Chapter 30The Detection of Volcanic Debris Avalanches(VDAs) Along the Hellenic Volcanic Arc,Through Marine Geophysical Techniques

P. Nomikou, D. Papanikolaou, A. Tibaldi, S. Carey, I. Livanos, K.L.C. Bell,F.A. Pasquare, and G. Rousakis

Abstract Recent marine oceanographic surveys using geophysical techniques haverevealed a number of volcanic debris avalanche deposits (VDAs) on the externalflanks of Antimilos, Santorini and Nisyros volcanoes in the South Aegean Sea.Swath bathymetry and side-scan sonar surveys led to the recognition of character-istic hummocky topography on all of these deposits. On seismic profiles the VDAsare identified by chaotic facies, with incoherent areas bordered by continuousundisturbed seismic reflectors. High-resolution examination of the morphologicalcharacteristics of the VDAs was accomplished by using Remotely OperatedVehicles (ROVs), in order to distinguish them from other clastic deposits. In somecases the VDAs can be traced upslope to horseshoe-shaped collapse depressionsand represent the expression of the complex evolution of these volcanic edifices.Recognition of VDAs at these volcanic centers has important implications forgeohazard assessments as flank collapses have the potential for triggering of

P. Nomikou (�) • D. Papanikolaou • I. LivanosDepartment of Geology and Geoenvironment, University of Athens, PanepistimiopoliZografou, 15784 Athens, Greecee-mail: evinom@geol.uoa.gr

A. TibaldiDepartment of Earth and Environmental Sciences, University of Milan Bicocca, Milan, Italy

S. CareyGraduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA

K.L.C. BellGraduate School of Oceanography, University of Rhode Island, Narragansett, RI, USA

Ocean Exploration Trust, Mystic, CT, USA

F.A. PasquareDepartment of Theoretical and Applied Sciences, Insubria University, Varese, Italy

G. RousakisInstitute for Oceanography, Hellenic Centre for Marine Research, Anavyssos, Greece

S. Krastel et al. (eds.), Submarine Mass Movements and Their Consequences, Advancesin Natural and Technological Hazards Research 37, DOI 10.1007/978-3-319-00972-8 30,© Springer International Publishing Switzerland 2014

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large-scale tsunamis. The relationship between the distribution/emplacementmechanisms of the VDA deposits and the source flank collapses remains an area ofongoing research.

Keywords Aegean volcanoes • Submarine landslide • Swath and seismic data •ROV exploration

30.1 Introduction

Repetitive eruptions at a single location frequently construct a volcanic edifice in theform of a subaerial stratovolcano or an ocean island volcano with highly unstableslopes (Manville et al. 2009). A variety of different mechanisms, such as magmaintrusion, phreatic eruption, earthquakes, sea-level changes or erosion, can triggerhuge sector collapses generating submarine volcanic landslides (McGuire 1996).Debris avalanches are blocky landslides originating directly from collapse of thesubaerial and/or submarine island flanks that can travel for tens of kilometers acrossthe sea floor (Siebert 1984). Globally, debris avalanches have been reported aroundvolcanic islands in subduction zones (Tibaldi 2001; Romagnoli et al. 2009; Wattet al. 2012) or large oceanic shield volcanoes such as the Canary islands (Urgeleset al. 1997; Gee et al. 2001; Krastel et al. 2001; Masson et al. 2002) and Hawaiianislands (Clague and Moore 2002; Mitchell et al. 2002).

The largest landslides from volcanic islands may generate disastrous tsunamis atan ocean-basin scale (Waythomas et al. 2009). Understanding how volcanic islandlandslides can destabilize adjacent seafloor sediment is important for understandinglandslide evolution and tsunami generating potential (Crutchley et al. 2013).Tsunamis may cause damage and destruction at coastal infrastructure and lossof lives. Thus identification of the products of such landslides, Volcanic DebrisAvalanche deposits (VDAs), on the seafloor is critical to evaluating the potentialtsunami hazard in coastal environments near island volcanoes.

Here we present the results of our study identifying VDAs along the Hellenicvolcanic arc in the South Aegean Sea, where five separate deposits have beendefined using marine geophysical techniques. This work is an extension of previousgeophysical investigations on the submarine flanks of Antimilos (Alexandri et al.2001), Santorini (Bell et al. 2013) and Nisyros (Tibaldi et al. 2008; Nomikou et al.2009; Livanos et al. 2012) volcanoes.

30.1.1 Geological Setting

The Hellenic Volcanic arc was formed along the convergent plate boundary of thenorthward-subducting African Plate underneath the active margin of the EuropeanPlate (McKenzie 1972; Le Pichon and Angelier 1979). This structure is an important

30 The Detection of Volcanic Debris Avalanches (VDAs). . . 341

supplier of volcaniclastic sediments to the Aegean Sea floor. The study of theTertiary volcanic rocks on several islands of the Aegean, has shown a southwardmigration of the Volcanic Arc since Late Eocene which started from the NorthAegean area and has progressed to its present day configuration along the southernmargin of the Cycladic platform and the northern margin of the Cretan Basin(Papanikolaou 1993). The migration rate has been estimated to be 10 km/Myr,since the total displacement has been approximately 400 Myr over the past 40 Myr(Royden and Papanikolaou 2011). According to geochronological data the actualHellenic Volcanic Arc has been active since the early Pliocene with significantactivity during the Quaternary (Pe-Piper and Piper 2002).

Four groups of volcanic complexes occur in the volcanic arc, including bothsubaerial and submarine outcrops (Nomikou et al. 2013). The Methana group,including Paphsanias submarine volcano, is located at the western edge of thevolcanic arc in western Saronikos Gulf. Both the Milos and Santorini groupstogether with Kolumbo volcanic chain northeast of Santorini, occur in the centralarc. At the easternmost end of the arc, the Nisyros group includes several submarinevolcanic features.

Nomikou et al. (2013) suggest that the volcanic centers are usually located withinneotectonic grabens formed by normal faulting that are sometimes overprinted bysubvertical dextral strike-slip structures, especially in the Santorini and Nisyrosvol-canic groups. This strike-slip component has been proposed to have a NE-SWdirection in the Santorini – Kolumbo volcanic line. Thus, the overall tectonic regimein the central and eastern part of the arc is better described as transtensional witha prevailing NE-SW structural direction. In contrast, the prevailing tectonic trendin the Milos and Methana groups is in the NW-SE direction, following the generalMiocene orientation of the Hellenic arc. This NW-SE tectonic trend was overprintedby younger E-W normal faults creating neotectonic horst and graben structures.

30.1.2 Methods

Bathymetric and seismic data were obtained during several cruises along thesubmarine volcanoes of the Hellenic Volcanic Arc with R/V Aegaeo and E/VNautilus. Additional ROV dives have been performed in 2010 and 2011 duringdifferent oceanographic cruises.

The multibeam bathymetric surveys were carried out by the R/V Aegaeo ofthe Hellenic Centre for Marine Research, using a SEABEAM 2120 swath system.GPS navigation (Trimble 4000) provided the average position of the ship to withinC/�10 m. High resolution seismic profiling of the Nisyros Volcanic Field, utilizeda 10 in3 air gun system coupled to a single channel streamer towed at 50 m andanalog recorder. Around the Santorini Volcanic Field, a 40 in3 air gun was usedwith the same streamer but with digital data acquisition. For the seismic survey SEof Nisyros, a 2 kJ sparker comprised by S.I.G.hydrophones, a receiver and a digitalrecording system, was used. For the ROV surveys, we used the E/V Nautilus’s

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Hercules and Argus ROVs. The Argus was outfitted with EdgeTech 4200 MPsidescan system with sonar frequency 300/600 kHz, for the acoustic survey of thehummocky area.

30.2 Results

30.2.1 Antimilos

The Milos-Antimilos volcanic group occurs at the southwestern margin of theCycladic platform, near the Myrtoon Basin, an asymmetric tectonic graben ofWNW–ESE direction with very steep morphological slopes (Fig. 30.1a) (Nomikouet al. 2013). Antimilos Island is a geometric volcanic dome formed exclusively ofandesitic and dacitic lavas, tuffs and various types of flows ending at abrupt seacliffs (Marinos 1961).

Fig. 30.1 (a) Shaded relief bathymetric map of Antimilos volcanic field based on multibeambathymetric data (Nomikou et al. 2013). The inset map shows the location of Antimilos volcano inthe Aegean Sea. (b) Detailed swath bathymetric map of the northern part of the Antimilos volcanicfield. Red lines indicate two surface profiles across VDA deposits and shown as insets in the upperright hand side of the figure

30 The Detection of Volcanic Debris Avalanches (VDAs). . . 343

The shaded relief bathymetric map reveals an area of rather irregular small-scalerough relief NW of Antimilos Island and a series assembly of three volcanic domesof decreasing size eastwards. Lobate volcanic deposits are emplaced at depthsbetween 200 and 500 m with diameter of about 100–300 m and maximum heightof the order of 50 m (Fig. 30.1b). Several channels (200 m wide and 4 km long)are cut into the seafloor from the shallow platform down to the slope, suggestinghigh transportation rates of volcaniclastic sediment from the island to the proximalpart of the VDA deposits, where the hummocks appear subdued and partially buried(Fig. 30.1b).

The large volcanic field covers about 260 km2 and appears to be very similarin character to the on-land outcropping volcanic features. The large scale of thehummocks, covering an area approximately 56 km2, may reflect a larger-volumevolcanic collapse of a pre-existing complex that may have been connected with thevolcanic structure of Antimilos, further work is necessary to confirm this potentialrelationship.

30.2.2 Santorini

Santorini is an active caldera system in the central part of the Aegean Volcanic Arc(Druitt et al. 1999). Historic volcanic centres in the region include the submarineKolumbo volcano, which lies 7 km north-east of Santorini (Sigurdsson et al. 2006;Nomikou et al. 2012), and the intra-caldera Kameni Islands which have been thesite of the most recent eruptions in the region.

Numerous offshore geophysical surveys reveal the spectacular seafloor morphol-ogy around Santorini, such as prominent hummocky morphology on the easternsubmarine flank of the island (Bell et al. 2013). The VDA deposit covers an area6 km wide by 20 km long and up to 75 m thick in the central region (Fig. 30.2a).The northern extent of the VDA appears to be bounded by a large ridge from theeastern slope of Santorini, and similarly the southern extent coincides with a channelrunning down the Santorini’s eastern slope. The extent of the layer to the south alsoappears to be bound by bathymetric features including the submarine slope of thenearby island of Anafi. Hummocks within the VDA are composed of individualblocks from a few meters to hundreds of meters in diameter that protrude up to tensof meters above the surrounding seafloor. The run-out distance of most blocks wasless than 9 km, but some traveled up to 15 km into water depths of 600 m.

A seismic profile from the southern margin of the VDA (Fig. 30.2b) reveals twoimportant features: (i) an upper layer characterized by chaotic undulating reflectors,and (ii) hyperbolic reflectors corresponding to large hummocks. The total volumeof the deposit is estimated to be approximately 4.4 � 109 m3 (Bell et al. 2013).This kind of landslide facies occurs near the island flanks and is associated with anirregular deposit surface. The hummocky seafloor is likely the result of a multi-stagelandslide event that was caused by one of several earthquakes or volcanic eruptions

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Fig. 30.2 (a) Three-dimensional swath bathymetric map of the eastern submarine flanks of San-torini, showing the extent of the hummocky seafloor. (b) The southernmost SW-NE longitudinalseismic profile showing a transect through the VDA deposit

that have occurred in and near Santorini after the Minoan eruption (1627–1600 BC)in the Late Bronze Age (Friedrich et al. 2006).

Side scan sonar data reveal a more complex morphology in the central part of theVDA. ROV observations reveal that some blocks have several areas of pumiceousscree and other areas of subvertical to vertical walls that are composed of pyroclasticflow deposits, sometimes covered by brown crust (Fig. 30.3a, d). Some others havecommunities of benthic megafauna, including corals (Callogorgia spp.) and sponges(Fig. 30.3b) and have flat tops (Fig. 30.3c).

30.2.3 Nisyros

The general trend of the recent volcanic centers between Kos and Tilos islands isENE-WSW, demonstrating the intense geodynamic processes of the eastern edge ofthe active Hellenic volcanic arc (Nomikou and Papanikolaou 2011). The island ofNisyros is composed mostly of Quaternary volcanic rocks represented by alternatinglava flows, pyroclastic layers and viscous lava domes, ranging in age from 160 to25 ka (Papanikolaou et al. 1991).

30 The Detection of Volcanic Debris Avalanches (VDAs). . . 345

Fig. 30.3 Mosaic of 300 kHz sidescan sonar data; high backscatter is black, low backscatter(shadow) is white. Bathymetric map at the upper left corner shows the locations of the airgunprofiles (black lines), ROV dive tracklines (red lines), sidescan survey tracklines (white lines) andcollected samples (red circles). Bottom photographs recorded on high definition video of selectedblock are shown as a, b, c, d in the mosaic map

In the area south of Nisyros, VDAs composed of three fields of large blocks andlongitudinal ridges (Fig. 30.4). VDA 1 extends 10 km offshore with a concentrationof large blocks at its terminus while VDA 2 extends further, out to 16 km from theisland and has a higher concentration of large blocks. VDAs 1 & 2 cover a total areaof 58 km2, extending down to 740 m depth. The largest elongated blocks are 520 macross in VDA 1 (with volume �0.7 km3), standing 60 m above the surroundingseafloor. A few smaller blocks (up to 400 m across) protrude 45 m above the seafloorat the central part of VDA 2 (with volume �0.37 km3). These deposits are borderedby NE-SW marginal fault zones of the South Nisyros Basin and may representa multi-staged landslide event. In the seismic profiles A-B and C-D (Fig. 30.4),the upper seismic unit that defines the VDA deposits, is characterized by convex-upwards rounded reflectors, diffraction hyperbolae, and wavy or broken reflectors,often with high amplitudes.

The hummocky topography at the SE flanks of Nisyros (VDA 3) forms a lobate-shaped deposit with southeast flow direction and covers an area of approximately4.8 km wide by 4.6 km long (about 22 km2) (Fig. 30.4). Most of the hummocksare located at the center of the VDA (250–370 m depth) having either elongated orirregular shape. Using an average thickness of 30 m, Livanos et al. 2012 estimatedthe total VDA volume to be 6.6 � 108 m3. ROV and side scan surveys over the blocksshow that most are covered by biological encrustation (Fig. 30.5a, A) while othersare rounded with black surface color (Fig. 30.5a, B). In some cases the northernslope of the hummock is smoother than the southern one, forming a ridge with NE-SW direction (Fig. 30.5a, C).

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Fig. 30.4 Gradient-shaded swath bathymetry map showing multiple VDA deposits, south ofNisyros Volcano. Red lines indicate fault zones. The inset map shows the regional location ofNisyros, in the eastern edge of Hellenic volcanic arc. Lower panel: Seismic profiles across VDAdeposits 1 (A–B) and 2 (C–D). Red lines indicate fault zones

Hyperbolic reflectors in the seismic profile (Fig. 30.5b) correspond to the locationof large blocks within the VDA 3 and illustrate the relationship of the avalanchedeposits to the recent stratigraphy of the sediments within Tilos Basin. The Tilosmarginal fault zone borders the VDA to the south whereas the volcanic basementof Nisyros Volcano, which is the source of the VDA, is observed to the North.Due to the shape characteristics and the uniform appearance along the lobate-shaped deposit, we believe that the flank collapse was a singular failure, involvinga rapid, virtually instantaneous movement of the entire slide mass into the sea.Unfortunately, there are no cores from the debris avalanche area that can be usedto date the event.

30.3 Discussion and Conclusion

A combination of marine geophysical techniques has allowed us to identify andcharacterize five volcanic debris avalanche (VDA) deposits from three volcaniccenters along the Hellenic Volcanic Arc in the southern Aegean Sea. All of the

30 The Detection of Volcanic Debris Avalanches (VDAs). . . 347

Fig. 30.5 (a) Mosaic of 300 kHz sidescan sonar data; The locations of side scan survey (blackline) and the seismic profile (red line) are indicated in the inset bathymetric map in the lowerright side of the figure. A, B, C: Bottom photographs recorded by high definition video of selectedblocks. (b) Seismic profile across the eastern part of VDA deposit

landslide deposits appear to have been associated with volcanic edifice collapse,because they contain large structureless blocks dispersed from a source around theisland flanks of Antimilos, Santorini and Nisyros (Lebas et al. 2011). While the agesof the VDAs cannot be precisely determined, those at Santorini appeared to be theyoungest (post Late Bronze Age) and the rest of the deposits may have occurredin the last 100 ka based on the absence of significant sediment cover and theirpenetration into recent sediments.

Two substantially different morphologies are observed in the submarine VDAdeposits. The first variety, “Type I” VDA deposit, includes large volume VDA, withelongate shape and less abundant and smaller blocks near the source zone, like theeastern flank of Santorini volcano. The second variety, “Type II” VDA deposit tendto be smaller in volume, exhibit lobate shapes, and are not as extensively dispersedfrom their source areas. This type contains many large scattered blocks over theentire surface of the deposit, like Antimilos VDAs and Nisyros VDAs.

Tsunamis around volcanic islands may be triggered less frequently by catas-trophic flank collapse than by earthquakes, but nevertheless they have the potentialto produce highly destructive events. Additional marine geophysical surveys aroundthese volcanic islands, coupled with sediment coring and dating, are necessary tobetter resolve the mechanisms of VDA formation and timing of these importantprocesses in the Hellenic Volcanic Arc of the Southern Aegean Sea.

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Acknowledgments The officers and the crew of the R/V Aegaeo and E/V Nautilus are gratefullyacknowledged for their important and effective contribution to the field work and sampling. JasonChaytor and Vasilis Lykousis are acknowledged for their useful and constructive comments.

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