The coral reef of South Moloka'i, Hawai'i - Portrait of a sediment-threatened fringing reef

Scientific Investigations Report 2007-5101

Edited by:
Michael E. Field, Susan A. Cochran, Joshua Logan, and Curt D. Storlazzi



Moloka'i is the fifth youngest island in the long chain of volcanoes and volcanic remnants that compose the Hawaiian archipelago (fig. 1). The archipelago extends from the Island of Hawai'i (the 'Big Island') in the southeast past Midway Island, to Kure Atoll in the northwest, for a total distance of about 2,400 km (1,500 mi). Beyond Kure Atoll, the chain continues as a series of submerged former islands known as the Emperor Seamounts, which extend to the Aleutian Trench off the coast of Alaska. Evolution of the entire Hawai'i-Emperor volcanic chain represents a time span of nearly 80 million years (Clague and Dalrymple, 1989). The volcanic chain is a result of gradual and persistent movement of the Pacific lithospheric plate (the sea-floor crust and rigid uppermost part of Earth's mantle) over a deep fracture (or hot spot) that extends down to the astenosphere, a less rigid part of the mantle (fig. 2). Plumes of molten lava flowed onto the sea floor, repeatedly creating massive shield volcanoes that exceed 10,000 m (33,000 ft) in relief above the surrounding sea floor. The growth of each volcano is a process that takes half a million years or more to construct most of its mass through sequential volcanic phases - submarine, explosive, and subaerial - of shield growth. Once formed, each massive island volcano is carried northwestward on the Pacific tectonic plate at rates of 8.6 to 9.2 cm/yr (Clague and Dalrymple, 1989). The postshield processes of alkalic volcanism, subsidence, landslides, rejuvenated volcanism, weathering, sediment deposition, and reef growth have all markedly influenced each volcano's present-day shape. Subsidence of each island is rapid at first (rates of 2 mm/yr or more; Moore and Campbell, 1987; Moore and Fornari, 1984; Campbell, 1986) in response to the extraordinary weight of large volumes of lava loaded onto the crust. As each island cools and slides northwestward with the sea-floor crust, it continues to subside at decreasing rates, down to the order of 0.02 mm/yr (Detrick and Crough, 1978). The sheer volume of rock that accumulates at each volcano ultimately leads to failure and partial collapse - each island has had spectacular landslides that are amongst the largest on earth (Moore and others, 1989). The large areas of irregular topography on the sea floor around the islands (for example, north of Moloka'i and northwest of O'ahu) attest to the magnitude of these events (fig.1). The normal processes of surface erosion and stream runoff modify volcano slopes early in an island's history. Those processes, along with development of soils, which occurs relatively quickly in humid volcanic terrain, lead to transport and deposition of sediment in alluvial fans, flood plains, and narrow coastal plains. The final process in island shaping is the establishment of coral reefs in shallow waters that are protected from large waves. Corals colonize exposed rock surfaces very quickly (Grigg and Maragos, 1974; Grigg, 1983), and it is likely that they become established early in the evolutionary history of each island. The development of coral reefs - the massive limestone structures capped by a living ecosystem that border many Hawaiian Islands - takes much longer (Grigg, 1987). Each reef is a thick (meters to tens of meters) packet of reefal limestone that likely accumulated over multiple stages of sea-level shifts (Grossman and others, 2006; Grossman and Fletcher, 2004; Sherman and others, 1999). In most locations in Hawai'i, modern coral cover is only a thin living veneer on top of older reef structures that formed during an earlier time under different conditions (Grigg, 1983, 1998)

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The coral reef of South Moloka'i, Hawai'i - Portrait of a sediment-threatened fringing reef
Series title:
Scientific Investigations Report
Series number:
Version 1.0
Year Published:
U.S. Geological Survey
Contributing office(s):
Coastal and Marine Geology Program
vi, 190 p.
United States
Other Geospatial:
South Molokai Reef
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