Bull Volcanol (2016)78:3 DOI 10.1007/s005-016-1027-2 SHORT SCIENTIFIC COMMUNICATION Dscovery of a trachyte gnmbrte sequence at Hualāla, Hawa Thomas Shea 1 & Jacquelne Owen 2 Receved: 2 December 2015 /Accepted: 29 March 2016 # Sprnger-Verlag Berln Hedelberg 2016 Abstract Ignmbrtes are common n many ntraplate ocean slands but have been mssng from the known geologcal record n Hawa. Durng a recent feld campagn, the remnants of a trachytc gnmbrte sequence have been dscovered at Hualāla volcano, fortutously preserved from subsequent basaltc lava flow cover. We provde a prelmnary descrpton of these deposts, as well as bulk and glass chemcal analyses to determne ther potental relatonshp wth other nearby trachytes from Pu u Wa awa a (PWW)andPu u Anahulu (PA). The results suggest that these gnmbrtes are from nether PWW nor PA, but nstead may relate to trachytes that are found as maar wallrock blocks some 20 km dstant. Therefore, despte beng rare overall n Hawa, the gnmbrtes and more generally trachytes were probably wdespread around Hualāla. Compared to other ntraplate ocean slands, the combnaton of a fast-movng plate, hgh magma supply, and erupton rates underneath Hawaan volcanoes may explan the scarcty of gnmbrtes preserved at the surface. Ther presence at Hualāla could reflect unusual condtons of edfce stress durng the transton from sheld to post-sheld volcansm. Edtoral responsblty: K.V. Cashman Electronc supplementary materal The onlne verson of ths artcle (do:10.1007/s005-016-1027-2) contans supplementary materal, whch s avalable to authorzed users. * Thomas Shea tshea@hawa.edu 1 2 Department of Geology and Geophyscs, SOEST, Unversty of Hawa, 96822 Honolulu, HI, USA Lancaster Envronment Centre, Lancaster Unversty, Lancaster, UK Keywords Pyroclastc densty currents. Ignmbrtes. Hawa. Hualāla volcano. Trachyte Introducton Ignmbrtes (here defned loosely as pyroclastc densty current BPDC^ deposts consstng domnantly of a poorly sorted mxture domnated by juvenle pumce and ash, e.g., Branney and Kokelaar 2002) are wdespread along arc, back-arc, and contnental ntraplate regons that erupt evolved magmas (e.g., phonoltes, trachytes, rhyoltes). Large gnmbrtes have also been dentfed at several oceanc ntraplate settngs, ncludng at the Canary Islands (Freundt and Schmncke 1995), the Azores (e.g., Duncan et al. 1999), and Cape Verde (Esele et al. 2015). Along the Canary Islands, ths type of volcanc actvty has produced many hundreds of cubc klometer of deposts. Evolved magmas typcally erupt n ntraplate oceanc slands durng post-sheld volcansm. Ther orgn s often attrbuted to fractonal crystallzaton (±replenshment) of alkalc basalts that produces phonoltes or trachytes, and/or mxng of basalts wth assmlated sedments or crustal ntrusons, whch typcally yelds rhyoltes (Freundt and Schmncke 1995; Freundt-Malecha et al. 2001; Hansteen and Troll 2003; Edgar et al. 2007; Sgmarssonetal.2013). In Hawa, exposed volcanc sequences nvolvng evolved magmas are comparatvely rare at the surface. Exceptons nclude the volumnous Pu u Anahulu-Pu u Wa awa a trachyte flow and cone assocaton at Hualāla, Hawa (Stearns and Macdonald 196), trachyte domes and flows on West Mau (e.g., Pu u Koae, Mt Eke, Pu u Launupoko, Stearns and Macdonald 192; Velde1978), and the rhyodactc Mt. Kuwale flow on Oahu (Van der Zander et al. 2010). To date, however, no gnmbrtes have been dentfed, n contrast wth other ntraplate stes such as the Canary Islands. Pyroclastc
3: 87) 6102( 3 Page 2 of 8 Bull Volcanol densty current deposts recognzed at Klauea are dstnct n beng basaltc and lthc-rch/juvenle-poor, wth an nferred phreatomagmatc orgn (McPhe et al. 1990; Swanson et al. 2012). In ths bref communcaton, we descrbe the frst dscovery of gnmbrte deposts n Hawa, at Hualāla volcano. We brefly speculate on the potental for other smlar deposts to be bured underneath the recent lava flow cover and the hazard mplcatons of such eruptons for Hawa. Descrpton of deposts The Pu u Anahulu (PA) trachyte flow on the north flank of Hualāla s a topographcally promnent feature ( 260 m from base to top) that has been lnked to the nearby trachyte pyroclastc cone of Pu u Wa awa a (PWW; Fg. 1; Stearns and Macdonald 196; Cousens et al. 2003; Shamberger and Hammer 2006). The Pu u Anahulu and Pu u Wa awa a trachytes are >100 ka (Clague 1987; Cousens et al. 2003), and by far the oldest subaeral Hualāla rocks exposed (Hualāla s next youngest exposed flows are 13 ka, Moore et al. 1987). Durng recent feld work, we dentfed a prevously undescrbed tephra sequence on top of the Pu u Anahulu flow. Ths sequence was preserved from eroson and weatherng at only a few stes (nne locatons are descrbed here, cf. Fg. 1), although the base of the depost stll blankets the flatter portons of some of the Pu u Anahulu flow lobes. The thckest secton (preservng the most comprehensve record of the gnmbrte deposts) was trenched about 1.5 km from the nearby Pu u Wa awa a trachyte cone (locaton 1) (Fg. 2a). The gnmbrte deposts were subdvded nto four major eruptve unts (-EU) based on abrupt changes n lthofaces relatonshps that could be recognzed between dfferent outcrops. The groupng nto dfferent eruptve unts reflects each nferred flow cycle. The Pu u Anahulu flow underneath the tephra sequence usually conssts of holocrystallne (mcroltes and mcrophenocrysts) trachyte blocks 10 50 cm n sze, enclosed wthn ether (1) a weathered mxture of ash to coarse ash-szed trachyte grans and sol, or (2) a basal pumce-bearng gnmbrte (massve lapll tuff lthofaces, ) contanng dsrupted lenses of sol, whch are nterpreted have been remoblzed by the gnmbrte (Fgs. 2a and 3c). Ths gnmbrte (eruptve unt ) has a fne ash matrx and a low KO MK HL ML KIL ML >11ka ML 3-5ka HL 5-11ka 5 km Huehue group Rft zone Hualāla Waha Pele group Legend 8 7 9 5 6 3 2 Pu u Anahulu flow HL >11ka ML 1.5-3ka Mauna Loa (ML) basalts Hualāla (HL) alkal basalts Hualāla vents <13 ka Pu u Wa awa a cone 1 ML 1859 Pu u Wa awa a-pu u Anahulu Trachytes Other Hualāla trachyte locatons Mnmum nferred extent of Pu u Anahulu flow Ignmbrte outcrops wells cones HL 3-5ka Hualāla summt HL 1.5-3ka 2 km N Fg. 1 Geologcal settng of Hualāla gnmbrtes. (Left) Dgtal elevaton model of Hualāla volcano wth promnent trachyte exposures (Pu u Wa awa a cone and Pu u Anahulu flow) on northern flank. Yellow stars show the locaton of other trachyte blocks found as lthcs n more recent basaltc products (BWaha Pele group^), red stars mark the locaton of wells n whch trachyte was recovered (BHuehue group^) (cf. Cousens et al. 2003). (Rght) Geologcal map of the Pu u Wa awa a-pu u Anahulu area (modfed from Sherrod et al. 2007). Only certan areas of the Pu u Anahulu trachyte reman uncovered by <13 ka post-sheld actvty. Numbers refer to man stratgraphc locatons descrbed n the text
Bull Volcanol (2016)78:3 cm a Page 3 of 8 3 LITHOFACIES DESCRIPTION b sol Post-gnmbrte deposts massve pumce lapll tuff pumce lapll unt coarse ash/fne lapll unt dark-clast-rch unt unt w/ color gradent trachyte blocks/flows basaltc tephra sol/weathered materal strongly ndurated /paleosol 150 mplt EU Basal bed comprses coarse pumce ash wth varable reddsh coloraton both at bottom and top. Grades nto weathered gnmbrte/paleosol. Locaton 1 dffuse stratfcaton Legend EU dffuse cross stratfcaton marked cross stratfcaton -pp dslt 100 dsplt Unt conssts of trpartte basal lthofaces wth gransze varatons (slght normal then nverse gradng), wth top bed ndurated, surrounded by dark tuff that propagates nto top gnmbrte va ppe structures. Top thrd of massve lapll tuff faces s ndurated. Base of unt s a coarse pumce ash-rch seres of beds dsplayng dffuse stratfcaton and beddng, wth EU2b pnchng-swellng. Lke unts below, grades nto gnmbrte wth ncreasng nduraton towards the top. erosve surface 2 gradual transton 5 50 xslt Unt marked by pnch and swell cross-stratfed beds wth hghly varable gransze and a few beds. Hgh EU2a pumce-rch abundance of dark/dense materal overall. Grades nto gnmbrte towards the mddle. -lenst 0 9 6 EU2a Pu u Anahulu trachyte flow 8 EU5? EU2b Ignmbrte unt wth slght reverse gradng of pumce grans and frequent ash/tuff lenses wth dfferent lthologes n lower half (pumce ash, obsdan+pumce ash, sol pods). Change n color from brown at base to lght grey/yellowsh, and to tan at top. Indurated towards the top. 7 3 PA Locaton 1 Fg. 2 a Stratgraphc column of pyroclastc densty current deposts at locaton 1. Lthofaces nomenclature used follows Branney and Kokelaar (2002), m = massve, LT = lapll-tuff or lapll-ash, p = pumce, ds = dffuse-stratfed, xs = cross-stratfed, lensl = pumce lapll lens. Lthofaces were grouped nto eruptve unts -EU (see text for explanaton). b Varatons n PDC stratgraphy across the dfferent locatons, ordered arbtrarly by dstance from the Hualāla summt. Stratgraphy and lthofaces are here represented as smplfed patterns, along wth nformaton pertanng to lthofaces contact, state of nduraton, and stratfcaton abundance of pumce lapll that reach 3 cm n sze at locaton 1. In addton to sol pods, lenses of fne lght gray ash or coarser dark gray ash are abundant near the contact wth the blocky surface of the Pu u Anahulu flow. The presence of remoblzed sol at the contact wth Pu u Anahulu blocks suggests that the PDC sequence and Pu u Anahulu were not produced by the same erupton. The gnmbrte s overlan by a cross-stratfed unt rch n obsdan and cryptocrystallne trachyte, nterbedded wth ndurated fne ash layers and coarser pumce lapll beds (Bcross-stratfed lapll-ash^ xslt lthofaces, Fgs. 2a and 3b, c, e). Ths subunt grades nto another gnmbrte and together comprse EU2a. EU2b s found at a few locatons and conssts of a seres of fnes-poor dffuse stratfed, coarse pumce lapll beds (Bdffuse stratfed pumce lapll-ash^ dsplt) that grade nto a thnner massve lapll tuff () unt (Fg. 3a). has a fantly stratfed fnes-poor coarse ash-rch base (dffuse-stratfed lapll tuff lthofaces dslt) that s farly dstnctve from other unts due to slght changes n color (darker toward the top, Fgs. 2a and 3d, e). At locaton 1, the top of ths unt s dark, hghly ndurated, and dsplays small ppe structures that progress nto the overlyng gnmbrte. Fnally, EU has a thn basal subunt composed of coarse pumce ash (normally then reversely graded Bmassve coarse pumce ash^ mpt) that agan transtons nto a massve lapll tuff () faces. Unts, EU2a, and EU are found n all locatons on the PA flows (Fg. 2b). Some locatons show low angle mbrcaton or bed-parallel fabrc marked by pumce clasts, typcally wthn the coarser-graned portons of the dslt and xslt lthofaces. Each repeated sequence of a fnes-poor, dffuse to crossstratfed unt gradng farly abruptly (usually over <15 cm) nto an gnmbrte s nterpreted here as a dstnct flow unt. Indvdual flow unts aggrade progressvely, startng as ether granular flud-based (dffuse-bedded unts) or tractondomnated (cross-stratfed unts) currents that transton nto
3: 87) 6102( 3 Page of 8 Bull Volcanol a b xslt EU2a EU2b dsplt plt c ppes d dslt xslt EU2 xslt 10 cm EU2b plt PA paleosol 10 cm PA block e EU2a Blocky mound on PA flow Fg. 3 Feld photographs of the Hualāla gnmbrte sequence. a gnmbrte and EU2a eruptve unt wth a stratfed to cross-stratfed fne lapll base nterbedded wth fne ash beds that transtons nto the overlyng gnbmrte. The color transton s farly abrupt but clasts from EU2 base clearly contnue nto the massve lapll tuff. b Unt EU2b wth coarse lapll-rch and dffuse-stratfed basal lthofaces dsplayng a clear pnch-and-swellng character. Note the abrupt color and gran sze transton between the fnes-poor plt base and the underlyng EU2a gnmbrte. c Faulted/deformed sequence wth dffuse-stratfed, normally then nverse-graded base wth dark-colored ndurated layer and two ppe structures. The dslt lthofaces grades progressvely nto a. d A slghtly more dstal locaton wth somewhat dffuse, dscontnuous Pu u Anahulu paleosol pods wthn gnmbrte. e Contact of the PDCs wth substrate and pnchng-swellng behavor. Interacton of the current wth small-scale topography (the trachyte blocks from underlyng Pu u Anahulu flow) results n much thnner and EU2 deposts (marked by two arrows) flud-escape domnated, more dlute current (massve lapll tuff) (e.g., Branney and Kokelaar 2002; Brown and Branney 200). The contact between nferred eruptve unts s domnantly erosve. Upward ncreases n nduraton and pnksh coloraton wthn each man gnmbrte unt could result from partal cementaton of the most tuff as gnmbrtes were overrdden by the subsequent hot, granular-flud based PDC currents, or, alternatvely, by mgraton of resdual gasses after emplacement. Numerous small peces of slcfed (.e., composed of pure SO 2 as verfed by energy-dspersve spectrometry) vegetaton
Bull Volcanol (2016)78:3 Page 5 of 83 were found wthn the EU2 gnmbrte lthofaces. The peces are generally dscontnuous and orented parallel to PDC stratfcaton and have an rregular tubular nternal structure that suggests they were branches of small trees and brush entraned durng PDC emplacement. Ths nterpretaton ndcates a mnmum of a few years between and EU2. Alternatvely, they may be rhzolths (slcfed roots), n whch case no nferences about the duraton/tmng of breaks n actvty can be made. A more thorough nspecton of the dstrbuton, orentaton and state (e.g., fragmented or contnuous n the tuff) of ths slcfed materal s requred to constran the tme gap between the frst two eruptve unts. Nature and orgn of pyroclasts Pyroclasts collected from the dfferent unts are almost exclusvely trachytc mcrovescular pumce, obsdan, and poorly vescular cryptocrystallne grans (Plate A1 n the Supplementary Materal). Crystallne clasts are strongly domnated by flowalgned alkal feldspar and are mostly phenocryst-free. A few ash partcles of submllmeter non-vescular mcrocrystallne basalts were found n thn sectons of and EU2 materal, but not n the overlyng unts. Glasses wthn pumce, obsdan, and cryptocrystallne clasts were analyzed usng the electron mcroprobe (see Supplementary Materal for analytcal condtons and correspondng data table) to (1) verfy ther composton and (2) assess ther orgn. Only domans that dsplayed less than about 10 % mcroltes were analyzed to avod glasses overly modfed by syn-eruptve crystallzaton. The glass analyses were compared to four Hualāla trachyte groups prevously analyzed for bulk composton usng XRF (Cousens et al. 2003; Shamberger and Hammer 2006): the Pu u Wa awa a pyroclastc cone, the Pu u Anahulu flow, and the trachytes of Waha Pele (found as lthc blocks co-erupted wth recent basalt) and Huehue (extracted from a water well). Hualāla gnmbrte glass analyses were also compared to glass analyses from Pu u Wa awa a pyroclasts (obsdan and pumce wth <10 % mcroltes). Despte ther generally smlar major element compostons wthn the array of eruptves at Hualāla (Fg. a), the four man trachyte groups show subtle dfferences n bulk chemstry (Fg. b, c, d). Somewhat surprsngly, the PDC glass compostons are dstnct from nearby Pu u Wa awa a or Pu u Anahulu trachytes and much more smlar to the Waha Pele blocks, found nearly 20 km to the south. At 103 ka, the Waha Pele trachyte s substantally younger than the Pu u Anahulu flow (11 ka) and probably derves from an effusve erupton as well (Cousens et al. 2003). Therefore, although the Hualāla gnmbrtes and the Waha Pele blocks may have orgnated from the same subsurface magma reservor, they lkely represent dfferent eruptons or, at least, dfferent eruptve phases. We also note a subtle compostonal change from to EU5. The early PDCs have lower MgO and TO 2 and hgher alkal contents compared to the later PDC phases. The later phases are also most smlar to the Waha Pele compostons. The magma feedng the Hualāla gnmbrtes may have therefore been slghtly chemcally zoned. In ths scenaro, effuson of Waha Pele trachytes would have followed the erupton of Hualāla gnmbrtes. Erupton style Wth so few outcrops, our record of stratgraphc varatons wthn ths gnmbrte sequence s naturally ncomplete. Therefore, makng nferences about depost area, thckness, and volume, whch may help characterze the ntensty of the erupton(s) that produced the Hualāla gnmbrtes, s not warranted at ths pont. Nevertheless, a few observatons can be made based on the nature of the deposts and the pyroclast characterstcs. The trachyte composton of all analyzed PDC grans and the general lack of basaltc wallrock wthn the eruptve unts suggest that the Hualāla gnmbrte eruptons were domnantly magmatc, wth lttle nvolvement of external water. These PDCs are therefore dfferent n nature from the other nstances of densty current deposts found n Hawa, whch are typcally generated durng phreatomagmatc actvty (Swanson et al. 2012). The presence of substantal amounts of non- or poorly-vescular obsdan and cryptocrystallne materal along wth the erupted pumce at the base of each PDC unt may be lnked wth the dsrupton of a cryptodome or dense condut plug (.e., vulcanan actvty). A broad range of relatve vesculartes and crystallntes s consstent wth a mxture of plug materal that was accompaned by subsequent emsson of hghly vescular pumce durng a more ntense erupton phase (e.g., Hobltt and Harmon 1993; Gachett et al. 2010). Thorough pyroclast componentry, gran-sze, and densty analyses are requred to verfy ths hypothess. Implcatons for trachyte volcansm at Hualāla and on the sland of Hawa Wdespread trachytes under Hualāla The possble cogenetc relatonshp between the Waha Pele trachyte and the gnmbrtes descrbed heren would mply that trachytc PDCs may have covered a large area of Hualāla volcano. Gravty studes by Moore et al. (1987) and Kauahkaua et al. (2000) also support the dea of volumnous trachyte deposts underneath the <13 ka basalt cover. These studes showed that Hualāla dffers from other nearby volcanoes n lackng clear hgh gravty anomales that are typcally assocated wth rft zone
3: 87) 6102( 3 Page 6 of 8 Bull Volcanol Na 2 O+K 2 O TO 2 16 1 12 10 8 6 2 0 5 0.8 0.6 Hualāla lavas Hualāla trachytes Phonotephrte Basante Hawate Basalt 50 Tephrphonolte Mugearte Alkalc Tholetc HH bulk Bulk compostons Basaltc Andeste Phonolte Benmorete Andeste Trachyte 55 60 65 SO 2 WP bulk PA bulk b Dacte a c Na 2 O + K 2 O MgO 15 1 13 12 11 61 0.8 0.6 2σ glass PWW glass EU5 EU PDC EU2 glass +0.5%Ox +0.5%Ol 2σ bulk +0.5%Bt HH bulk PWW bulk PA bulk +10%An WP bulk +0.5%Ol +0.5%Bt HH bulk +0.5%Ox 62 63 6 65 SO 2 +10%An WP bulk PA bulk PWW bulk b d 0. PWW bulk 0. 0.2 +0.5%Ol +0.5%Bt +0.5%Ox +10%An 2σ glass 2σ bulk 3.2 3.6. FeO tot Fg. Geochemcal characterstcs of Hualāla trachytes. a Bulk compostons of all Hualāla products (blue symbols and orange feld) showng the clear gap n composton between erupted lavas. b, c, andd Major-element plots of Pu uwa awa a and Hualāla gnmbrtes glasses, compared wth bulk analyses of Pu u Wa awa a (PWW), Pu u Anahulu (PA), Waha Pele (WP), and Huehue (HH) samples (PWW and PA compostons are from an upcomng study, WP and HH from Cousens et al. 2003). Error bars show average standard devatons for both glass and bulk analyses. Gray arrows are dfferentaton paths for fractonal 0.2 2σ glass 2σ bulk 3.2 3.6. FeO tot crystallzaton of dfferent mneral phases typcal of the Hualāla trachytes (An anorthoclase, Ox oxde, Bt botte, Ol olvne). Note that SO 2 and MgO are usually lower and the alkals hgher n glasses from PWW pyroclasts (green trangles) when compared wth ther bulk compostons (blue feld). Ths s because most samples analyzed are not 100 % glassy (at least a few mcroltes are always present). Smlar dfferences are observed between Hualāla gnmbrte glasses (yellow, orange, and red crcles) and Waha Pele bulk compostons (purple feld) ntrusves and cumulate bodes. Instead, most of the western and northern regons of Hualāla dsplay low gravty anomales (cf. Fg. 8 n Denlnger and Morgan 201). Gven that Hualāla possesses rft zones and cumulate bodes smlar to those of neghborng volcanoes (e.g., Shamberger and Hammer 2006), the low gravty anomales must represent sgnfcant volumes of low densty materal under the recent basalt. As proposed by Moore et al. (1987), ths low-densty materal may be trachyte. Our fndng that trachyte PDCs may have mantled a large area of Hualāla agrees well wth gravty observatons and lends further support to the dea of wdespread 90 120-ka trachyte under the alkal basalt cap. The compostonal varablty of the dfferent Hualāla trachytes could be attrbuted to (1) chemcal zonng n a large magma reservor, (2) at least four dfferent small trachyte reservors scattered underneath Hualāla, or (3) delays between eruptons that would allow trachyte magma to dfferentate. The second hypothess appears the most realstc smply because the chemcal dfferences between the dfferent trachytes are not easly accommodated by ether smple fractonaton models nvolvng a sngle dfferentatng magma (e.g., Cousens et al. 2003) or magma mxng models nvolvng two magma end-members (.e., the four Pu u Anahulu,Pu u Wa awa a, Huehue, Waha Pele bulk compostons do not lne up on a unque fractonal crystallzaton or mxng trend, Fg. ). Why then, dd Hualāla erupt uncharacterstcally large volumes of trachytes compared to other Hawaan volcanoes? The unusual tmng of trachyte producton and erupton, both occurrng pror to the post sheld stage of Hualāla, may ndcate unusual condtons of crustal stress, volcanc load and/or rft-zone organzaton. Such specal condtons may be brought about by accelerated edfce slumpng or large-scale collapses (e.g., Denlnger and Morgan 201). The tmng of the North Kona slump on the western submarne sde of Hualāla ( 130 ka, Moore and Clague 1992) and the Alka landsldes produced by collapse(s) of Mauna Loa ( 112
Bull Volcanol (2016)78:3 Page 7 of 83 128 ka, McMurtry et al. 1999) roughly concde wth the onset of trachyte volcansm at Hualāla. Whether these events may have nfluenced the structure and/or stress condtons of subaeral Hualāla drectly remans to be demonstrated. Further hgher spatal resoluton gravty surveys of the area may help determne whether the subsurface dstrbuton of trachytes s coherent wth eruptons controlled by regonal volcano tectoncs. Occurrence of gnmbrtes n Hawa The relatve scarcty of gnmbrtes n the geologcal record of Hawa compared to other locatons such as Atlantc ocean slands s probably due to a combnaton of several factors. Hawa s the Earth s most actve hotspot, wth a hgh magma supply rate, underlyng a fast-movng plate. Potentally, the hgh magma supply, even after a volcano enters ts postsheld stage, prevents large volumes of magmas from dfferentatng and reachng trachyte compostons (e.g., Shaw 1985; Clague 1987), favorng nstead the formaton of large volumes of alkal basalts wth subordnate hawates, mugeartes, and benmoretes (Spengler and Garca 1988; Frey et al. 1990; Sherrod et al. 2007). In contrast, lower supply rates and slower-movng plates at Atlantc hotspots may allow large bodes of magmas to dfferentate and erupt as large gnmbrte sheets. Another consequence of the hgh magma supply and erupton rates n Hawa s that volcanoes such as Hualāla can be completely resurfaced by lava flows n a matter of <15 thousand years (Moore et al. 1987). Therefore, exposure of gnmbrtes at the surface s also less lkely n Hawa than at other locatons where resurfacng rates are substantally slower. As a result, the smaller-volume, thnner Hawaan gnmbrtes may smply be hard to detect. Hazard mplcatons The gnmbrtes descrbed here are probably of much smaller volume than those that erupted at other ocean slands such as those n the Canares, whch often reach several tens of meters n thckness (e.g., Freundt and Schmncke 1995; Troll and Schmncke 2002; Brown and Branney 200; Sgmarsson et al. 2013). Nonetheless, the Hualāla gnmbrtes were probably volumnous and wdespread enough to cause sgnfcant regonal destructon. Ths type of event s not lkely to occur n the future around Hualāla, because trachyte volcansm appears to have stopped at about 92 ka. Such eruptons are stll nevertheless possble at other Hawaan volcanoes (e.g., Mauna Loa, Mauna Kea), snce trachytes seem to erupt both n between the sheld and post-sheld volcanc stages (Hualāla) and durng the late post-sheld stages (Kohala, Island of Hawa, and West Mau, Island of Mau). Acknowledgments The feld work was greatly facltated by Ellott Parsons and the Hawa Expermental Tropcal Forest (HETF), the Hawa Dvson of Forestry and Wldlfe, and the Department of Land and Natural Resources. We thank Stephen Worley and The Bg Island Country Club for provdng access and vehcles to examne some of the deposts. John Snton s also acknowledged for the numerous frutful dscussons on Hawa trachytes. Ths paper benefted from the helpful revews of Davd Clague and Valentn Troll, as well as the nsghtful edtoral comments of Kathy Cashman. Ths project s supported by a Natonal Scence Foundaton grant EAR 1250366 to Thomas Shea and an AXA Postdoctoral Fellowshp to Jacquelne Owen. References Branney MJ, Kokelaar BP (2002) Pyroclastc densty currents and the sedmentaton of gnmbrtes. Geol Soc Lond Mem 27 Brown RJ, Branney MJ (200) Event-stratgraphy of a caldera-formng gnmbrte erupton on Tenerfe: the 273 ka Pors Formaton. Bull Volcanol 66:392 16 Clague DA (1987) Hawaan xenolth populaton, magma supply rates, and development of magma chambers. Bull Volcanol 9:577 587 Cousens BL, Clague DA, Sharp WD (2003) Chronology, chemstry, and orgn of trachytes from Hualala Volcano, Hawa. Geochem Geophys Geosyst (9) Denlnger RP, Morgan JK (201) Instablty of Hawaan volcanoes. In Poland MP, Takahash TJ, Landowsk CM, eds, Characterstcs of Hawaan volcanoes: U.S. Geologcal Survey Professonal Paper 1801, 29 p. Duncan AM, Gaspar JL, Guest JE, Wlson L (1999) Furnas Volcano, São Mguel, Azores. J Volcanol Geotherm Res 92:1 209 Edgar CJ, Wolff JA, Oln PH, Nchols HJ, Pttar A, Cas RAF, Reners PW, Spell TL, Martí J (2007) The late quaternary Dego Hernandez formaton, Tenerfe: volcanology of a complex cycle of volumnous explosve phonoltc eruptons. J Volcanol Geotherm Res 160:59 85 Esele S, Freundt A, Kutterolf S, Ramalho RS, Kwasntschka T, Wang K- L, Hemmng SR (2015) Stratgraphy of the Plestocene, phonoltc Cão Grande Formaton on Santo Antão, Cape Verde. J Volcanol Geotherm Res 301:20 220 Freundt A, Schmncke H-U (1995) Petrogeness of rhyoltetrachytebasalt composte gnmbrte P1, Gran Canara, Canary Islands. J Geophys Res 100:55 7 Freundt-Malecha B, Schmncke H-U, Freundt A (2001) Plutonc rocks of ntermedate composton on Gran Canara: the mssng lnk of the bmodal volcanc sute. Contrb Mneral Petrol 11:30 5 FreyFA,WseWS,GarcaMO,WestH,KwonS-T,KennedyA (1990) Evoluton of Mauna Kea volcano, Hawa: petrologc and geochemcal constrants on postsheld volcansm. J Geophys Res 95:1271 1300 Gachett T, Drutt TH, Burgsser A, Arbaret A, Galven C (2010) Bubble nucleaton, growth, and coalescence durng the 1997 Vulcanan explosons of Soufrere Hlls Volcano, Montserrat. J Volcanol Geotherm Res 193:215 231 Hansteen TH, Troll VR (2003) Oxygen sotope composton of xenolths from the oceanc crust and volcanc edfce beneath Gran Canara (Canary Islands): consequences for crustal contamnaton of ascendng magmas. Chem Geol 193:181 193 Hobltt RP, Harmon RS (1993) Bmodal densty dstrbuton of cryptodome dacte from the 1980 erupton of Mt. St. Helens, Washngton. Bull Volcanol 55:21 37 Kauahkaua J, Hldenbrand T, Webrng M (2000) Deep magmatc structures of Hawaan volcanoes, maged by three dmensonal gravty models. Geology 28:883 886
3: 87) 6102( 3 Page 8 of 8 Bull Volcanol McMurtry GM, Herrero-Bervera E, Cremer M, Resg J, Sherman C, Smth JR, Torresan ME (1999) Stratgraphc constrants on the tmng and emplacement of the Alka 2 gant Hawaan submarne landslde. J Volcanol Geotherm Res 9:35 58 McPhe J, Walker GPL, Chrstansen RL (1990) Phreatomagmatc and phreatc fall and surge deposts from explosons at Klauea Volcano, Hawa, 1790 A.D.: Keanakako Ash Member. Bull Volcanol 52: 33 35 Moore JG, Clague DA (1992) Volcano growth and evoluton of the sland of Hawa. Geol Soc Am Bull 10:171 18 Moore RB, Clague DA, Rubn M, Bohrson WA (1987) Hualala volcano: a prelmnary summary of geologc, petrologc, and geophyscal data, n Volcansm n Hawa, edted by RW Decker, TL Wrght, and PH Stauffer, USGS Prof Pap 1350:571 585 Shamberger P, Hammer JE, (2006) Leucocratc and gabbroc xenolths from Hualala Volcano, Hawa. J Pet 7:1785 1808 Shaw HR (1985) Lnks between magma-tectonc rate balances, plutonsm, and volcansm. J Geophys Res 90(11275):1288 Sherrod DR, Snton JM, Watkns SE, Brunt KM (2007) Geologc Map of the State of Hawa. USGS Open Fle Report 2007-1089. Sgmarsson O, Laporte D, Carpenter M, Devouard B, Devdal J-L, Mart J (2013) Formaton of U-depleted rhyolte from a basante at El Herro, Canary Islands. Contrb Mneral Petrol 165:601 622 Spengler SR, Garca MO (1988) Geochemstry of the Haw lavas, Kohala volcano, Hawa. Contrb Mneral Petrol 99:90 10 Stearns HT, Macdonald GA (192) Geology and groundwater resources of the sland of Mau, Hawa. Hawa Dv Hydrography Bull 7 Stearns HT, Macdonald GA (196) Geology and groundwater resources of the sland of Hawa. Hawa Dv Hydrography Bull 7:3 Swanson DA, Rose TR, Fske RS, McGeehn JP (2012) Keanakāko Tephra produced by 300 years of explosve eruptons followng collapse of Kīlauea s caldera n about 1500 CE. J Volcanol Geotherm Res 215 216:8 25 Troll VR, Schmncke H-U (2002) Magma mxng and crustal recyclng recorded n ternary feldspar from compostonally zoned peralkalne gnmbrte BA^, Gran Canara, Canary Islands. J Pet 3:23 270 Van der Zander I, Snton JM, Mahoney JJ (2010) Late sheld-stage slcc magmatsm at Wa anae Volcano: evdence for hydrous crustal meltng n Hawaan volcanoes. J Pet 51:671 701 Velde D (1978) An aengmatte-rchterte-olvne trachyte from Puu Koae, West Mau, Hawa. Am Mneral 63:771 778