Vanished Islands and Hidden Continents of the Pacific

3: Islands That Vanished Long Ago

The periodic disappearance of islands and continents is part of the natural evolution of the earth’s surface. It happened before people were present to witness it and, as will be seen later in the book, it happened after they arrived. There is no need to invest the process with undue importance simply because it was witnessed by the first species that would come to recognize it for what it was, then record it and ponder its meaning. Questions concerning the presence or absence of large continental landmasses in the ocean basins at particular times in the past are understandably easier to prove or refute than questions concerning smaller landmasses, like islands. There are innumerable islands whose highest parts currently lie beneath the ocean surface but that rose above it in the past. The change may have occurred because the islands either subsided or were drowned by sea-level rise (or both). Ascertaining the time(s) when these sunken islands were emergent is often problematical , yet critical, for example, to the reconstruction of transocean biogeographic tracks and for linking the existence of such islands to oral traditions concerning their disappearance. This chapter gives an account of those islands in the Pacific that were formerly emergent but are now submerged. It focuses exclusively on those islands that were submerged before humans settled the region in large numbers. The purpose of this exercise is to show that island submergence, for a variety of reasons, has been a relatively common phenomenon during the history of the Pacific. This supports the contention that incidences of island disappearance recalled by humans through oral traditions (discussed in chapters 6 and 9) are not necessarily fanciful or even wildly exaggerated versions of something that actually happened. At the outset, we need to distinguish islands from continents. Unfortunately, this has never really been done successfully although continents are generally considered to be larger, older, and geologically more complex than islands. As a working definition, this has to suffice.1 Islands go up and down for many reasons, and sea-level changes also cause some is3Islands That Vanished Long Ago 36 Vanished Islands and Hidden Continents of the Pacific lands to become either emergent or submergent. The purpose of this chapter is to show, using a number of well-documented examples, how such processes led particular islands to vanish. Islands That Have Disappeared into Ocean Trenches Ocean trenches, those asymmetrical steep-sided chasms etched into the ocean floor, are often portrayed as smooth-sided: places where one plate slides effortlessly beneath the other, part of the great engine that is plate tectonics. The reality is very different. Oceanic crust is everywhere marred by irregularities—islands, ridges, swells—that are carried into ocean trenches and distort or sometimes disrupt the process of subduction. Some of the best examples in the Pacific come from lines of islands that have been carried along the moving ocean floor, pulled down into ocean trenches, and finally thrust underneath the overlying plates. The Louisville Ridge is an underwater ridge in the South Pacific with several high spots that were once probably islands. It collided obliquely with the TongaKermadec Trench around six million years ago and has since been dragged down beneath the Tonga Island Arc (see Figure 2.4c). The section of the Tonga Island Arc beneath which the northern part of the Louisville Ridge has disappeared is noticeably wider than what is regarded as normal for this island arc. Specifically in this section there is a 60-kilometer gap between the nonvolcanic arc, which includes islands like Tongatapu and those in the Ha‘apai Group, and the active volcanic arc, including islands like Tofua, that is attributed to the subduction of the Louisville Ridge.2 Farther west, plate convergence is occurring along the Vanuatu (New Hebrides) Trench west of the islands of Vanuatu (shown in Figure 9.5a). In the central part of this trench, the D’Entrecasteaux Ridge is being dragged down into the trench and subducted in much the same way as the Louisville Ridge. But something quite different is happening in Vanuatu. The D’Entrecasteaux Ridge is not slipping down beneath the overlying plate quite as readily as we imagine the Louisville Ridge to have done. Perhaps because the D’Entrecasteaux Ridge is thicker, perhaps because the slope down into the trench is steeper, it is clear that the D’Entrecasteaux Ridge is bulldozing into the central Vanuatu Arc, pushing it eastward at around 50 millimeters/year. In the process, the...