Journal of Volcanology and Geothermal Research 94Ž. 1999 323±340 www.elsevier.comrlocaterjvolgeores Flank collapse triggered by intrusion: the Canarian and Cape Verde Archipelagoes Derek Elsworth a,), Simon J. Day b,c a Department of Energy and Geo-EnÕironmental Engineering, PennsylÕania State UniÕersity, UniÕersity Park, PA 16802-5000, USA b Department of Geography and Geology, Cheltenham and Gloucester College of Higher Education, UK c Benfield Greig Hazard Research Centre, UniÕersity College London, London, UK Received 10 May 1999 Abstract The potential to develop kilometer-scale instabilities on the flanks of intraplate volcanoes, typified by the Canary and Cape Verde Archipelagoes, is investigated. A primary triggering agent is forced injection of moderate-scale dikes, resulting in the concurrent development of mechanical and thermal fluid pressures along the basal decollement, and magmastatic pressures at the dike interface. These additive effects are shown capable of developing shallow-seated block instabilities for dike thicknesses of the order of 1 m, and horizontal lengths greater than about 1 km. For dikes that approach or penetrate the surface, and are greater in length than this threshold, the destabilizing influence of the magmastatic column is significant, and excess pore fluid pressures may not be necessary to initiate failure. The potentially destabilized block geometry changes from a flank-surface-parallel sliver for short dikes, to a deeper and less stable decollement as dike horizontal length builds and the effects of block lateral restraint diminish. For intrusions longer than about 1 km, the critical basal decollement dives below the water table and utilizes the complementary destabilizing influences of pore fluid pressures and magma ``push'' at the rear block-scarp. In addition to verifying the plausibility of suprahydrostatic pressures as capable of triggering failure on these volcanoes, timing of the onset of maximum instability may also be tracked. For events within the Cumbre ViejaŽ. 1949 and FogoŽ. 1951, 1995 pre-effusive episodes, the observation of seismic activity within the first 1 week to 4 months is consistent with the predictions of thermal and mechanical pressurization. q 1999 Elsevier Science B.V. All rights reserved. Keywords: aquathermal; poromechanics; flank instability; Fogo; Cumbre Vieja 1. Introduction these volcanoes vary in angle from less than 88 to more than 208. At the lower end of this range, in Large scale lateral collapses, with volumes of up particular, the causes of these massive failures are to thousands of cubic kilometers, are a ubiquitous enigmaticŽ Iverson, 1991, 1992; Voight and Elsworth, feature of oceanic island volcanoes in their most 1992. since the destabilizing gravitational stresses active, shield-building, stage of growthŽ e.g., Moore, generated on basal failure planes are much less than 1964; Moore and Fiske, 1969. The overall slopes of resisting frictional forces, for reasonable values of friction coefficients. Models for the kinematics of preserved gravita- ) Corresponding author. Tel.: q1-814-863-1643; fax: q1-814- tional slumps assumed typical for oceanic volcanoes, 865-3248; E-mail: [email protected] including the Hilina slide system in HawaiiŽ Swan- 0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S0377-0273Ž. 99 00110-9 324 D. Elsworth, S.J. DayrJournal of Volcanology and Geothermal Research 94() 1999 323±340 son et al., 1976; Ando, 1979; Buchanan-Banks, 1987; cussed below. An alternative, that could reduce Lipman et al., 1985. , are difficult to justify as the strength sufficiently, is mechanical and thermal initial geometries of catastrophic long-runout slides, pore-fluid pressurizationŽ Elsworth and Voight, 1995, since motion must occur updip on landward dipping 1996; Day, 1996; Voight and Elsworth, 1997; Hurli-È interfaces. Slides must initiate on primarily seaward mann et al., 1997; van Wyk de Vries and Francis, dipping interfaces if sufficient momentum is to be 1997. According to these models, instability may be generated for long-runout avalanches. In addition to triggered by a reduction in strength on the decolle- the well-known occurrences of such deposits around ment through an increase in pore fluid pressures in the Hawaiian Islands, long-runout debris avalanche the volcanic edifice. deposits have been observed on the ocean floor Here, we consider how a relatively small volume surrounding the Canarian ArchipelagoŽ Holcomb and of magma, emplaced as dikes in a volcanic rift zone, Searle, 1993; Masson, 1996; Teide-Group, 1997; can destabilize the edifice through the mechanical or Urgeles et al., 1997. , together with aborted collapse thermal pressurization of pore fluids. structures on the flanks of El HierroŽ Day et al., 1997. , and incipient collapse structures on Cumbre ViejaŽ. La Palma and Fogo in the Cape Verde 2. Field evidence for magmagenic fluid pressures IslandsŽ. see Day et al., 1999a-this volume . Indica- tions of the importance of including pore fluid pres- These postulated pressurization mechanisms have sure effects in analysis of the stability of these been observed. Mechanical pressurization has been steep-sided oceanic island volcanoes can be found in observed in a series of intrusive events at Krafla, the patterns of flank deformation associated with Iceland, with pore pressures rapidly reaching 70 m of certain recent eruptions. These are the 1949 eruption water-head in a confined aquifer 4.3 km from the of the Cumbre Vieja volcano, La PalmaŽ Canary intrusion siteŽ Brandsdottir and Einarsson, 1979; Islands.Ž Day et al., 1999a-this volume . ; the 1951 Tryggvason, 1980; Stefansson, 1981. Similarly, eruption on FogoŽ.Ž Cape Verde Islands Day et al., thermal pressurization is likely the cause of long-term 1999b-this volume. ; and possibly also the 1995 erup- anomalous flow from a borehole observed during the tion of FogoŽ. Heleno da Silva et al., 1999 . eruption of HeimaeyŽ. BjornssonÈ et al., 1977 . These Flank failure is only feasible if processes are processes of generating excess fluid pressures are not capable of significantly increasing destabilizing limited to mid-ocean ridge environments, with at forces, or reducing strength of a decollement. Alter- least one example reported for island-arc volcanics nate mechanisms to describe mechanistic triggers for Ž.Watanabe, 1983 . The potential to overpressurize shallow debris avalanches include consideration of rocks within the core of a volcanic edifice appears fluid pressurization resulting from lava accretion, clear from these observed occurrences, and it is surface saturation by rainfallŽ. Iverson, 1996 or the possible to match response directly through appropri- presence of a soft underlying basementŽ Clague and ate mechanistic models of staticŽ. Sigurdsson, 1982 Denlinger, 1994. Although plausible, the first set of and migratingŽ. Elsworth and Voight, 1992 intru- triggers are rejected by IversonŽ. 1996 as insuffi- sions. ciently strong. The presence of an extensive underly- Paleo-evidence for aquathermal pressurization by ing magmatic plumbing system appears plausible, dikes is present in the exhumed oceanic island although it is appliedŽ. Clague and Denlinger, 1994 Seamount Series in the Barranco de Las Angustias, to the landward-dipping geometry of the Hilina La PalmaŽ. Staudigel, 1997 . These rocks were up- Ž.Kilauea slide, and therefore is classified as kine- lifted above sea level and deeply incised both before matically incapable of developing into an avalanche. and after the growth of overlying subaerial volcanic This mechanism also implies that deformation should rocks, resulting in exposures of pillow lavas cut by continue between eruptions; although this is the case an intense dike swarm. Both intrusive and extrusive in Hawaii, Fogo and Cumbre Vieja show little evi- rocks were affected by contemporaneous hydrother- dence of deformation in inter-eruptive periods, but mal activity. Pipe-like zones of hydrothermal mineral significant deformation in eruptive periods, as dis- Ž.mainly zeolite -cemented breccia along dike mar- D. Elsworth, S.J. DayrJournal of Volcanology and Geothermal Research 94() 1999 323±340 325 Fig. 1.Ž. a Irregular western margin of N±S-trending aphyric dike with intrusive horn structure and irregular breccia zoneŽ extending to right of picture. at contact in centre of field of view. Host rock is older, altered olivine±phyric basaltic dike rock. Thickness of main breccia body ca. 30 cm: pen in centre of field of view is ca. 15 cm long.Ž. b Detail of main breccia zone showing zeolitic veining of host rock and brecciated mass of strongly chilled aphyric basalt dike rock. Note irregular, sharply-defined chilled margin at left side of brecciated rock mass. 326 D. Elsworth, S.J. DayrJournal of Volcanology and Geothermal Research 94() 1999 323±340 gins result, typically at jogs and breached bridges in sures; their potential occurrence and influence are the dikesŽ. Fig. 1a . The breccias cut both the host examined in the following. rocks and the broad, well-defined chilled margins Ž.Fig. 1b of the dikes. The brecciated dike rock 3. Collapse mechanisms contains a finely-divided hyaloclastitic matrix in ad- dition to the hydrothermal mineralization, suggesting Both mechanical and thermal fluid pressurization that the brecciated zones formed soon afterŽ but not mechanisms may be quantified. Mechanical behavior during. dike emplacement. The explosive brecciation is represented by a migrating volumetric dislocation