Topographic History ofthe Maui Nui Complex, Hawai'i, and Its Implications for Biogeography 1 Jonathan Paul Price 2,4 and Deborah Elliott-Fisk3 Abstract: The Maui Nui complex of the Hawaiian Islands consists of the islands of Maui, Moloka'i, Lana'i, and Kaho'olawe, which were connected as a single landmass in the past. Aspects of volcanic landform construction, island subsi­ dence, and erosion were modeled to reconstruct the physical history of this complex. This model estimates the timing, duration, and topographic attributes of different island configurations by accounting for volcano growth and subsi­ dence, changes in sea level, and geomorphological processes. The model indi­ cates that Maui Nui was a single landmass that reached its maximum areal extent around 1.2 Ma, when it was larger than the current island of Hawai'i. As subsi­ dence ensued, the island divided during high sea stands of interglacial periods starting around 0.6 Ma; however during lower sea stands of glacial periods, islands reunited. The net effect is that the Maui Nui complex was a single large landmass for more than 75% of its history and included a high proportion of lowland area compared with the contemporary landscape. Because the Hawaiian Archipelago is an isolated system where most of the biota is a result of in situ evolution, landscape history is an important detertninant of biogeographic pat­ terns. Maui Nui's historical landscape contrasts sharply with the current land­ scape but is equally relevant to biogeographical analyses. THE HAWAIIAN ISLANDS present an ideal logic histories that can be reconstructed more setting in which to weigh the relative influ­ easily and accurately than in most regions. ences of ecological phenomena (concerned Having been extremely isolated in the Pacific with local conditions and species interactions) since its inception, the Hawaiian biota has and historical phenomena (concerned with evolved in situ from a limited number of col­ dispersal and speciation) on the composition onists, achieving very high levels of ende­ of species assemblages. Environmental gra­ mism (Carlquist 1980). Hawai'i's biota is dients such as elevation and moisture are very therefore best examined in terms of how cur­ readily studied in the Hawaiian Islands, as rent conditions and island history relate to compared with many regions. The Islands the dispersal, evolution, and extinction of also span a remarkable time line of geo- organisms. The Hawaiian Islands are volcanic in ori­ gin, going through a life cycle with well­ I This work was supported by NSF Doctoral Disser­ tation Improvement Grant no. 9900933 and a grant from defined stages (Macdonald and Abbott 1970, the ]astro-Shields Foundation at University of California, Moore and Clague 1992). Upon formation on Davis. Manuscript accepted 1 May 2003. the seafloor, a volcano grows until it forms 2 Department of Botany, MRC-166, National Mu­ a gently sloped volcanic shield above the sea seum of Natural History, Smithsonian Institution, P.O. surface. During shield building and for some Box 37012, Washington, D.C. 20013-7012. 3 Department of Wildlife, Fisheries, and Conserva­ time after completion, weight of the volcanic tion Biology, University of California, Davis, 1 Shields mass on Earth's crust causes it to subside. Avenue, Davis, California 95616. Despite some postshield volcanism, subse­ 4 Corresponding author. quent erosion and subsidence further reduce the volcano to sea level until it ultimately be­ Pacific Science (2004), vol. 58, no. 1:27-45 comes an atoll or seamount. Volcanoes of the © 2004 by University of Hawai'i Press Hawaiian Archipelago increase in age to the All rights reserved northwest, exhibiting a linear progression of 27 28 PACIFIC SCIENCE· January 2004 GKaUa'i-4.7 Ni'ihau - 5.2 ai'anae~3'0 , Ko'olau - 2.6 Oahu L :A. / £:estMoloka'i _2.0 East Moloka'i - \.8 :.to. :A. Penguin Bank _2.2 /). 2Yest Maui - \.5 _ ,. ~ :A.:A. East Maui - \.2 Lana I- 1.5 (Haleakalli) ~ Kaho'olawe - 1.2 Hawai'j Hualalai - 0.4 0 50 100 150 200 Kilometers I MaunaLoa (near end of 0 25 50 75 100 125 Miles shield-building) FIGURE 1. Volcanoes of the major Hawaiian Islands. Ages given in myr are from Clague (1996) and reflect the esti­ mated end of the shield-building stage. these stages (Figure 1). Although this age (Moore 1987). The only estimate of the na­ gradient is the primary historical variable, the ture and timing of changes in Maui Nui's islands of Maui, Moloka'i, Lana'i, and Ka­ configuration due to subsidence is that by ho'olawe share a distinct history in having Carson and Clague (1995). They postulated once formed a single island, "Maui Nui." the following sequence: (1) Penguin Bank Stearns and Macdonald (1942) first sug­ Volcano was connected to the island of O'ahu gested that four islands of the Maui Nui via a land bridge; (2) newer volcanoes in the complex were conjoined before subsidence. Maui Nui complex coalesced with older ones, Age and duration of this formation remained ultimately forming a single landmass larger vague, however. Later understanding of sea­ than the current island of Hawai'i; (3) as level history and bathymetry surrounding the subsidence ensued, submerging saddles be­ Islands revealed that they were connected re­ tween its volcanoes, this landmass gradually cently during the low sea stands of glacial divided into separate islands as Moloka'il periods into a landmass referred to as "Maui Lana'i separated from Maui/Kaho'olawe less Nui," Hawaiian for "Big Maui" (Macdonald than 300,000 to 400,000 years ago (ka); and and Abbott 1970, Nullet et al. 1998). Detailed (4) finally, the complex consisted of four dis­ bathymetry and marine geological explora­ crete islands by less than 100 to 200 ka. tion made possible by submersible vehicles However, concurrent with these events, sea­ illuminated the magnitude of subsidence: level changes reunited the fragments periodi­ areas surrounding the Islands have subsided cally, as pointed out by Asquith (1995). Thus, as much as 2000 m as evidenced by former multiple processes have resulted in a complex shoreline features existing at extreme depths history. Topographic History of the Maui Nui Complex . Price and Elliott-Fisk 29 Our current understanding of volcanism, grid of values representing the elevation at subsidence, and sea-level change, along with each point, the resolution (lateral spacing of advances in submarine surveying and avail­ points) of which was 300 m. able Geographic Information System (GIS) Key features were identified from this base technology, makes possible a detailed recon­ DEM in conjunction with published sources struction of how the spatial and topographic and consultation with geologists (David characteristics of the Maui Nui complex have Clague, David Sherrod). These features, in­ changed over time. Also, because climate in cluding former shorelines and volcanic fea­ the Hawaiian Islands is largely a function tures, defined five components of a composite of topographic attributes of an island (e.g., model, each detailing a given physical char­ height of mountain masses, aspect with re­ acteristic or process. The major components spect to weather systems), details of past of the model are as follows: (1) assessment topographic attributes should help resolve of age and spatial distribution of volcanic climate history, thus further enhancing un­ shields; (2) reconstruction of East Maui's derstanding of evolutionary history. The (Haleakala Volcano) topography before and complex history presented here provides a after extensive postshield volcanism; (3) esti­ framework on which to base detailed biogeo­ mation of the extent and timing of major graphical hypotheses. erosion and landslides; (4) determination of the timing and spatial variability of tectonic subsidence; and (5) consideration of glacio­ MATERIALS AND METHODS eustatic sea-level change. The overall model The first step in modeling past landscapes adjusts the base DEM by accounting for the of the Maui Nui complex is accurate com­ processes in each component to create new pilation of geographic data. Topographic DEMs approximating the topography of the and bathymetric (seafloor topography) data Maui Nui complex through time. Each com­ were obtained from several sources. High­ ponent is detailed here. precision sonar data were supplied by James Gardner (U.S. Geological Survey, Menlo Shield-Building Volcanism Park, California) and David Clague (Monte­ rey Bay Aquarium Research Institute, Moss Volcanoes in the Hawaiian Islands form in Landing, California). A less detailed but more response to hot-spot magmatism deep below spatially extensive composite data set came the lithosphere. As a volcano is moved away from the University of Hawai'i, School of from the hot spot by motion of the Pacific Ocean and Earth Science and Technology. tectonic plate, it ceases volcanic activity and a This was used for areas where detailed sonar new vent forms (Wilson 1963). Thus a chain data were unavailable or had sparse coverage, of volcanoes forms along the direction of as well as for all areas above sea level. The plate motion, with younger volcanoes near area between Moloka'i and Lana'i was not the position of the hot spot. As volcanoes accurately represented by any data set, and so emerge above the sea surface, they form a spot depths from a National Oceanic and At­ gently sloping volcanic shield composed of mospheric Administration nautical chart were tholeiitic basalt. The period from when a new used; the lower precision of these data is ac­ volcano breaks the sea surface to the end of ceptable given the shallow depth of the area shield building is estimated to last between (<100 m), which makes them sufficiently ac­ about 0.5 million yr (myr) (Moore and curate. In scattered locations, elevations were Clague 1992) and 1.0 myr (Guillou etal. 2000). interpolated from nearby data in small areas Available radiometric ages of tholeiitic lavas where no accurate data were available.