Geological Society of America Bulletin, published online on 2 May 2014 as doi:10.1130/B30936.1 Ka‘ena Volcano—A precursor volcano of the island of O‘ahu, Hawai‘i John M. Sinton1,†, Deborah E. Eason1, Mary Tardona1, Douglas Pyle1,§, Iris van der Zander1,#, Hervé Guillou2, David A. Clague3, and John J. Mahoney1,* 1University of Hawai‘i, Department of Geology and Geophysics, 1680 East-West Road, Honolulu, Hawaii 96822, USA 2Laboratoire des Sciences du Climat et de L’Environment, Commissariat à l’Energie Atomique et aux Energies Alternatives/Centre National de la Recherche Scientifi que, Gif sur Yvette, 91198, France 3Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA ABSTRACT O‘ahu, which we call Ka‘ena Volcano. After island of O‘ahu, and Ka‘ena Ridge appears to emergence, Ka‘ena Volcano tilted ~2° to the form a general prolongation of that volcano. Ka‘ena and Wai‘alu Ridges form promi- south. We estimate a total volume of 20–27 × However, Moore et al. (1989) alluded to the nent submarine ridges NW of the island of 103 km3 for Ka‘ena Volcano, taking into ac- possibility of a “third volcano forming Ka‘ena O‘ahu, Hawai‘i. We evaluate whether or count overlapping geometry of concurrently Ridge” that might have contributed debris to not either one of these ridges represents a active volcanoes. Sample compositions from the Wai‘anae slump, and Smith (2002) drew sub marine extension of Wai‘anae Volcano the Ka‘ena landslide deposit are entirely con- attention to several enigmatic aspects of its on O‘ahu using new bottom observations, sistent with derivation from Ka‘ena, whereas morphology, including its large size in the geophysical surveys, and geochemical data most samples from the Wai‘anae slump are middle of the Kaua‘i Channel, and the pres- acquired on new samples from the region. likely derived from Wai‘anae Volcano. Uni- ence of “cone-shaped topographic highs” on it. Wai‘alu Ridge has the morphology of a sub- formly oriented dikes in the Wai‘anae NW Although Coombs et al. (2004) left the relation marine rift zone but is too shallow for its rift zone likely refl ect buttressing by a pre- of Ka‘ena Ridge to Wai‘anae unresolved, their distance from the O‘ahu shoreline; Ka‘ena exist ing Ka‘ena Volcano. Unusual isotopic fi gure 1 refl ects the possibility that it might Ridge also is unusually shallow and is sur- compositions of some Wai‘anae samples, in- be an extension of one of Wai‘anae Volcano’s mounted by two topographic shields. Ka‘ena cluding unique hydrous silicic lavas, probably rift zones, and Robinson and Eakins (2006) and Wai‘alu Ridges have similar magmatic refl ect interaction with underlying Ka‘ena included Ka‘ena Ridge in their calculation of and volcanic evolutionary histories, begin- crust. A newly recognized lava fl ow fi eld on the volume for Wai‘anae Volcano. ning ca. 5 Ma with a submarine, shield phase the southern fl ank of Ka‘ena Ridge extends The morphology of Ka‘ena Ridge differs of volcanism that produced high-SiO2, low- the previously known distribution of sec- from that of most Hawaiian rift zones, which FeO* tholeiites with higher 208Pb/204Pb than in ondary volcanism in the Kaua‘i Channel. steadily increase in depth with distance from the the adjacent Wai‘anae Volcano. Late-shield Putative submarine volcanic activity in the shoreline (Fig. 2) and have a hummocky appear- volcanism included transitional and alkalic region in 1956 cannot have built a large edi- ance refl ecting constitution predominantly of rock types, with lower SiO2 and enrichment fi ce and is unlikely to have produced pumice pillow lavas (Smith, 2002; Smith et al., 2002). in incompatible elements, especially P2O5, that was found on O‘ahu shores. This erup- The lower part of Wai‘alu Ridge has the pro- Nb, Zr, Ti, and light rare earth elements. The tive activity therefore remains unconfi rmed. fi le and hummocky appearance of other sub- transition from shield to late-shield stage oc- marine rift zones (Figs. 1 and 2), although the curred as the edifi ce was beginning to emerge INTRODUCTION overall profi le is unusually shallow for its dis- from the sea. Geological observations and tance from the O‘ahu shoreline (Fig. 2), and it K/Ar ages indicate that Ka‘ena emerged A large region of shallow bathymetry appears to radiate away from the north-central above sea level ca. 3.5 Ma, reaching a maxi- extends almost 100 km WNW from Ka‘ena part of Ka‘ena Ridge, and not from either of the mum height of ~4000 m above the abyssal Point, the western tip of the island of O‘ahu, sub aerial volcanoes of O‘ahu (Fig. 1). Ka‘ena ocean fl oor and 1000 m above sea level. Rela- Hawai‘i (Fig. 1). First revealed in early bathy- Ridge has a much smoother profi le, is generally tively weak gravity anomalies, topographic metric mapping around the Hawaiian Islands shallower at a given distance from shore than lineaments, and the orientation of dike com- (e.g., U.S. Navy, 1939), it was referred to by other ridges, and is surmounted by two topo- plexes indicate a volcanic structure that is Stearns (1978) as the Waho Shelf but desig- graphic highs (Fig. 2). independent of Wai‘anae Volcano. Thus, nated Ka‘ena Ridge on later maps (e.g., Wilde Resolution of the question whether Ka‘ena volcanic structure, geochemistry, and age all et al., 1980). Most submarine ridges in the and Wai‘alu Ridges represent submarine exten- indicate a precursor volcano to the island of Hawaiian chain are extensions of volcanic rift sions of Wai‘anae Volcano, or possibly one zones (e.g., Fiske and Jackson, 1972; Smith or more previously unrecognized volcanoes, †E-mail: sinton@ hawaii .edu. et al., 2002; Lipman and Calvert, 2011), but the has important implications for understanding § Current address: U.S. Embassy, Unit 3240 Box #7, relation of Ka‘ena Ridge to the rest of O‘ahu is vol canic construction of the young end of the DPO, AA 34021-0007, Managua, Nicaragua. #Current address: Environmental Science Inter- uncertain. Ka‘ena Point (Fig. 1) is composed Hawaiian chain, and especially for the struc- national Inc., Kailua, Hawaii 96734, USA. of lava fl ows of the Wai‘anae Volcano, the tural and geochemical evolution of the island of *Deceased. older of the two volcanoes that make up the O‘ahu. Volcano spacing among the 17 youngest GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–26; doi: 10.1130/B30936.1; 14 fi gures; 6 tables; Data Repository item 2014194. For permission to copy, contact [email protected] 1 © 2014 Geological Society of America Geological Society of America Bulletin, published online on 2 May 2014 as doi:10.1130/B30936.1 Sinton et al. 22° 10 km Kaua‘i O‘ahu Moloka‘i 21° Maui Lāna‘i Kaho‘olawe 20° Hawai‘i 19° −3000 160° 159° 158° 157° 156° 155° D7 −4000 J2−380 Ka‘ena Slide 22° J2−305 Wai‘alu Ridge D1 −3000 00′N J2−556 D8 T−328 T−327 D2 −2000 D5 J2−251 Ka‘ena Ridge −1000 D9 X -2000 −1000 J2−306 T−325 −1000 −3000 D6 F4 F2 −4000 J2−551 Ka‘ena Pt. J2-557 D4 2500 −1000 −30−3000 21° J2−381 J2−560 30′N 21° 34′ Wai‘anae Slump 3000 3 km T−326 21° −3500 −2000 32′ 158°40′ 158°38′ 159°00′W 158°30′W Figure 1. Bathymetric map of the seafl oor west of O‘ahu, Hawai‘i; contour interval = 200 m, labeled every 1000 m. Box in upper-left inset shows map location relative to main Hawaiian Islands. Dredge tracks for R/V Kilo Moana cruise KM06–03 (prefi x D) and R/V Farnella cruise F11–88-HW (prefi x F) are shown as thick black lines; track for D8 is beneath lower part of dive J2–556. Blue (Ka‘ena and Wai‘alu Ridge) and red (secondary alkalic) dots are new samples collected by remotely operated vehicle (ROV) Jason from this study; green circles are ROV Tiburon sample locations (prefi x T) from Coombs et al. (2004), and white dots are sample locations from ROV Jason dives 305 and 306 from Greene et al. (2010). The outlines of the Ka‘ena and Wai‘anae landslide deposits are shown with light shading. Circled X shows approximate location of the “disturbance” of 22 May 1956 (from Macdonald, 1959). Inset in lower-left corner (see box on main fi gure for location; contour interval every 500 m) shows the constructional mounds of secondary alkalic pillows and lobate lavas on the lower slopes of Ka‘ena Ridge near 158°39′W, 21°34′N. 2 Geological Society of America Bulletin, Month/Month 2014 Geological Society of America Bulletin, published online on 2 May 2014 as doi:10.1130/B30936.1 Ka‘ena Volcano 100 km A WR Figure 2. Bathymetric profi les along sub marine 22°N ridges in the Hawaiian province. (A) Map KR showing locations of profi les along the sub- PW marine Ka‘ena Ridge (KR), Wai‘alu Ridge (WR), Pa‘uwela Ridge (PW, probable exten- sion of East Moloka‘i east rift zone), Hāna Ridge (HR), Hilo Ridge (HiR), Puna Ridge HR (PR, Kīlauea east rift zone), and Ka Lae Ridge (KL, Mauna Loa SW rift zone). (B) Profi les along ridges shown in A; horizontal scale is in km from the shoreline; vertical exaggeration is 20°N HiR ~6:1.