Barbara H. Keating

Age and origin of Truk Atoll, eastern Caroline Islands: Geochemical, radiometric-age, and paleomagnetic evidence

Geochemical, K-Ar age, and paleomagnetic data are presented for volcanic rocks from Truk Lagoon (7°20′N, 151° 15′E). Petrographic observations and 52 analyses for major and trace elements reveal 2 shield-building magma types—a mildly alkalic Truk Main Lava Series (TMLS) and a less alkalic Truk Transitional Lava Series (TTLS); and a post-erosional, highly silica-undersaturated, and incompatible-trace-element–enriched Truk Nephelinite Series (TNS). The TMLS and TTLS shield-building lavas give a mean K-Ar age of 10.9 m.y.; post-erosional TNS lavas are dated at 4.8 m.y. B.P. Paleomagnetic results for 15 stably magnetized sites give a paleopole of declination and inclination of 359.3°E and 8.8°30′, respectively, which suggests that Truk was formed 4.4° north of the equator. These data are consistent with a hot-spot origin for the volcanics of Truk Atoll.

Instability and structural failure at volcanic ocean islands and the climate change dimension

Abstract Keating and McGuire (2000) [Island edifice failures and associated tsunami hazards. In “Landslides and Tsunamis”, Birkhauser, Boston, pp. 899–956] presented and examined evidence for ubiquitous island edifice failures and their associated tsunami hazards. In this follow-up review, we examine the status of landslide, debris flow and tsunami research and find that significant progress has been made in placing constraints on physical parameters that will facilitate numerical modeling of tsunami, landslide and debris flow movements. Similarly, physical modeling has provided an important contribution to our understanding of slope failure and debris transport, with many features generated in laboratory experiments clearly identifiable in sonar images of mass wasting events. In particular, we address published evidence and proposed models supporting the rule of climate change toward the generation of ocean island and continental margin landslides. Evidence presented from the Canary Islands and the Madeira Abyssal Plain is supportive of such a relationship, while that from the Hawaiian archipelago provides a weak argument for this association, primarily due to age-dating limitations. Notwithstanding these, a comparison of the known rainfall pattern for the Hawaiian Islands and the distribution of mapped edifice failures, however, reveal no obvious link between either wet or dry conditions and the disposition of debris avalanches and slumps. Furthermore, contemporary observations from the Hawaiian archipelago show that sub-aerial landslides are common under both wet and dry conditions. A correlation between climate-related sea-level change and continental margin slope failures is supported via the mechanism of methane venting. A comparable link between variations in sea level and instability and collapse at coastal and island volcanoes may also be vindicated by correlation between the intensity of volcanic activity in marine settings and the rate of sea-level change during the late Quaternary. Our review reiterates strong evidence for abundant slope failures on the flanks of ocean islands. The existence and nature of a climate change link remains, however, to be firmly established. It is likely that this will need to await better constrained age determinations for submarine slope failures, provided—for example—by an extensive program of ash-layer dating within sequences associated with landslides, slumps and debris flows within the marine environment. We conclude by addressing the hazard implications of future major collapse in the marine environment, drawing attention to increasing evidence for a serious future threat to coastal zones from major landslide-induced tsunamis.

The Geology of the Samoan Islands

The Samoan chain consists of high volcanic islands, atolls, and submerged reef banks near the southwest margin of the Pacific plate. The chain is unusual, particularly when compared with the Hawaiian chain, because the islands are volcanically active on both the eastern and western ends of the chain, the islands are larger westward, the easternmost edifice is an atoll not an active seamount, and the chain consists dominantly of alkali rather than tholeiitic lavas. While geological studies of the Samoan group are limited, the existing results are consistent with a hot spot origin similar to Hawaii, complicated by continued reactivation of volcanism on Savaii. The continuing volcanism on Savaii is believed to be the result of deformation of the margin due to lithospheric dilation, as the plate bends where it approaches the Tonga Trench subduction zone. The dominance of alkalic volcanism in this island chain has recently been associated with a geochemical heterogeneity in the underlying mantle.

Insular Geology of the Line Islands

The Line Islands seamount chain consists of a series of ridges and seamounts which extend 4800 km across the Central Pacific Basin (Fig. 1). The seamounts form the longest intraplate submarine mountain chain in any ocean, rising from a sea floor in excess of 5000 m deep. Only twelve seamounts reach sea level to form low islands or atolls; all are presently bordered by fringing reefs. Some seamounts extend above sea level to form typical atolls, having shallow salty lagoons and a rim occasionally broken by channels to the sea, e.g., Fanning, Palmyra and Caroline. Still other seamounts reach sea level and form islands with unbroken land surfaces, e.g., Jarvis, Vostok and Flint Islands. All of the islands are low and nearly flat. Christmas Island and Johnston Island display the highest elevations with sand dunes reaching 12 m.

Side‐scan sonar images of submarine landslides on the flanks of atolls and guyots

Geological studies of oceanic islands indicate that landslides play a large role in modifying the size and shape of islands. Sea floor mapping of the slopes of these islands shows that submarine landslides are common and widespread. Because surveys have concentrated on young volcanic islands (less than 28 My), little information is available about landslides on older atolls and guyots. Side‐scan sonar surveys are presented here of a Cretaceous age (100–70 My) guyot and an atoll. Both surveys show that landslides altering the carbonate caps are present on these features. Since the carbonate caps were formed long after volcanism ceased and the volcanic edifices had subsided, their modification suggests that landsliding continues to be a significant factor changing the shape and size of atolls and guyots. Early studies of atoll shapes indicate that few atolls are true ring reefs (about 5%). It is suggested that submarine landslides are the likely mechanism by which ring reefs are modified to irregularly shap...

Contributions of the 1838-1842 U.S. Exploring Expedition

The years 1988–1992 mark the 150th anniversary of the U.S. Exploring Expedition, one of the first scientific missions undertaken by the United States to explore the oceans of the world. It circumnavigated the world and mapped large portions of the coast of Antarctica, the islands of the central Pacific Ocean, and the northwest Pacific coast of North America. The explorers were the first to sight land on the Antarctic continent. The eastern Antarctic coastline was mapped by the expedition and later named Wilkesland in honor of the commander of the U.S. Exploring Expedition. The expedition mapped most of the island groups in the central Pacific Ocean, eliminating the confusion over the number and position of islands in this region. The important legacies from the expedition include the establishment of the U.S. National Museum (Smithsonian Museum) to house the collections of the cruise and the establishment of the United States as an internationally recognized scientific leader in the area of botanical, zoological, and marine studies, and as a leader and contributor to international ocean-going expeditions.