnole Hills such as aprevious volcanic summit, Nīnole Hills rift zone, or faulting and landslides but rather can be explained morecomprehensively by rift zone migration or reorganization of Mauna Loa’s SWRZ. The migration of Hawaiianrift zones was first suggested by Swanson et al. [1976]. Lipman [1980] followed with detailed work on thepossible migration of Mauna Loa’s SWRZ supported by geologic field evidence. Rift zone migration was alsoinvoked to explain the large bends in Hawaiian rift zones [e.g., Lipman, 1980; Swanson et al., 2014].

Bouguer gravity surveys along Kīlauea’s East Rift Zone clearly show an asymmetrical gravitational fieldcentered on the present axis of the rift zone due to rift zone migration [Broyles et al., 1979]. If the migration

Figure 4. (top left) Island wide Bouguer gravity data from Kauahikaua et al. [2000] near the summit of Mauna Loa. (top right)Pwave isosurfaces of seismic tomography from Figure 6 in Park et al. [2007]. (bottom) Residual Bouguer gravity from this study.Solid black lines represent the approximate locations of Mauna Loa’s SWRZ and NERZ. Dashed lines outline the range of thelocation of the suggested proto Southern rift zone. Elevation contour interval of 250m.

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of Mauna Loa’s SWRZ was slow and steady we would expect to see a constant westward increase in thegravitational field due to the shallowing of intrusives as one reached the axis of the modern rift zone.However, a constant increase in the gravitational field toward the modern (or current) rift zone axis wasnot observed. Instead, we see two gravitational highs beneath the modern SWRZ and the Nīnole Hills.Furthermore, the imaged anomaly directly beneath the Nīnole Hills could be up to 1200 km3 in volume.

If continuous rift zone migration is the cause of the high-density structure beneath the Nīnole Hills, it wouldrequire long-lived activity. Sustained activity in the vicinity of the Nīnole Hills would build a large intrusivecomplex and potentially produce the observed gravity signal. However, there is no supporting evidence tosuggest why the Nīnole Hills would have been a preferential pathway for magma ascent or why it would haveceased to be one as the rift zone migrated. If rift zone migration does not occur at a constant velocity, thenrelatively rapid migration would leave behind areas with larger intrusive volumes creating a positive Bougueranomaly in the gravitational field toward the axis of the rift zone.

Gravitational instability of volcanic piles has been shown to cause edifices to spread through slow deepseated gravitational slope deformation, faulting, and landsides [e.g., Lipman et al., 1988; Clague andDenlinger, 1994; Delaney et al., 1998; Benz et al., 2002; Park et al., 2009]. Variations in rift zone migration velo-cities necessitate rapid changes to the gravitational stability of the edifice. While not yet recognized forHawai’i volcanoes, a geologically instantaneous reorganization of rift zones of Anaga volcano (Tenerife,Canary Islands) has been suggested due to a large mass wasting event [Walter et al., 2005]. Based on40Ar/39Ar dates, geologic mapping, and aerial photograph lineament mapping, Walter et al. [2005] arguesthat Anaga volcano originally had straight east west rift zones. A large landslide occurred on the northernflank of the volcano, which caused the rift zone to bend as it conformed to the scarp. A third rift likely formedlater due to the stresses associated with a curved rift zone. Large landslides have been discovered off thecoast of many Hawaiian Islands [Moore et al., 1989] and have been recognized as being associated with vol-canic spreading [e.g., Oehler et al., 2005]. Applying this new information to Mauna Loa’s Southwest Rift Zonesuggests that it was positioned directly under the Nīnole Hills 100–200 ka ago. An event capable of changingthe edifice stress field, such as a large mass wasting event like the Ālika debris flows, then caused a geologi-cally instantaneous reorganization of Mauna Loa’s SWRZ. Given the number of large mass wasting depositsidentified around the ocean island volcanoes, we suggest that reorganization and rapid movement of riftzones are not uncommon, however generally unrecognized.

Events capable of causing drastic changes to Mauna Loa’s internal stress field in the last 200 ka includelarge mass wasting events and possibly the buttressing of Mauna Loa by Kīlauea Volcano [e.g., Lipmanet al., 1988; Coombs et al., 2006]. Mass wasting, such as the Ālika-1 and Ālika-2 debris flows, on the westflank of Mauna Loa, has together transported 200 to 600 km3 of material [e.g., Lipman et al., 1988] and beendated at 200–240 ka [Felton et al., 2000; Rubin et al., 2000] and 127 ka, respectively [McMurtry et al., 1999].These are thus approximately contemporaneous with the Nīnole Basalt (100–200 ka [Lipman et al., 1990;Jicha et al., 2012]). The removal of more than 100 km3 of material due to either of the Ālika debris flows[Lipman et al., 1988; Felton et al., 2000; Morgan et al., 2010] would have changed the stress/strain on theedifice potentially causing Mauna Loa’s SWRZ to jump northward or migrate faster than before the event.The effect of Kīlauea buttressing Mauna Loa’s south flank would have also significantly affected thestress/strain fields within Mauna Loa, potentially changing deformation patterns causing relatively rapidchanges to any SWRZ migration. The most recent model for the age of Kīlauea suggests that it is ~275 ka[e.g., Calvert and Lanphere, 2006] and the timing of buttressing is unconstrained. It is not currently possibleto determine which mechanism, buttressing, landslides, or both could have caused the change in gravita-tional stability of Mauna Loa to lead to changing its rift zone orientation. However, of the above processes,instability due to mass wasting would be the most catastrophic and consequential.

Geologically fast rift zone migration or orientation change can also explain the preservation of the NīnoleHills, as a stable rift zone will create a topographic high along its axis. When the rift zone moves westward,the old rift axis will shelter land seaward from lava inundation. Although it is not possible to conclusivelydetermine the mechanism responsible for the imaged density anomaly beneath the Nīnole Hills, we interpretthe gravity data as evidence of an early Mauna Loa Southwest Rift Zone [Lipman, 1980; Kauahikaua et al.,2000]. This study suggests that migration or offsetting of rift zones by large landslides is likely more importantto the development and modification of basaltic island volcanism then previously recognized.

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AcknowledgmentsThis study was supported by a NSERCDiscovery grant to G. Williams-Jones.Mahalo to Lis Gallant for help with datacollection and to Mike Poland forfacilitating GPS acquisition. Additionalthanks to Peter Lipman, StefanieHautmann, and Mike Poland for theirconstructive reviews. This study wouldnot have been possible without thesupport from the Hawaiian VolcanoObservatory.

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