Coastal evolution on volcanic oceanic islands: A complex interplay between volcanism, erosion, sedimentation, sea-level change and biogenic production
Introduction
Coastlines are the ever-changing boundary between the land and the sea. They are complex threshold-driven, non-linear dynamical systems (Naylor and Stephenson, 2010) whose evolution is the product of mutual adjustments in topography and fluid dynamics in response to changes in external conditions (Wright and Thom, 1977, Trenhaile, 1997, Woodroffe, 2002). Few places on our planet experience more dramatic and rapid changes in topography and external conditions than the coasts of hotspot islands, which experience the effects of volcanism, flank collapses, and exposure to open ocean. Additionally, coastlines on oceanic island volcanoes have a clear beginning (through volcanic emergence) and a predictable end (through island subsidence/erosion), and evolve as the edifices themselves evolve. Oceanic island volcanoes are, thus, prime natural laboratories to study the different processes that interplay in a complex manner to shape coastlines.
Oceanic hotspot islands are prominent, dynamic geological features that rise from the deep seafloor by a combination of volcanic, intrusive and tectonic processes. Coastlines are established as soon as oceanic island edifices breach sea level, and they become the forefront of a raging battle that, in the long term, is lost. This confrontation is, essentially, a competition between volcanic (and biogenic) construction on one side, and erosive destruction on the other. This balance, or more appropriately this imbalance, of powers varies in space and time as island edifices evolve. In this paper we offer an overview on the main mechanisms that shape oceanic island coastlines, and how these mechanisms act differentially throughout the successive stages of island evolution. An analysis of the various factors that control coastal evolution on oceanic island volcanoes is also presented here and discussed, albeit in a qualitative manner.
Section snippets
Oceanic hotspot island evolution and development of coastlines
Oceanic island volcanoes frequently follow an evolutionary trend that exhibits some basic similarities across different hotspot systems (Schmincke, 2004, Ramalho, 2011). This evolutionary trend is essentially caused by variations in the rate of magma-supply through geological time. This, in turn, is a function of plate motion relative to the hotspot melting source, plate age/thickness, proximity to a plate boundary, and melt source characteristics (Ramalho, 2011). Internal factors such as those
Factors controlling coastal evolution
Several factors interplay in a complex manner to shape coastlines at oceanic island volcanoes. As one or more of the factors change, feedback mechanisms immediately cause adjustments in the other factors, driving the system back towards equilibrium (though it never reaches this state). These factors are, chiefly: volcanism, tectonics, mechanical properties of shoreline lithologies, wave energy parameters, the amplitude of eustatic change, mass wasting, subaerial erosion, sediment production and
Volcanic growth and synvolcanic erosion and sedimentation
Oceanic hotspot islands are essentially very large polygenetic volcanic edifices and so volcanism is, inevitably, the foremost agent of insular growth and shoreline expansion. The style of volcanism, however, dictates the type of morphology created by volcanic activity. Eruptive style, in turn, is mainly controlled by magma composition and the mixing mass ratio of water/magma in the system (Wohletz and Sheridan, 1983); since hotspot magmas are overwhelmingly basaltic in composition, it is
Coastal erosion
Oceanic island volcanoes are exposed to the destructive forces of the ocean from the moment they breach the sea surface until they (eventually) are drowned. The relative importance of marine erosion processes in shaping oceanic island coasts greatly depends upon the role and character of the other agents (e.g. volcanism, coral reef growth). Marine erosion is the dominant agent (or at least an important one) of coastal evolution during: the emergent island stage of any island, especially when
The role of biogenic processes
Living organisms established on coastlines may act as erosive agents (e.g. lithophagous organisms), constructors of large lithic structures that further enlarge and protect coasts from erosion (e.g. coral reefs, coralline algae), or sediment suppliers (e.g. organisms with mineral shells, such as mollusks and crustaceans); they may also simultaneously assume a combination of any of these functions (e.g. echinoderms) (Taylor and Wilson, 2003, Wilson, 2007, Wisshak et al., 2010, Davidson, 2011).
Sedimentation along coastlines and insular shelves
Oceanic islands are, by nature, exposed to wave action and thus sediment dynamics along their coasts and insular shelves are typically wave- to storm-dominated. The presence of reefs, however, as we previously argued, changes sediment dynamics considerably. When coral reefs are present, carbonate sedimentation increases significantly, either from erosion of the reef framework or from direct deposition as skeletal components on islands with fringing (Calhoun et al., 2002, Harney and Fletcher,
Edifice slope failure (landsliding)
A distinction is made here between massive failure and lava delta collapse, both of which typically occur during the shield-building stage, and seacliff failures occurring wherever surf erosion generates unstable cliffs.
Structural and tectonic control on coastline evolution
On many oceanic island volcanoes, coastal morphology is partly controlled by active or inherited tectonic features and/or volcano-tectonic structures. These result from either regional tectonic stresses or local volcano-tectonic and/or gravitational stresses affecting the edifices (Fiske and Jackson, 1972, Dieterich, 1988, Walker, 1993). Structural control on coastal morphology is especially evident on island edifices that are located at or near plate boundaries, i.e. island edifices built by
Conclusions
Oceanic island volcanoes are very dynamic landscape systems and they constitute prime localities to look at how different processes and agents of change interact in complex ways to shape coastal evolution. Volcanic island coasts are also one of the few places where constructional processes can be observed over short time scales, frequently occurring side by side with destructive processes. Coastal processes typically operate more rapidly on islands than on their continental counterparts.
Acknowledgments
We would like to acknowledge: Tim Orr, Mike Poland and the Hawaiian Volcano Observatory (USGS) for their support in obtaining some of the photos used in this work; Tim Orr also for his support during a field trip to the Kalapana coast and for helpful insights on the formation of lava deltas and littoral cones; Dave Kennedy, Paulo E. Fonseca, Carlos M. da Silva and Fernando Cardigos for kindly providing some of the photos used in this work; George Helffrich and Tine Thomas for their general
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Formerly at Institut für Geophysik, Westfälische Wilhelms-Universität, Münster, Germany.