Roger Urgeles

Slope failures on the flanks of the western Canary Islands

Abstract Landslides have been a key process in the evolution of the western Canary Islands. The younger and more volcanically active Canary Islands, El Hierro, La Palma and Tenerife, show the clearest evidence of recent landslide activity. The evidence includes landslide scars on the island flanks, debris deposits on the lower island slopes, and volcaniclastic turbidites on the floor of the adjacent ocean basins. At least 14 large landslides have occurred on the flanks of the El Hierro, La Palma and Tenerife, the majority of these in the last 1 million years, with the youngest, on the northwest flank of El Hierro, as recent as 15 thousand years in age. Older landslides undoubtedly occurred, but are difficult to quantify because the evidence is buried beneath younger volcanic rocks and sediments. Landslides on the Canary Island flanks can be categorised as debris avalanches, slumps or debris flows. Debris avalanches are long runout catastrophic failures which typically affect only the superficial part of the island volcanic sequence, up to a maximum thickness of 1 to 2 km. They are the commonest type of landslide mapped. In contrast, slumps move short distances and are deep-rooted landslides which may affect the entire thickness of the volcanic edifice. Debris flows are defined as landslides which primarily affect the sedimentary cover of the submarine island flanks. Some landslides are complex events involving more than one of the above end-member processes. Individual debris avalanches have volumes in the range of 50–500 km3, cover several thousand km2 of seafloor, and have runout distances of up to 130 km from source. Overall, debris avalanche deposits account for about 10% of the total volcanic edifices of the small, relatively young islands of El Hierro and La Palma. Some parameters, such as deposit volumes and landslide ages, are difficult to quantify. The key characteristics of debris avalanches include a relatively narrow headwall and chute above 3000 m water depth on the island flanks, broadening into a depositional lobe below 3000 m. Debris avalanche deposits have a typically blocky morphology, with individual blocks up to a kilometre or more in diameter. However, considerable variation exists between different avalanche deposits. At one extreme, the El Golfo debris avalanche on El Hierro has few large blocks scattered randomly across the avalanche surface. At the other, Icod on the north flank of Tenerife has much more numerous but smaller blocks over most of its surface, with a few very large blocks confined to the margins of the deposit. Icod also exhibits flow structures (longitudinal shears and pressure ridges) that are absent in El Golfo. The primary controls on the block structure and distribution are inferred to be related to the nature of the landslide material and to flow processes. Observations in experimental debris flows show that the differences between the El Golfo and Icod landslide deposits are probably controlled by the greater proportion of fine grained material in the Icod landslide. This, in turn, relates to the nature of the failed volcanic rocks, which are almost entirely basalt on El Hierro but include a much greater proportion of pyroclastic deposits on Tenerife. Landslide occurrence appears to be primarily controlled by the locations of volcanic rift zones on the islands, with landslides propagating perpendicular to the rift orientation. However, this does not explain the uneven distribution of landslides on some islands which seems to indicate that unstable flanks are a ‘weakness’ that can be carried forward during island development. This may occur because certain island flanks are steeper, extend to greater water depths or are less buttressed by the surrounding topography, and because volcanic production following a landslide my be concentrated in the landslide scar, thus focussing subsequent landslide potential in this area. Landslides are primarily a result of volcanic construction to a point where the mass of volcanic products fails under its own weight. Although the actual triggering factors are poorly understood, they may include or be influenced by dyke intrusion, pore pressure changes related to intrusion, seismicity or sealevel/climate changes. A possible relationship between caldera collapse and landsliding on Tenerife is not, in our interpretation, supported by the available evidence.

Submarine Mass Movements and Their Consequences

Submarine mass movements represent major offshore geohazards due to their destructive and tsunami-generation potential. This potential poses a threat to human life as well as to coastal, near shore and offshore engineering structures. Recent examples of catastrophic submarine landslide events that affected human populations (including tsunamis) are numerous; e.g., Nice airport in 1979 (Dan et al. 2007), Finneidfjord in 1996 (e.g., L’Heureux et al., this volume, Steiner et al., this volume), Papua-New Guinea in 1998 (Tappin et al. 2001), Stromboli in 2002 (Chiocci et al. 2008), and the 2006 and 2009 failures in the submarine cable network around Taiwan (Hsu et al. 2008). The Great East Japan Earthquake of March 2011 also generated submarine landslides that may have amplified effects of the devastating tsunami as shown in Fryer et al. (2004). Given that 30% of the World’s population lives within 60 km of the coast, the hazard posed by submarine landslides is expected to grow as global sea level rises. In addition, the deposits resulting from such processes provide-various types of constraints to offshore development (Shipp et al. 2004), and have significant implications for non-renewable energy resource exploration and production (Weimer and Shipp 2004; Beaubouef and Abreu 2010).

Deep sea-floor evidence of past ice streams off the Antarctic Peninsula

The past existence of a giant ice stream off the northern end of the Antarctic Peninsula is suggested by a convex-upward, elongated sediment body now at a mean water depth of 1000 m. Because of its morphological characteristics, i.e., a set of parallel to subparallel ridges and grooves to 100 km long, with an overall width of 25 km, we call this depositional body a bundle structure. We hypothesize that bundle structures form by accumulation of basal deformation till under their parent ice streams. From its location, size, and overall characteristics the bundle structure described here constitutes the best preserved, largest, deepest, and relatively low latitude evidence of giant ice streams that flowed offshore Antarctica during glacial maxima. Bundle structures reveal the very dynamic behavior of ice caps in the northern Antarctic Peninsula during the last glacial maximum, with catchment areas draining rapidly under marine-based critical subglacial conditions. Bundle structures represent a new megascale streamlined glacial landform.

The most recent megalandslides of the Canary Islands: El Golfo debris avalanche and Canary debris flow, west El Hierro Island

Two major landsliding events have been identified west of the island of El Hierro: The El Golfo debris avalanche and the Canary debris flow. These landslides were identified from swath bathymetry, seismic reflection, and TOpographic PArametric Sonar (TOPAS) data obtained in December 1994 during a cruise on board the Spanish R/V Hesperides. The El Golfo debris avalanche originated subaerially on the western flank of the island of El Hierro and has an associated 150 km3 rock debris deposit on the base of slope. The Canary debris flow, which dislocated some 400 km3 of sediment, resulted from a different failure originated between 3200 and 3700 m depth at the base of slope of the island of El Hierro. According to the studied data set, its source area seems to have been covered by the El Golfo debris avalanche deposit. The triggering of the El Golfo debris avalanche (between 136 and 21 ka) is related to tensional stresses on the rift zones of the island. These rift zones control the emplacement and morphology of the landslide scar. In the Canary Islands, a relation between landslide ages and island ages can be established, indicating a link between subsidence history, age of shield phases, and giant landslides.

The morphology of the submarine flanks of volcanic ocean islands: A comparative study of the Canary and Hawaiian hotspot islands

The submarine flanks of volcanic islands are shaped by volcanic constructional processes, landslides, erosion, sediment deposition and tectonic movements. We use a newly acquired multibeam sonar dataset from the westerly Canary Islands (El Hierro, La Palma and Tenerife) to develop a comparison with the Hawaiian Islands, which suggests differences in the processes constructing and modifying their flanks. Landslides affect the flanks of both island groups. Debris avalanches (fast-moving shallow landslides) have left smooth chutes and blocky deposits in both cases, but blocks within some Hawaiian avalanche deposits are markedly larger. We attribute the larger block sizes in the Hawaiian Islands to the fact that their avalanches were relatively unconfined, whereas many Canary and Hawaiian avalanches with small block sizes appear to have been constrained down narrow chutes, forcing interactions between blocks within the flows and encouraging disintegration. Furthermore, the Hawaiian avalanches with the largest blocks initiated near sea-level, whereas many of the Canary avalanches initiated above sea-level, so hydraulic resistance of water entering cracks may be an additional factor in resisting block disintegration during flow. Slow-moving deep-seated slumps or volcanic spreading have produced submarine benches and tabular escarpments due to thrust faulting adjacent to several Hawaiian rift zones, but are not well-developed in the Canaries. Although volcanic morphology is partly obscured by sedimentation in the Canaries, we are able to interpret lava terraces around the deep flanks of El Hierro which are similar to those found in the Hawaiian Islands. However, cones rather than terraces are the most common volcanic forms in the Canary Islands, implying that flank eruptions have involved magma with significant volatile contents, assuming that volatile contents dictate whether cones or terraces are formed. These differences may ultimately originate from the different building rates of the two island groups. For example, the lack of evidence for high-level magma chambers in the Canaries, associated with their lower outputs, implies that there is less possibility for degassing of magma below the summit before lateral intrusion down rift zones, hence cones rather than lava terraces are more commonly observed. The apparent lack of slumping or volcano spreading could also reflect a lack of driving pressure from extensive high-level magma chambers in the Canaries.

Recurrent large-scale landsliding on the west flank of La Palma, Canary Islands

A large area of debris avalanche deposits has been discovered on the western submarine flanks of the island of La Palma. Multibeam bathymetry and its derivative backscatter data, Towed Ocean Bottom Instrument (TOBI) sidescan sonar images, and 3.5 kHz and airgun seismic reflection data have been used to identify at least two, and possibly as many as four, major landslide events. The youngest of the events, the Cumbre Nueva Debris Avalanche, extends onshore into the valleys bounded by the Caldera de Taburiente and Cumbre Nueva Ridge, which mark the degraded collapse scars. Radiometric dating of the volcanic flows in the headwall indicate an age of between 536 and 125 ka for this landslide. The debris avalanche covers an area of 780 km2, has a maximum thickness of 500 m, and has an estimated volume of 95 km3. Older deposits, collectively referred to as the Playa de la Veta Debris Avalanche Complex, are probably, as the name indicates, an amalgamation of at least two or three events rather than the result of a single catastrophic failure. The Playa de la Veta Debris Avalanche Complex is associated onshore with an unconformity dated as late Matuyama (1 Ma to 800 ka). It covers an area of 1200 km2, has a maximum thickness of 1300 m, and may represent a total volume of up to 650 km3. The greater thicknesses and limited areas occupied by debris avalanches on the western flank of La Palma, compared to other landslides in the Canary Archipelago, suggest that the La Palma landslide masses have relatively low mobility. The different debris avalanche lobes formed by each landslide event are separated by channels 2–2.5 km wide. The clear relationship between channel position and the boundaries of each debris avalanche lobe indicates that debris avalanches control later channel formation and pathways. The relief of the submarine flanks of the La Palma volcanoes, in the areas of island slope unaffected by landslides, is mainly the result of constructional volcanic processes. However, the older submarine slopes, such as in the northern Taburiente volcano, may also have been modified by smaller-scale submarine mass wasting and sediment flows.

Seafloor evidence of a subglacial sedimentary system off the northern Antarctic Peninsula

Swath-bathymetry data and high-resolution seismic reflection profiles allow us to portray a subglacial sedimentary system off the northern tip of the Antarctic Peninsula, in the Central Bransfield Basin, during the Last Glacial Maximum with unprecedented detail. Postglacial reworking and sedimentation are weak enough for the subglacial morphology of the Last Glacial Maximum to be preserved on the present seafloor. The studied sedimentary system extends 250 km, from ∼1000 m above sea level to ∼2000 m water depth. The data set supports a model for subglacial sedimentary systems that consists of: (1) an upper ice catchment or erosional zone on the innermost continental shelf, extending onshore; (2) a transitional erosional-depositional zone on the inner shelf with drumlinized seafloor; (3) a depositional outer shelf zone with mega-scale bundle glacial lineations; and (4) a debris apron on the continental slope and base of slope formed under floating ice shelves with debris delivery linked to grounding lines along the shelf break.

The Balearic Promontory geomorphology (western Mediterranean): morphostructure and active processes

In this paper, a detailed study of the submarine geomorphology surrounding the Balearic Promontory (western Mediterranean), a northeast prolongation of the Neogene Betic Range in southern Spain, is presented from a series of high-resolution tools including swath bathymetry and seismic reflection profiling. The study identifies the main features of the continental shelf, slope and basins surrounding the Balearic Islands. We show a variety of seafloor relief that owes its origin to several geologic processes, which ultimately control the transport of sediment from the shallower areas to the deep basin. The most important processes are erosion of the shelf and upper slope (terraces associated with different Quaternary sea-level stands and canyons), transport and sediment deposition in the lower slope and base-of-slope by turbidity currents, volcanism and instability processes (landslides scarps and debris lobes). The swath data show that tectonics plays an important role in shaping the submarine slopes of Eivissa and Formentera, the two southernmost islands, as well as its interplay with sedimentary processes, especially mass wasting. Finally, several areas show evidence of pockmarks, which indicate that fluid migration take place in the sediments, probably conditioning several other processes such as mass wasting.

The July 1996 flood deposit in the Saguenay Fjord, Quebec, Canada: implications for sources of spatial and temporal backscatter variations

In July 1996 a major rainstorm and flood took place in the Saguenay Fjord. Backscatter strength measurements made with a Simrad EM1000 multibeam echosounder in 1993, 1997 and 1999 have shown spatial and temporal variations, which are interpreted in relation to the occurrence of the flood. After empirical calibration of the different maps the data show an overall diminution of 5 dB in backscatter strength in 1997, 1 yr after the flood took place, while the data sets obtained in 1993 and 1999 show comparable levels of backscatter strength. The different data sets show a similar pattern of low and high backscatter patches, which represent variations of the backscatter strength of a few decibels. Several grain size and water content measurements were also carried out on sediment box core and grab samples from the Fjord bottom in 1997 and 1999. These have shown that the areas with higher acoustic backscatter correspond to the finer sediments, while the low backscatter patches correspond to the coarser material. The data show little relation between water content and backscatter strength, thus indicating a poor dependence between the impedance terms (bulk density and sound speed) and the backscatter. Having taken this into account, the major contribution to backscatter strength is assumed to result from differences in surface and volume roughness of the sediment. Since finer grain sizes offer higher backscatter the grain size of the sediment is not considered as being a major contributor to roughness and hence, backscatter strength. The major factor that seems to control roughness generation in the Saguenay Fjord is considered to be bioturbation. The areas where geological processes most physically disturb the bottom, which coincide with the areas where also most accumulation takes place and the coarser grain sizes are deposited, are more sparsely colonized by organisms. This results in a lower degree of bioturbation, lower roughness and thus, backscatter strength. This hypothesis helps explain the variation in backscatter strength observed between the different measurement years. The several million tons of sediment deposited in the Fjord bottom after the 1996 flood buried the benthic fauna, which was still recovering 1 yr later. The less bioturbated sediment at this time would present lower roughness with respect to the 1993 and 1999 data resulting in a lower backscatter strength.

Analysis of slope failures in submarine canyon heads: An example from the Gulf of Lions

To improve understanding of evolution of submarine canyons, a three-dimensional slope-stability model is applied to Bourcart Canyon in the western Gulf of Lions in the Mediterranean Sea. The model builds on previous work by Chen and others, and it uses the upper bound theorem of plasticity to calculate the factor of safety of a kinematically admissible failing mass. Examples of three-dimensional failure surfaces documented in the literature were used to test the model formulation. Model application to Bourcart Canyon employed the results of a detailed stratigraphic analyses based on data acquired by swath bathymetry, sub-bottom profiling, high-resolution seismic reflection surveys, and piston coring. The sediment layers were also characterized using in-situ geotechnical measurements and laboratory tests. The effects of three loading scenarios were analyzed: (1) earthquake shaking, (2) hemipelagic sedimentation, and (3) axial incision. These three mechanisms influenced the predicted volumes and shapes of slope failures along the flanks of Bourcart Canyon, and comparison of these predictions with failure geometries inferred from seafloor morphology showed that mass failures could account for the observed morphology along the canyon walls as well as a mechanism of canyon widening