Quantitative morphology, recent evolution, and future activity of the ...

For permission to copy, contact [email protected]© 2006 Geological Society of America

253

Geosphere; August 2006; v. 2; no. 5; p. 253–268; doi: 10.1130/GES00028.1; 16 fi gures, 9 tables, Data Repository 2006120.

ABSTRACT

Linking quantitative measurements of lava fl ow surface morphology with histori-cal observations of eruptions is an important, but underexploited, route to understanding eruptions of silicic magma. We present here a new, high-resolution digital elevation model (DEM) for the intracaldera Kameni Islands, Santorini, Greece, which reveals the potential of high-resolution imaging (at ~1 m per pixel) of lava-fl ow fi elds by airborne light detec-tion and ranging laser radar (LiDAR). The new DEM has an order-of-magnitude better resolution than earlier models, and reveals a wealth of surface morphological information on the dacite lava fl ows of the Kameni Islands. In turn, this provides quantitative constraints on the bulk rheology of the emplaced lava fl ows. When combined with a reanalysis of contemporary eruption accounts, these data yield important insights into the behavior of dacite magma during slow effusive erup-tions on Santorini and elsewhere, and allow the development of forecasts for the style and duration of future eruptions.

Kameni Island lava fl ows exhibit classic surface morphologies associated with vis-cous magma: levées and compression folds. Levée heights and fl ow widths are consis-tent with a Bingham rheology, and lava yield strengths of 3–7 × 104 Pa. Compres-sion folds have long wavelengths (15–25 m), and change only a little downstream; this is consistent with observations of other terres-trial silicic lava fl ows. The blocky a‘a dacite lava-fl ow margins show a scale-invariant morphology with a typical fractal dimen-sion that is indistinguishable from basaltic Hawaiian a‘a, confi rming that the fractal

dimension is insensitive to the composition of the fl ow.

Dome-growth rates during eruptions of the Kameni Islands in 1866 and 1939 are consistent with a model of slow infl ation of a dome with a strong crust. Lava domes on the Kameni Islands have a crustal yield strength (4 × 107 Pa) that is lower by a factor of 2–4 than the domes at Pinatubo and Mount St. Helens. The dome-height model combined with the apparent time-predictable nature of volcanic eruptions of the Kameni Islands allow us to suggest that should an eruption occur during 2006, it will last for more than 2.7 yr and produce a dome ~115–125 m high.

Keywords: digital elevation model, lava dome, lava rheology, Aegean volcanic arc, dacite lava.

INTRODUCTION

A long-standing goal in volcanology is to develop ways of extracting quantitative infor-mation about past eruptions that will allow us to develop forecasts of the nature and timing of future activity. In the case of lava fl ows, a key area of investigation is understanding the rela-tionship between the surface morphology of lava fl ows, and the bulk dynamics of the erupt-ing material (e.g., Hulme, 1974; Fink 1980; Kil-burn, 2004). Understanding this link is critical in general because of the importance of lava in planetary resurfacing, and, specifi cally, because it allows reconstruction of the detail of past eruptions from the morphology of the emplaced lavas, and underpins forecasts of how future lava fl ows will evolve.

Here, we present a new high-resolution digi-tal elevation model (DEM) for the volcanic Kameni Islands (Santorini, Greece) based on a light detection and ranging laser radar (LiDAR) aerial survey carried out in April 2004. This is one of the fi rst applications of an aerial LiDAR survey to a volcano, and the unprecedented spatial resolution that the technique offers

opens up many new avenues for future work. In particular, the new data reveal details of the sur-face morphology (on 1–100 m length-scales) of young dacite lava fl ows, cones and domes, from which important rheological information can be extracted. In combination with a reanalysis of historical eruption accounts, we show how this information can be used to understand the emplacement processes of viscous silicic lavas.

Santorini Volcano, Greece

Santorini is one of the active volcanoes of the South Aegean volcanic arc. It has a rich volca-nic history, with more than 12 major explosive eruptions recognized over the past 250,000 yr (Druitt et al., 1989, 1999). Typically, episodes of caldera formation on Santorini have been followed by extended periods of lava effusion, leading to the intercalations of andesite to dacite lava piles and andesite to rhyodacite tephra formations that are exposed within the caldera cliffs at the present day (Druitt et al., 1999).

Currently, Santorini volcano is in an effu-sive phase, with the focus of intracaldera vol-canism for the past 2200 yr being the dacitic Kameni Islands, which represent the emergent top of a 2.5 km3 volcano that has a basal area of ~3.5 km2 and that rises 500 m from the fl oor of the fl ooded caldera (Druitt et al., 1999). These islands, of which there are currently two (Palea and Nea Kameni, Fig. 1), have been the sub-jects of extensive petrological, geochemical, and textural investigations over the past thirty years because of their unusually uniform chemi-cal compositions and their contrastingly het-erogeneous population of exotic xenoliths and cognate enclaves (e.g., Nicholls, 1971; Barton and Huijsmans, 1986; Higgins 1996; Zellmer et al., 2000; Holness et al., 2005; Martin et al., 2006). There are considerable ongoing efforts to characterize and monitor the state of the Kameni Islands, in particular, the seismicity (Dimitria-dis et al., 2005), hydrothermal and fumarolic activity, and ground deformation (Stiros and

Quantitative morphology, recent evolution, and future activity of the Kameni Islands volcano, Santorini, Greece

David M. Pyle*†

John R. Elliott†

Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK

*Corresponding author e-mail: [email protected].

†Now at Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK.

Pyle and Elliott

254 Geosphere, August 2006

Nea Kameni

Palea Kameni

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Akrotiri

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Figure 1. General location (A) and geological map (B) of Santorini, and the intracaldera Kameni Islands, after Druitt et al. (1999) and Martin et al. (2006). Part A shows the shallow submarine Colombos bank (NE of Santorini), which was the location of an erup-tion in 1650 A.D., and the “Kameni line,” which marks the NE-SW trend of a region of preferred vent locations on the Kameni Islands (Druitt et al., 1989, 1999).

Morphology of Kameni Island Lavas, Santorini, Greece

Geosphere, August 2006 255

Chasapis, 2003; Vougioukalakis and Fytikas, 2005). Although the islands are currently in a state of intereruptive repose, there is no reason to suppose that there will not be future eruptions of a similar nature, perhaps within decades, and certainly within centuries.

Since the extensive work of Fouqué (1879), Kténas (1926, 1927), Georgalas and Liatsi-kas (1925a, 1925b, 1936a, 1936b), and Reck (1936a), which documented the course of many of the historic eruptions in great detail (Table 1), little attention has been paid to the physical form, or posteruptive morphology and evolution, of the lavas and ash cones that make up the Kameni Island group. Indeed, many of these original works have been overlooked, and the wealth of relevant data they contain has been forgotten.

Historical Eruptions of the Kameni Islands

The historical activity of the Kameni Islands is well known from contemporary written records (Table 1). Modern compilations (e.g., Georga-las, 1962; Fytikas et al., 1990) generally begin with the work of Fouqué (1879), who produced one of the classic modern scientifi c accounts of an active volcano and its evolution. Fouqué, in turn, drew on material collated by Pègues (1842), who published an extended account of the historical activity of the islands, and by Leycester (1851), who also published a detailed

bathymetric map of the caldera. The 1866–1870 eruptions were closely documented by Fouqué, based in part on his own observations, as well as on the many books and pamphlets that were published within a year or two of the start of the 1866 activity (e.g., Virlet d’Aoust, 1866; von Seebach, 1867; Reiss and Stübel, 1868). The 1866 eruptions clearly drew considerable scientifi c interest at the time; for example, the eruption was responsible for the earliest medical work on the health effects of volcanic eruptions (da Cologna, 1867). Together, the many pub-lished works that describe the major eruptions of the eighteenth, nineteenth, and twentieth cen-turies present an excellent basis (Table 2) from which to develop a quantitative analysis of the evolution of an intracaldera volcano.

The salient features of the Kameni Islands eruptions and their products are summarized in Tables 1–4. The Kameni Islands make an excellent case study since the eruptions have exclusively involved dacite lava (Table 2), in contrast to the many active lava-forming sys-tems that have been studied in recent decades, which are predominantly basaltic (e.g., Etna, Hawaii; Walker, 1973; Pinkerton and Sparks, 1976). Eruptions typically include both dome-forming and mildly explosive (Vulcanian) phases, and the steady effusion of lava. During the most closely observed eruptions of the twen-tieth century, fl ows typically extended to more

than 500–1000 m over durations of 30–200 d (e.g., Kténas, 1926; Georgalas and Papastama-tiou, 1951, 1953; Georgalas, 1953), with fl ow-front advance rates ranging from 10−3 ms−1 in the early stages to <10−4 ms−1 after 3 mo, and aver-aged effusion rates on the order of 0.5–2 m3s−1 (Fig. 2; Tables 3 and 4). The erupting lavas are, therefore, classical examples of creeping vis-cous fl ows, with low Reynolds numbers (imply-ing laminar fl ow) and moderate Peclet num-bers, similar to those of slow-growing domes (Griffi ths, 2000).

DATA COLLECTION

Airborne data were collected over the Kameni Islands, Santorini, in April 2004 dur-ing an overfl ight by the UK’s Natural Environ-ment Research Council and airborne remote-sensing facility (ARSF) Dornier 228 aircraft. This aircraft was equipped with a WILD RC-10 camera, and Cambridge University’s Airborne Laser Terrain Mapper (ALTM) system (Model 3033, Optech Inc., Canada). The aircraft survey took place at an altitude of 650–780 m above sea level (asl) and at a ground speed of 60–75 ms−1. The ALTM LiDAR was operated at 33.3 kHz and used a side-to-side scanning mecha-nism, which combined with the forward motion of the aircraft to provide swath coverage of the ground surface below. Data from an onboard

TABLE 1. HISTORICAL ACTIVITY OF THE KAMENI ISLANDS

Eruption date Location References and notes

10 January–2 February 1950 Nea Kameni Georgalas (1953).20 August 1939–early July 1941 Nea Kameni Georgalas and Papastamatiou (1951, 1953).23 January–17 March 1928 Nea Kameni Kténas and Kokkoros (1929); Georgalas and

Liatsikas (1936b); Reck (1936c).11 August 1925–January 1926 Nea Kameni Kténas (1925a, 1925b, 1925c, 1926, 1927);

Washington (1926a, 1926b); Georgalas and Liatsikas (1936a); Reck (1936b).

26 January 1866–15 October 1870 Nea Kameni Described by Fouqué (1879); accounts of 1866 activity in von Fritsch et al. (1867) and Reiss and Stübel (1868).

23 May 1707–14 September 1711 Nea Kameni Described by Goree (1710) and Tarillon (1715a). Eruption end date is from Tarillon (1715b); misquoted as 11 September in Georgalas (1962) and subsequent catalogues.

1570 or 1573 Mikra Kameni The eruption was in either 1570 or 1573 (Fouqué, 1879).

1457 Palea Kameni No eruption, but a collapse of part of Palea Kameni (Fouqué, 1879).

726 Palea Kameni Northeast side of Thia Island46–47 Palea Kameni (Thia) Eruption of Thia, about 400 m from Hiera, to form an

island of ~5600 m circumference (Fouqué, 1879; Stothers and Rampino, 1983). Now Palea Kameni.

19 A.D. Thia Reported by Pliny, but not regarded as an eruption by later authors (e.g., Pègues, 1842; Fouqué, 1879).

199–197 B.C. Hiera (or Lera) Island that formed in 4 d, with a circumference of 2200 m (Stothers and Rampino, 1983). Presumed to have been eroded below sea level. Fouqué (1879) suggested Hiera was the Bancos reef, which was northeast of Nea Kameni and was covered with lava during the 1925–1926 eruption.

TABLE 2. TYPICAL FEATURES OF THE KAMENI DACITE LAVAS

Parameter Value

SiO2 content 64–68 wt%Inferred eruption temperature 900–950 oCPhenocryst content* 15%Inferred viscosity†, anhydrous

magma, η(4–6) × 104 Pas

Effective viscosity† of porphyritic magma

(6–9) × 104 Pas

Bulk lava density† (vesicle free), ρ

2680–2725 kgm–3

Flow characteristics

Typical fl ow rates§, V 10–5–10–3 ms–1

Effective Reynolds number#, Re 10–5–10–3

Peclet number**, Pe 2 × 102–2 × 104

Prandtl number††, Pr 2 × 107

*Phenocrysts (1–2 mm) of plagioclase and clinopyroxene (Huijsmans, 1985; Higgins, 1996).

†Viscosity and density calculated using Ken Wohletz’s ‘Magma’ (http://www.ees1.lanl.gov/Wohletz/Magma.htm), based on the Bottinga and Weill (1972) model.

§From fi eld observations by Kténas (1926) and Georgalas and Papastamatiou (1951, 1953).

#Re = ρVh/η, assuming a typical fl ow depth, h, of 20 m.

**Pe = Vh/κ; κ, thermal diffusivity = ~10–6 m2s–1.††Pr = η/(ρκ)

Pyle and Elliott


global positioning system (GPS) and the local GPS base station (at Akrotiri) were used to derive the precise fl ight path. First pulse and last pulse data were recorded for each airborne laser measurement and converted to location (x, y, z) and intensity data by Gabriel Amable of the Cambridge University Unit for Landscape Modeling. During the overfl ight, 21 aerial pho-tographs and 4.52 million light-imaging and

detection-ranging (LiDAR) measurements were made. In addition, Airborne Thematic Mapper (ATM) data were collected across 11 spectral bands.

LiDAR

The processed LiDAR data comprised 4.52 million measurements over a surveyed area of

~8 km2, including a ground area of 3.86 km2. Measured points have a nominal accuracy of 1.0 cm, 1.2 cm, and 5.9 cm in their x, y, and z directions, respectively. The data were imported into a geographic information system (GIS) application (ARCMAP), and anomalous data points (due to cloud cover) were removed. A large area of absorption due to low cloud cover appeared on one fl ight line (Fig. 3), so for this area there is no precise height information. Instead, this area was patched with a 15-m-reso-lution digital elevation model (DEM; from Inte-gralGIS). The LiDAR data were used to con-struct a high-resolution DEM by interpolating the LiDAR points to an ~1 m post spacing using a cubic spline function and fi tting a surface to these data.

A hill-shade rendition of the DEM is pre-sented in Figure 4, and a contoured DEM is pre-sented in Figure 5. In addition, maps of slope (not shown) were generated to help visualize surface morphological features. This digital elevation model may be referenced either to the UTM (Universal Transverse Mercator) grid, or to standard latitude and longitude coordinates (as shown in Figs. 3 and 4). It should prove a useful starting point for any future analysis of either pre-eruptive deformation of the Kameni Islands or of posteruptive morphological evolu-tion. For the present time, and the purposes of this paper, the DEM allows us to proceed with a detailed quantitative investigation of the surface morphology of a number of key aspects of the Kameni Island lava fl ows and domes.

TABLE 3. DEVELOPMENT OF EIGHTEENTH- AND NINETEENTH-CENTURY FLOW FIELDS

Eruption Flow length(m)

Lobe area(km2)

Typical fl ow thickness

(m)

Approximate fl ow duration

(d)

Inferred effusion rate

(m3s–1)

1707–1711* 1200 0.9 > 30–60 300–600 1–21866†, Aphroessa fl ow (NNW lobe) 1000 0.2 20–50 90 0.5–1.31866–1869†, Giorgios fl ows 1200 1.7 30–70 600–1000 1–1.4

*Accounts suggest that most lava extrusion occurred in 1707–1708 (Tarillon, 1715a, 1715b; Goree, 1710; Fouqué, 1879).

†See Fouqué (1879) and Georgalas (1962).

TABLE 4. DEVELOPMENT OF TWENTIETH-CENTURY FLOW FIELDS

Eruption Maximum length (m)

Area (km2)

Typical thickness

(m)

Duration (d)

Averaged effusion rate

(m3s–1)

Data source

August 1925, N branch 728 0.24 60 80 2.2 Kténas (1926)

August 1925, E branch 1200 0.61 60 150 2.8 Kténas (1926)September 1939, NW and SW lobes

407–620 0.13 20–30 52 0.15–0.25 Georgalas and Papastamatiou

(1951)November 1939–April 1940, N, E, SW lobes

320–620 0.38 20–30 95–230 0.4–0.7 Georgalas and Papastamatiou

(1953)

1925 N branch

1925 E branch

August 1939 SW

August 1939 NW

November 1939 E branch

November 1939 SW branch

0

200

400

600

800

1000

1200

1 10 100 1000

Time elapsed (d)

Flo

w le

ng

th (

m)

10-3 ms-1 10-4 ms-1

Figure 2. Flow lengths as a function of time for the eruptions in 1925–1926 and 1939–1941, based on fi eld maps and obser-vations in Kténas (1926) and Georgalas and Papastamatiou (1951, 1953). Time-averaged fl ow rates typically range between 10−5 and 10−3 ms−1 (contoured).



Aerial Photographs

During the overfl ight, a set of 21 color ana-logue aerial photographs were captured using the onboard Wild RC-10 camera. Together, these images give a mosaic that covers the entirety of the Kameni Islands. Images were scanned digitally at high resolution, internally referenced using fi ducial points, and linked to the x, y, z coordinates of identifi able ground control points on the digital elevation model. In turn, this allowed the photographs to be corrected for distortion and terrain effects. An orthorectifi ed DEM-referenced aerial pho-tograph mosaic is shown in Figure 6. This image is complete, save for the easternmost part of one lobe of the 1925 lava fl ow, which was obscured by a cloud. This mosaic image demonstrates clearly the ease with which the surface textures of the lava fl ows and ash cones can be identifi ed, and with which fl ows of dif-ferent age can be distinguished. We used the aerial photographs, combined with the DEM, in the following sections to develop an updated interpretation of the geological map of the Kameni Islands.

Geological Map of the Kameni Islands

There have been several modern interpre-tations of the surface geology and eruption sequence of the Kameni Islands, based primar-ily on the observations and interpretations of Georgalas (1962). More recently, maps were published by Pichler and Kussmaul (1980), Huijsmans (1985), and Druitt et al. (1999). We present an interpreted map in Figure 7, which was prepared using a combination of feature mapping (e.g., using the slopes visualized in the DEM) and visual imagery. The new map is properly georeferenced to the UTM and lati-tude-longitude grids, and is offset by 280 m (S) and 225 m (W) from the UTM coordinates of the Kameni Island map of Druitt et al. (1999).

The main difference in the new map, com-pared to that of Druitt et al. (1999), is the inter-pretation of the lava-fl ow sequence from the 1939–1941 eruptions in the western portion of Nea Kameni. This is shown in the detail in Figure 8A, along with extracts from two aerial photographs of the same region: Figure 8B is a monochrome photograph taken in May 1944 by a British reconnaissance aircraft, and Fig-

ure 8C is the same view from the 2004 over-fl ight. The new lava fl ows that erupted during 1939–1941 activity can be clearly seen in the two photographs, as can the minor changes in crater morphology associated with the 1950 eruption, and the emplacement of the very small Liatskias dome and lavas, as described by Georgalas (1953).

A second feature of the map is that, in contrast to the earlier mapping of Pichler and Kussmaul (1980), we see no evidence for exposed surfi cial faults on Nea Kameni. In our map, we do not explicitly identify the areas covered or partially covered by younger ash deposits (e.g., the por-tions of the 1866–1870 lava fl ows covered with ash from the 1925–1928, 1939–1941, and 1950 activity), but these areas can be identifi ed from the surface texture and coloration on the aerial photograph mosaic.

Lava-Flow Surface Textures: Folding

The high-resolution DEM (Figs. 4 and 6) reveals a wealth of lava-fl ow surface textural information, most strikingly, the forms of lava-fl ow levées and fold patterns. Both of these

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Figure 3. Light detection and ranging laser radar (LiDAR) return intensity map. Return intensity is qualitatively related to surface smoothness; there are strong returns from the old surfaces of Palea Kameni, for example. The central strip with no return intensity is the fl ight line that was affected by low cloud cover. The raw LiDAR data (x, y, z, intensity) are available in the GSA Data Repository item accompanying this paper (see text footnote 1).

Pyle and Elliott


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Figure 4. Hill-shaded digital map of the Kameni Islands, based on light detection and ranging laser radar (LiDAR) data. A coarse-resolution patch (data provided by P. Moore, IntegralGIS) was used to aug-ment the missing strip. This rendition clearly picks out sur-face morphological features of the lava fl ows and domes, including prominent levées and lava channels. Note the contrast between the rough surfaces of the blocky a‘a dacite lava fl ows and the smoother ash cones of north-central Nea Kameni.

Figure 5. Elevation map of the Kameni Islands, shown as ele-vation above mean sea level.



sets of features, in principle, have scales that are thought to refl ect the rheological properties of the underlying fl ow, and there are widely used models for both approaches (e.g., levées: Hulme, 1974; folds: Fink and Fletcher, 1978). The resolution of the DEM (with a point spac-ing on the order of 1 m) means that the meter-scale blocks that are often the most obvious surface feature of these and other silicic lavas in the fi eld cannot be resolved.

One of the prominent features of the lava-fl ow morphology that is visible both from aerial photographs and the DEM is the abundance of gently arcuate ridges on the fl ow surfaces, which are convex downstream (away from the vent) and lie approximately perpendicular to the fl ow direction. These ridges are best devel-oped on fl ows (such as the 1707–1711 fl ow) that also have well-defi ned levées. Ridges such as these have been described from a number of terrestrial and planetary volcanic settings (e.g., Fink 1980; Gregg et al., 1998). Analogue experiments, for example, using cooling wax fl ows, strongly suggest that ridge development happens early in the fl ow history as the fl ow surface develops a crust (whether by cooling or by crystallization; Griffi ths et al., 2003). In the case of the Kameni fl ows, fi eld observations

show clearly that the ridges formed very close to the vent, essentially as the lava fl ow was extruded. In the case of the 1925–1926 erup-tions, this much is also clear from the contem-porary eruption reports (Kténas, 1926; Reck, 1936b). We quantify and interpret each of these morphological features in subsequent sections.

Shapes of Lava Lobes: Levées and Folds

The shapes of lava fl ows, and the forms of lava-fl ow fi elds, are complex functions of the rheological properties of the fl uid and the emplacement conditions. It remains a continuing goal of physical volcanologists to understand and quantify these functions (e.g., Griffi ths, 2000; Blake and Bruno, 2000). One widely used approximation of the behav-ior of viscous lavas is that they behave as Bingham fl uids, with a yield strength that must be exceeded before fl ow can occur and a plastic viscosity. Assuming a Bingham rheology, one may use the widths of fl ows and the dimen-sions of their levées to gauge the apparent yield strength of the fl uid (Hulme, 1974; Hulme and Fielder, 1977). For a fl ow of bulk density ρ and depth h fl owing down a slope of angle α, the yield strength Y is

Y hg= ρ αsin( ) . (1)

For a levée of width w, this is equivalent to

Y gw= 2 2ρ αsin ( ) , (2)

while for a whole fl ow width of W,

Y

gh

W= ρ 2

. (3)

Dimensions of fl ow levées and surface folds were determined by taking transverse and lon-gitudinal (fl ow-parallel) sections across fl ows (Table DR11). A typical cross-sectional profi le of a fl ow is shown in Figure 9. This exhibits the classical form associated with a fl ow levée (e.g., Sparks et al., 1976). Levées are typically

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0 500 1000 1500 2000250m

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Figure 6. Orthorectifi ed aer-ial photograph mosaic of the Kameni Islands, April 2004. Particularly prominent fea-tures that can be seen include the twentieth-century lava fl ows (darker, rough surfaces in the east and west of Nea Kameni) and the discoloration, due to persistent low-grade fumarolic activity of the ash-covered regions of central Nea Kameni. The modern tourist trail that crosses the northern part of Nea Kameni can also be very clearly seen. The two small islands that lie between Nea Kameni and Palea Kameni are the May islands, which erupted in May 1866, and are now just submerged. A high-resolution version of this fi gure is avail-able from the GSA Data Repos-itory (see text footnote 1).

1GSA Data Repository item 2006120, text fi le with the raw LiDAR data (UTM coordinates, zone 35N), a high-resolution aerial photomosaic of the Kameni Islands, a table summarizing eruption volumes, and fi gures showing the locations of sections used for fl ow shape analysis, is available online at www.geosociety.org/pubs/ft2006.htm, or on request from [email protected] or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.

Pyle and Elliott


12–30 m high, 30–60 m wide (Table 5), with outer slopes of 25–35°. While measurement errors associated with the DEM are small, more signifi cant errors result from the subjectivity of identifying levée margins, from the addi-tional complications where levée dimensions change downstream, or where fl ows bifurcate, and from uncertainty in the slope down which the fl ow was emplaced. The apparent viscosity and yield-strength estimates based on lava-fl ow morphology should only be regarded as order-of-magnitude estimates of fl ow parameters.

Estimates of yield strengths derived using the three complementary approaches are sum-marized in Table 6. The yield-strength esti-mates derived from the gross-fl ow morphology appear to be the most internally consistent and suggest yield strengths on the order of 30 kPa (1925 fl ows) to ~70 kPa (1940 fl ows). These values are consistent with yield-strength esti-mates for dacites elsewhere (Hulme and Fielder, 1977; Wadge and Lopes, 1991). Yield-strength estimates based on levée height are similar, in terms of order of magnitude, to those based on whole-fl ow morphology, while those based on

levée width are substantially smaller. The poor agreement between the yield-strength estimates based on levée width and those based on gross-fl ow morphology is consistent with observations elsewhere (e.g., Etna; Sparks et al., 1976) and most likely refl ects the fact that the processes important in levée formation and evolution (including accretion, avalanching, and cooling, for example; Sparks et al., 1976) violate the assumptions required for Hulme’s (1974) analy-sis to work.

In addition to the levées, another prominent surface morphological feature of the Kameni lava fl ows that can be seen clearly in the DEM and aerial photo images is folding. A typical longitudinal section along a fl ow, corrected for the general slope of the fl ow fi eld, is shown in Figure 10. This shows folding with wavelengths on a 10–100 m scale, defi ned by a train of arcu-ate ridges that runs perpendicular to fl ow direc-tion. Folds of these sorts have been recognized on a number of length scales on lava fl ows of all compositions, from basalt to rhyolite, and the usual interpretation is that they form as a result of buckling of a more rigid surface layer

(e.g., lava crust) during fl ow of the underly-ing fl uid. Detailed descriptions and analyses of examples of folding have been presented by Fink and Fletcher (1978), Fink (1980), and Gregg et al. (1998), among others; the motiva-tion for this work, again, was the possibility of inferring fl ow rheology and compositional parameters for extraterrestrial examples (e.g., Warner and Gregg, 2003).

Nine lava fl ows were selected for surface folding analysis, based on the visible ridges seen in the DEM and aerial photographs. For each fl ow, fi ve parallel profi les, running per-pendicular to the axis of the folds, spaced 2 m apart, were taken along the middle sections of each fl ow. Fold scales were investigated by fast Fourier transform (FFT) analysis in MatLab using a custom series of scripts to investigate fold wavelength characteristics. Data were prepared for FFT analysis by interpolating the data to unit spacing, detrending the data stream, and applying a cosine taper (Kanasevich, 1975; Bracewell, 2000) to remove the baseline shift and sinc convolutions, which would otherwise have reduced the data quality. Typical examples

46-47

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Agios Nikolaoslavas

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Dafni lavas

Kténas lavas

Fouqué, Reck& Smith lavas

Niki lavasLiatsikasdome

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0 500 1000 1500 2000250m

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Figure 7. Interpreted geological map of the Kameni Islands, based on the new imagery and on previous maps compiled by Georgalas (1962), Huijsmans (1985), and Druitt et al. (1999). The names of selected lava fi elds follow the conventions started by Fouqué (1879) and subsequent authors (Georgalas, 1962). The Liatsikas dome is the youngest extrusive element on the Kameni Islands.



of a contoured periodogram are shown in Fig-ure 11, which shows the relative power (arbi-trary scale) of fold wavelengths along the length of a 1925 lava fl ow. This gives a rapid visual assessment of the extent to which fold dimen-sions change, or otherwise, along a fl ow. In this case, the fold wavelength is ~20 m close to the vent and ~30–40 m further downstream.

Most of the fl ow profi les share a number of common features: folds form close to the origi-nal eruptive vent. Folding is most prominent close to the center-line of the fl ow, and in many cases there is a consistent pattern of fl ow gen-erations that develops downstream, i.e., several fold generations of differing wavelength can be recognized. Results are summarized in Table 7. It is clear from Table 7 that most of the fl ows that show folding have one generation of folds with wavelengths (λ

1) on the order of 15–25 m.

When multiple generations of folds exist, sec-ond-generation folds (λ

2) have wavelengths

on the order of 25–65 m (or 1.5 ± 0.2λ1), and

third-generation folds have wavelengths of 30–110 m, or 1.8 ± 0.3λ

2. This pattern of low

ratios (<2) between subsequent generations of folds is consistent with the few data that exist for other dacite (r = 2.1 ± 0.3) and rhyolite (1.8 ± 0.4) fl ows (Gregg et al., 1998), and contrasts strongly with the high ratios (5.1 ± 1.1) that characterize basaltic fl ows (Gregg et al., 1998). This confi rms the potential of fl ow fold analy-sis for the identifi cation of “silicic” fl ows by remote sensing.

The scale of the folds qualitatively constrains the minimum apparent viscosity of the underly-ing fl uid (Fink, 1980; Gregg et al., 1998). Data from the Kameni Island fl ows are consistent with the view that in slow, effusive eruptions of silicic magmas, crust formation on the fl ow by cooling is much faster than crustal thickening by strain, and consequently the folds that form are of long wavelength. The slow effusion of the fl ows is consistent with the observation that the folds develop close to the vent.

Planform of Kameni Dacite Lava Flows

One motivation for the study of the quanti-tative morphology of terrestrial lava fl ows of known composition is that it may reveal fea-tures that allow investigators to make inferences about the compositions of fl ows on distant plan-ets, simply from the remote observations of morphology. One such approach that appears to have some promise was proposed by Bruno et al. (1992, 1994), which is to determine the frac-tal dimension, D, of the fl ow shape in planform. The technique relies on measuring the gross linear distance around an object as a function of the length scale of the measuring rod. On a

Lava ages

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Figure 8. (A) Expanded map section showing the revised interpretation of the 1939–1941 lavas, with the inferred fl ow paths shown with arrows (+ symbols show the UTM grid inter-sections). The extracts from a 1944 Royal Air Force reconnaissance air photograph (TARA ACIU 42090 3080) (B) and a 2004 air photo (C) show the subtle changes in the summit cra-ters due to the minor explosive activity in 1950, and the emplacement of the Liatsikas dome.

Pyle and Elliott


log-log plot of distance versus length scale, data following a fractal distribution should defi ne a trend with a linear slope of 1 – D. Bruno et al. (1992) showed that the parameter varies with lava type (a‘a versus pahoehoe) in basalts, but also showed (Bruno et al., 1994) that many more chemically evolved lavas do not show a consistent fractal behavior.

The Kameni lavas present an excellent oppor-tunity to test whether this model is applicable to blocky dacite lavas.

Lava fl ows with well-defi ned margins were digitized from the orthorectifi ed aerial photo-graphic images to obtain an outline with a spa-tial resolution of better than 1 m. Spatial data were exported into a program (Fractals; http://www.nsac.ns.ca/envsci/staff/vnams/Fractal.htm; Nams, 1996) to determine the gross length as a function of length scale, for length scales of 1–100 m. Most of the analyzed lava fl ows had margins that could be traced for ~1 km, which limited the upper range of length scale that could be used. A typical plot of gross distance against ruler length, confi rming a fractal distribution, is presented in Figure 12. All of the lava fl ows that were analyzed showed clear evidence for a fractal distribution, and, surprisingly, there was no detectable difference between the fractal

0 50 100 150 200 250 300 350 40030

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X: 190Y: 68.67

X: 17.77Y: 31.9

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X: 96.65Y: 68.23 X: 292.2

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Distance across levée (m)

Hei

ght a

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id (

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l = 3

2 m

abo

ve g

eoid

)

Vertical exaggeration x6

Flow width = 331 m

Flow height = 36 mFlow height = 35 m

Levée angle = 25°

Levée width = 79 m

Figure 9. Cross section showing a classical levée structure across the 1866 SW lava fl ow. Levées tend to be 15–30 m high, while fl ows are 100–300 m wide (Table 5).

TABLE 5. LEVÉE AND FLOW DIMENSIONS

Flow* N† Levée height(m)

Levée width(m)

Flow width(m)

Surface slope§

(o)

1707 16 23.1 ± 2.3 53.7 ± 6.6 224 ± 22 3.1 ± 0.71866 SSW 2 20.3 ± 2.0 32.9 ± 4.0 196 ± 17 4.3 ± 0.61866 SW 5 25.2 ± 3.4 61.6 ± 5.5 310 ± 16 4.2 ± 0.5

1925 NE 4 12.2 ± 1.5 29.5 ± 3.4 133 ± 4 1.5 ± 0.41925 SE 3 18.8 ± 1.6 51.5 ± 1.6 201 ± 6 2.0 ± 0.21925 W 5 21.2 ± 3.5 39.4 ± 9.3 119 ± 12 0 ± 0.71940B 5 18.0 ± 1.4 27.7 ± 1.8 121 ± 18 8.3 ± 2.01940 SW 12 20.9 ± 2.1 44.9 ± 3.7 193 ± 13 2.6 ± 0.91940 W 4 16.4 ± 2.1 25.4 ± 3.3 103 ± 1.2 6.7 ± 1.0

*Sections across which measurements were taken are shown in Figure DR2 (see text footnote 1).†N—number of sections measured. Quoted error is the standard deviation of the measurements.§Slope is slope at present day. This may not have been the slope down which the fl ow was emplaced.

TABLE 6. YIELD STRENGTHS, ASSUMING BINGHAM RHEOLOGY

Yield strength(×103 Pa)

Flow Flow width method Levée width method Levée height method

1707 64 ± 14 8 ± 4 64 ± 61866 SSW 57 ± 12 10 ± 3 75 ± 71866 SW 55 ± 15 18 ± 5 93 ± 131925 NE 30 ± 8 1.1 ± 0.6 23 ± 31925 SE 48 ± 8 3.4 ± 0.7 35 ± 31940B 72 ± 16 32 ± 15 134 ± 101940 SW 61 ± 13 5 ± 3 58 ± 61940 W 71 ± 18 19 ± 6 107 ± 14



dimensions of younger fl ows (<200 yr old) that form the coastline and those with margins that are entirely exposed inland. The only example that was clearly nonfractal was the profi le for the A.D. 726 fl ow on Palea Kameni, which presum-ably has been deeply eroded. The fractal dimen-sions for all of the lava fl ows that were analyzed are plotted in a histogram in Figure 13. It is clear from this graph that the blocky a‘a dacite lavas of the Kameni Islands have a fractal dimension, D (1.067 ± 0.006, n = 10), that is indistinguish-able from that of Hawaiian a‘a, at least on the 1–100 m scale. Thus, fractal dimension alone may be a good discriminator for gross-fl ow type (a‘a versus pahoehoe), but is not necessarily a good discriminator for composition.

Dome Growth on the Kameni Islands

One typical feature of all of the historical eruptions of the Kameni Islands is the growth of lava domes. Field observations of the chang-ing height of the summit of the 1866–1870 (Giorgios) dome and the 1939 (Fouqué) dome were recorded, respectively, by Fouqué (1879), and Georgalas and Papastamatiou (1953).

Figure 10. Surface buckling shown in a topographic section along the 1925 SE lava fl ow, with the underlying slope removed. Note the prominent folding, which has 10–100-m-scale wavelengths and 2–3 m amplitude. Data from fl ows are summarized in Table 7.

100 200 300 400 500 600 700 800 900 1000-5

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ght f

rom

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ar tr

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Figure 11. Contour map of the spec-tral power (arbitrary scale) of the dif-ferent fold wavelengths in a set of fi ve parallel profi les, spaced 2 m apart, along the length of the 1866 SW lava fl ow. Window 1 is at the fl ow toe, and 11 is at the fl ow origin. Buckling with an ~20 m wavelength can be observed near the vent; downstream, a 30–40 m wavelength becomes dominant.

Pyle and Elliott


Figure 12. Graph showing the relation-ship between the length of the measuring rule (x axis) against the gross length of the planform for the 1925 N fl ow. The data can be described by a power law with a slope of −0.054 (±0.001), which confi rms that the shape of the margin of this lava fl ow is self-similar on a 1–100 m length scale (see Mandelbrot, 1967). The data have a fractal dimension of 1.054.

TABLE 7. WAVELENGTHS OF FOLDS

Flow* N† Fold generation 1(m)

Fold generation 2(m)

Fold generation 3(m)

1707 21 19.9 ± 0.9 24.8 ± 0.9 35.5 ± 2.41866 SSW 11 23.1 ± 0.4 32.7 ± 2.1 72 ± 51866 SW 11 23.5 ± 1.5 35.4 ± 1.01925 NE 21 15.4 ± 1.1 21.0 ± 1.3 28.4 ± 2.21925 SE 21 26.0 ± 1.5 48.9 ± 2.0 93 ± 191925 W 11 41.9 ± 1.7 63 ± 71940B 11 21.5 ± 1.7 28.5 ± 1.1 53 ± 61940 SW 11 35.5 ± 2.2 48.8 ± 4.11940 W 11 38.5 ± 3.1 64 ± 6 109 ± 12

*Sections along which measurements were made are shown in Figure DR1 (see text footnote 1).†Number of measurements

.

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.6 1.8 2.0

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Kameni blocky a'aGalapagos a'aHawaii a'aHawaii

Figure 13. Comparison of fractal dimensions determined for Nea Kameni blocky a‘a fl ows with data from basaltic a‘a and pahoehoe fl ows from Hawaii and Galapagos (Bruno et al., 1992, 1994). Fractal dimension appears to be an excellent discrimi-nant of gross fl ow type, independent of composition.



Surprisingly, the importance (indeed, the existence) of these data seems to have been overlooked, and they are summarized for ref-erence in Table 8. The dome-growth data are plotted in Figure 14. Both dome-forming erup-tions show closely similar growth rates in terms of dome height (H) with time (t), which paral-lel those of the well-known lava domes of the Soufrière volcano of St. Vincent (1979 eruption; Huppert et al., 1982), and Mount St. Helens (1980–1986; Swanson and Holcomb, 1990). The power-law dependence of dome height with time is close to the t1/4 dependence predicted by models of domes that have growth controlled by a crustal yield strength (e.g., Fink and Griffi ths, 1998; Griffi ths 2000), and the best-fi t curve with a t1/4 dependence is H = 1.2t1/4. Follow-ing Griffi ths (2000), this suggests that the yield strength of the crust of the Kameni dacite domes is ~4 × 107 Pa, which is somewhat lower than for the domes of St. Vincent (1.5 × 108 Pa), Mount St. Helens (1.3 × 108 Pa), or Pinatubo (9 × 107 Pa; Griffi ths, 2000).

It is worth noting that while the dome-height data are consistent with a t1/4 time dependence, alternative relationships cannot yet be ruled out. For example, measurement errors are realisti-cally likely to have been of the order of 1 m, and the start time of the eruption (as opposed to the time when magma emerged above sea level) is not necessarily well known. In addition, Kameni domes often show a transition from endogenous growth early in the eruption, to modifi cation by Vulcanian explosions at later stages (e.g., Kténas, 1926). For these reasons, the possibil-ity of a different time dependence (e.g., t1/3 for a fi xed dome shape and a uniform eruption rate) cannot be ruled out; and in the event of a future eruption, careful quantitative observations of dome growth will be of great value.

The close similarity of the behavior of the 1866–1870 and 1939 domes allied with the uni-formity of the composition of the Kameni Islands over the past 2200 yr suggests that we may use the heights of domes from earlier historic eruptions to infer the eruption durations. We summarize the results of this analysis in Table 9. The predictions of both the t1/4 model and a time model based simply on the empirical fi t to the height data are consistent with reports of these early eruptions. Both models predict that the dome will grow to a height of ~20 m within 1 d, and ~30 m within 4–5 d. Thus, for a submarine eruption originating on the Kameni-Banco plateau (at ~20 m below sea level), the dome should emerge above the sur-face within a day or two of the start of activity. This is consistent with descriptions of the start of the 1707 eruption (Goree, 1710; Tarillon, 1715a; Fouqué, 1879). For domes emerging from this submarine base level to the heights recorded in

1875 of ~70 m (the 1573 dome), or 101 m (the 1707 dome), expected eruption durations are on the order of 0.3–1 yr and 1–3 yr, respectively. The latter is, again, consistent with the contemporary records of the 1707–1711 eruption.

Simply on the basis of observed height of the Mikra Kameni (1573) dome, it is plausible that the eruption did indeed last for only 1 yr or less. This supports our suggestion that the quoted

TABLE 8. GROWTH OF DOMES DURING THE 1866 AND 1939 ERUPTIONS

1866 Giorgos Dome* 1939 Fouqué Dome†

Growth started 2 February 1866 Growth started 13 November 1939

Elapsed time(d)

Dome height(m)

Elapsed time(d)

Dome height(m)

3 20 3 23.55 30 7 30.535 50 9 40.547 50 28 45.5108 70 65 56.5396 108 150 78.51519 118

*1866 dome-growth parameters from Fouqué (1879).†1939 dome-growth parameters from Georgalas and Papastamatiou (1953).

10

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1.0E+05 1.0E+06 1.0E+07 1.0E+08 1.0E+09

Elapsed time (s)

Dom

ehe

ight

(m)

Kameni 1866 dome

Kameni 1939 dome

St. Vincent 1979

Mount St. Helens 1981 - 1986

Figure 14. Lava dome height (m) as a function of growth time, showing data from the 1866 and 1939 eruptions of the Kameni Islands (1866 Giorgios dome; 1939 Fouqué dome; Table 8), the Soufrière volcano of St. Vincent eruption (1979; Huppert et al., 1982), and Mount St. Helens (Swanson and Holcomb, 1990). The dependence of dome height on elapsed time for the Kameni and other domes is close to t1/4, which is expected from analysis of domes with a crustal yield strength (Fink and Griffi ths, 1998; Griffi ths, 2000). The solid line is the best-fi t curve through the Kameni data with t1/4 dependence: H = 1.217t1/4.

TABLE 9. INFERRED ERUPTION LENGTHS FROM DOME HEIGHT

Eruption Dome height above base level

(m)

Inferred eruption duration

(d)

1573 70–90 134–368 (Griffi th)119–296 (Empirical)

1707–1711 100–120 560–1165 (Griffi th)433–835 (Empirical)

Pyle and Elliott


1570–1573 age range for this eruption derives simply from an uncertainty in the calendrical date of the eruption, and does not refl ect the true dura-tion of the event.

Future Activity of the Kameni Islands

Apart from the anomalous event of 1950, all of the historic intracaldera eruptions for which there are at least adequate records (those since the eighteenth century) have shared a number of important common features: pre-eruptive uplift of parts of the submarine Kameni edifi ce, and the eruption of magma from at least two eruptive vents. All have also involved the early formation of lava domes, which later act as a focus for vig-orous, intermittent explosive activity, as well as acting as the vent from which lavas emerge. There is no reason to suspect that a future erup-tion would not be preceded by the same general phenomena—including general uplift of the edi-fi ce, and discoloration of the sea—and so, given the current monitoring of the caldera, it is highly likely that the next eruption will be anticipated some days to weeks in advance. Some further inferences about a future eruption on the Kameni Islands can also be gained from a consideration of the vent distribution and intereruptive periods.

Vent Distribution

The distribution of volcanic vents across the Kameni Islands strongly suggests that there is an underlying tectonic control on the supply of magma toward the surface. As previous authors (e.g., Fytikas et al., 1990; Druitt et al., 1989, 1999) noted, the vent pattern defi nes a narrow NE-SW trend (Fig. 15) that extends northeast of Nea Kameni to include the submarine high (including the former Bancos bank), which divides the fl ooded caldera basin into two parts. Many of the prehistoric explosive eruptions also have tephra dispersal patterns that are consistent with vent locations along the same trend (Druitt et al., 1989, 1999), and the locations of previ-ous (pre-1866) and current hydrothermal vents around the Kameni Islands also lie along this same trend. Purely on the basis of the activity of the past 1000 yr, one would anticipate that the next intracaldera eruption would start in the region between Palea Kameni (UTM zone 35N coordinates 40291, 3542) and northern Nea Kameni (40302, 3568), and, as long as the erup-tion lasted more than 2 mo, it would involve two or more eruptive vents.

Forecasting Future Events

The present state of the Kameni Islands can most simply be interpreted as a part of the

intereruptive shield-building phase, which is a typical feature of the evoluti on of Santorini over the past 300,000 yr (e.g., Druitt et al., 1999). Despite the considerable work that has been completed on the dating of past erup-tions of Santorini (Druitt et al., 1999), several of the major explosive eruptions still lack a good age control. Consequently, there are insuf-fi cient data from which to quantify the pattern of intereruptive shield-building periods, for example, using a rank-order approach (Pyle, 1998). Instead, we can use the known ages of a few key eruptions to infer the mean interval between all events. The last phases of seven explosive eruptions were separated, on aver-age, by 28 k.y. of repose. Within this sequence, major lava shields were constructed between ca. 67 and 55 ka (the Skaros shield) and ca. 50 and 21 ka (the Therasia dome complex; Druitt et al., 1999). These crude constraints imply that the shield-building phase might last for tens of thousands of years. Assuming a mean interval between explosive eruptions of 20–30 k.y., then from Poisson statistics, there is a 50% probabil-ity that the post-Minoan shield-building phase will last between 14 and 23 k.y.

The relationship between eruption length and intereruptive interval for the last 5 eruptions of the Kameni Islands is shown in Figure 16. If we regard the 1950 eruption as anomalously short, and interpret this as a minor extrusion follow-ing the 1939–1942 activity, the notable feature is that the remaining four events lie along a line of best fi t described by: eruption length (days) = 7.4 × (intereruptive interval, yr) + 578. This striking relationship is consistent with a model for the Kameni Islands of a constant time-aver-aged deep supply of magma, and with erup-tion lengths that are determined by available magma volume. Estimates of erupted volumes (Table DR1, see footnote 1) are consistent with this model; the largest recent eruption (1866–1870) followed the longest pre-eruptive repose period. If this empirical time-dependent relationship holds in the future, then the next Kameni Island eruption will last for more than 2.7 yr (in 2006) to more than 4 yr (in 2070).

CONCLUSIONS

A combination of high-resolution digital mapping and aerial photography with archived

Figure 15. Graph showing the areal distribution of known vents from the Kameni Islands. Error bars indicate the scale of the vent, crater, or dome. There is a very tight clustering of vents about an approximately NE-SW trend, which agrees with the Kameni line identifi ed previously by Druitt et al. (1989). This trend is thought to mark the locus of a tectonically (fault?) controlled magmatic plumbing system, and future activity is therefore likely to be initiated at a point, or points, along the same trend.

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contemporary eruption reports reveals a wealth of detail relating to the emplacement of viscous, blocky dacite a‘a lava fl ows on the Kameni Islands, Santorini, Greece.

Lava fl ows from recent eruptions of the Kameni Islands exhibit the classic surface morphologies associated with viscous a‘a lava-fl ow emplacement: levées and compression folds. Levée structures, tens of meters wide and tens of meters high, develop close to the vent. Channelized fl ows within the levées show prominent ridges with ~20–40 m wavelength and 1–4 m amplitude. Ridges show variable, but limited, evolution in terms of scale down-stream, with secondary and tertiary fold wave-lengths only 50–100% longer than the fi rst-formed folds. Levée heights and fl ow widths are consistent with a Bingham rheology, and lava yield strengths are on the order of 3–7 × 104 Pa.

Analysis of the shapes of fl ow edges con-fi rms that the blocky a‘a dacite lava fl ows show a scale-invariant morphology, on length scales of 1–100 m, with a typical fractal D value of 1.067 ± 0.006, which is indistinguishable from Hawaiian a‘a. On these short length scales, at least, there may be limitations to the use of frac-tal dimension for inferring lava composition.

Dome-forming eruptions of the Kameni Islands in 1866–1870 and 1939–1940 showed similar patterns of behavior, with progressive increases in dome height with time (height = 1.2t1/4) that are consistent with a model of slow infl ation of a dome with a strong crust. Lava

domes on the Kameni Islands have a crustal yield strength (4 × 107 Pa) that is lower by a factor of 2–4 than the domes at Pinatubo (1991), Mount St. Helens (1981–1986), or St. Vincent (1979). The dome-height model enables us to infer the durations of early historic eruptions on the Kameni Islands. This, combined with the appar-ent time-predictable nature of volcanic eruptions of the Kameni Islands since 1573 A.D., allows us to suggest that the next eruption of the Kameni Islands will last for more than 2.7 yr (in 2006) or more than 4 yr (by 2070 A.D.), and may involve the formation of a dome of ~115–125 m and 130–145 m height, respectively.

ACKNOWLEDGMENTS

Data were collected during the Natural Environ-ment Research Council (NERC) Airborne Remote Sensing Facility (ARSF) campaign to the eastern Mediterranean in April 2004. Light detection and ranging laser radar (LiDAR) data were expertly pro-cessed in Cambridge, Department of Geography Unit for Landscape Modelling, by Gabriel Amable. We thank David Cobby, Carl Joseph, Ivana Barisin, and the ARSF team, and George Vougioukalakis (IGME), Stathis Stiros, and Aris Chasapis for assistance with data collection; and Vikki Martin for discussion. Pyle thanks staff of the Earth Sciences and University Libraries, Cambridge; the British Library; and Keele University’s ‘Evidence in Camera’ for their able assis-tance with archive material. Elliott thanks Shell (UK) for support toward fi eld costs, and Zoë Rice for fi eld assistance. Part of this work formed John Elliott’s fi nal-year undergraduate dissertation. We thank Ricky Herd and Steven Anderson for their detailed and help-ful reviews.

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0

400

800

1200

1600

2000

100500 150 200

Intereruption interval (yr)

Eru

pti

on

len

gth

(d

)1866

1707

1925

1939

1950

Figure 16. Relationship between eruption length and intereruptive interval for the last fi ve eruptions of the Kameni Islands (1707–1711, 1866–1870, 1925–1928, 1939–1941, 1950). The 1950 eruption appears to be anomalously short, and may be best interpreted as a minor extrusion following the 1939–1942 activity. The remaining four events lie along a line of best fi t described by: eruption length (days) = 7.4 × (intereruptive interval, yr) + 578. Based on this relation, the next Kameni Island eruption is expected to last for at least 2.7 yr (in 2006, interval = 56 yr, eruption length = 991 d).

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