Boulder Ridges on the Aran Islands (Ireland): Recent Movements Caused by Storm Waves, Not Tsunamis

Abstract

A B S T R A C TIreland's Aran Islands are an excellent place to test whether coastal boulder deposits-including individual rocks weighing several tens of tonnes near sea level and clasts weighing several tonnes transported at tens of meters above sea level-require a tsunami for emplacement or whether storm waves can do this work. Elongate deposits of cobbles, boulders, and megagravel are strung along the Atlantic coasts of the Aran Islands. No tsunamis have affected this region in recent centuries, so if these deposits are forming or migrating at the present time, they must be storm activated. We find a diverse range of evidence for recent ridge activity. First, shells of Hiatella arctica (subtidal rockboring bivalves preserved in life position within ridge boulders) yield radiocarbon ages from ≈200 AD to modern (post-1950 AD). Second, recent motion is attested to by eyewitness accounts that pin the movement of several individual 40-80-t blocks to a specific 1991 storm and by repeat photography over the last few field seasons (2006)(2007)(2008)(2009)(2010)(2011) that captures the movement of boulders (masses up to ≈10.5 t) even in years without exceptionally large storms. Finally, geographic information system comparison of nineteenth-century Ordnance Survey maps with twenty-firstcentury orthophotos shows that in several areas the boulder ridges have advanced tens of meters inland since the mid-nineteenth century, overrunning old field walls. These advancing ridges contain boulders with masses up to 78 t at 11 m above high-water mark, so wave energies sufficient to transport those blocks must have occurred since the 1839 survey. Thus, there is abundant evidence for ridge activity since the 1839 mapping, and as there have been no tsunamis in the northeastern Atlantic during that time period, we conclude that the Aran Islands boulder ridges are built and moved by storm waves.Online enhancement: appendix.

Localized tsunamigenic earthquakes inferred from preferential distribution of coastal boulders on the Ryukyu Islands, Japan

“…Coastal boulders provide valuable information at places where the preservation of sandy tsunami deposits are less expected, such as at rocky shorelines or coral reefs. Similar research can be done where many boulders have been reported, such as the islands in the Pacifi c Ocean (e.g., Frohlich et al, 2009) and other regions, to explore the tsunami histories and the possible source models of causative earthquakes, submarine volcanic eruptions, and landslides.…”

“…Differentiation of boulders according to whether they were deposited by tsunami or storm waves is controversial because many enigmatic boulders are present along rocky coasts or reefs throughout the world (e.g., Switzer and Burston, 2010;Buckley et al, 2012;Cox et al, 2012;Weiss, 2012). For the Ryukyu Islands, it was proposed (Goto et al, 2010a) that boulders of tsunami and storm-wave origin can be differentiated based on the different wavelengths of tsunamis and storm waves.…”

Block and boulder transport in Eastern Samar (Philippines) during Supertyphoon Haiyan

“…The size of individual clasts and in particular the dimensions of block ESA 9 (9.0 × 4.5 × 3.5 m 3 ), in combination with the documented vertical and lateral transport distances, exceeds any existing literature account, including the often-cited boulder at Sydney's Bondi Beach (Süssmilch, 1912;6.1 × 4.9 × 3.0 m 3 ), and clasts moved during TCs in Japan (Goto et al, 2011) and Jamaica (Khan et al, 2010) as well as during Atlantic winter storms (Williams and Hall, 2004;Regnauld et al, 2010;Cox et al, 2012) (Tables 1, 2). According to the pedestal found at its pre-Haiyan position, block ESA 9 was stationary for a considerable period of time prior to Typhoon Haiyan (cf.…”

Extreme Waves

“…That said, some general characteristics can be used as broad indicators of either storm or tsunami deposition. This context allows storm waves to gain access to the coast with minimal attenuation and thus retain the capacity for substantial erosion and landward transport of a surprising range of clast sizes (Williams and Hall, 2004;Cox et al, 2012). This tendency is mirrored at altitude on cliff-top sites where the cliffs are fronted by deep water.…”