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Geohazards Explained 2 GEOHAZARDS Geohazards explained 2 Collapsing volcanoes: the sleeping giants’ threat Dormant volcanoes are typically thought of as relatively safe environments to live around. However, under certain geological conditions, such volcanoes can be prone to collapse and may generate massive rockslide avalanches reaching tens of kilometres from the volcano. On Earth and other planets, volcanoes are extremely emitted gas volumes and compositions at the sur- Thomas Shea1 & dynamic systems within fairly short geological times- face, abrupt transitions in underground seismicity or cales. Erupted lava and pyroclastic debris build up ground deformation accompanying the rise of magma Benjamin van Wyk volcanic landforms, and the threat the associated towards the surface. Unfortunately, when flank col- de Vries2 processes cause to surrounding populations are one lapses are not associated with eruptions, precursory 1Department of Geology of the central focuses of volcanology. Through ob- indications are mostly absent, and hence pose a big- and Geophysics, University servation of eruptions at the surface, measurements ger threat to unprepared human populations living of Hawaii, USA of gas and seismicity, or through the study of the around volcanoes. This threat becomes more and [email protected] products they generate, we have made substantial more of a reality as field scientists are finding that 2Laboratoire Magmas et progress towards understanding and reducing haz- almost every volcano on Earth has undergone one Volcans, Université de ards associated with volcanoes. Less well documented or more flank failures. To better understand and find Clermont-Ferrand II, France are destructive events triggered by volcanic flank col- ways to anticipate flank collapses, there is a growing [email protected] lapses that produce massive and extremely destruc- need to investigate the processes that lead to me- bpclermont.fr tive rockslide avalanches. Rockslide avalanches are chanical failure, and the differences these will make huge masses of rock sliding down at velocities exceed- in terms of avalanche size and transport capacities. ing 250 km/h, often travelling tens of kilometres and Volcán Mombacho, in Nicaragua, is a perfect field covering areas of up to hundreds of square kilome- laboratory to pursue these investigations. tres. The devastation they may cause is particularly significant when considering their capacity to trans- form into secondary lahars (i.e. volcanic mudflows) The puzzling transport properties of rockslide provided that sufficient water is present within the avalanches moving rockmass, or their ability to enter lakes or Large-scale mass movements involving rock material oceans and produce large-scale tsunamis. Mount St possess several intriguing properties, the most peculiar Helens erupted in 1980 and gave a glimpse of how of all probably being the distance travelled horizon- huge flank collapses can be: the summit elevation tally in comparison with the height from which the was reduced by about 500 m and the volcanic edifice rockmass fell. Under Coulomb friction laws, granular lost approximately one third of its total volume. More materials should stop moving horizontally at about recently, a much smaller portion of Volcán Casita, in 1.6 times the collapse height. Remarkably, collapses Nicaragua, failed as a consequence of heavy rainfalls of over 1 km3 will typically reach distances 10–15 associated with the passing of Hurricane Mitch in times larger than their fall height. Possible explana- 1998; however, because such large amounts of water tions for this include the presence of air trapped under were involved in the collapse, the rockslide quickly the bulk of the mass, basal fluidization or lubrication turned into a mudflow that almost instantly took the by air or water, energy released by progressive mate- lives of 2500 Nicaraguans. rial fragmentation, or vibrational energy produced In terms of hazard mitigation, volcanic eruptions by solid particle interactions. To date, however, no often grant windows of opportunity prior to their cli- model is widely received and as a result, the trans- max through precursory signals such as changes in port properties of such rockslides are still poorly un- 72 © Blackwell Publishing Ltd, The Geologists’ Association & The Geological Society of London, Geology Today, Vol. 26, No. 2, March–April 2010 GEOHAZARDS derstood. In order to derive a robust emplacement model, a certain amount of information must be known on the kinematics and dynamics of transport. Fig. 1. Satellite images of small Fortunately for us, almost every well-preserved rock- areas of the Andean Tata Sabaya slide avalanche deposit possesses a detailed record of (left) and Socompa (right) the deformation suffered during transport. While the rockslide avalanche deposits. base of rockslide avalanches is stipulated to be the Note the contrasting surface agent reducing frictions during transport, the bulk morphologies: hummocky (left) of the mass behaves as granular material in the fric- and ridged (right). tional regime; as a result, deformation is preserved in variations: dominantly extensional (normal faults the form of surface faults that extend into the inte- cover most of the surface), dominantly compressive rior of the deposit. Recent studies have shown that (thrust faults cover most of the surface), hummocky fault structures can truly be considered fingerprints, (materials are very heterogeneous) and ridged (ho- unique to each avalanche, which, when deciphered, mogenous materials). These laboratory analogues yield invaluable information about the type of de- may provide key information on how transport and formation suffered (extension/compression), its loca- emplacement occur in nature. Such structures seem tion in space (rockslide front/back, centre or sides), to only form under certain conditions: the base of the and in time (chronology of fault formation). Figure 1 rockslide has to experience low-friction (lubrication or shows two deposits displaying the two main surface fluidization) while the bulk of the mass forms a brittle, morphologies encountered on rockslide avalanches: deforming carapace, much like in a plug-flow model. one is full of individual hummocks with circular to The following sections offer superb examples of two elliptical bases (Tata Sabaya, Bolivia), and the other is such rockslide avalanches in a single volcanic envi- Fig. 2. Laboratory analogue covered by ridges that are fault structures (Socompa, ronment, and illustrate how field observations can be models of rockslide avalanche deposits. Hummocky Chili/Argentina). Within the interior of most ava- directly linked to initiation, transport and emplace- topographies (left) are lanches, three types of fault structures are detected: ment properties. normal (extension), thrust (compression) and strike- reproduced using layered materials with different slip (transform) faults. Simple laboratory analogue cohesions while ridged experiments involving the slide of stratified sand on a topographies (right) are best low friction ramp are able to physically replicate the replicated using sands with two structural types (hummocky vs. ridged) as well homogeneous size and cohesion. as the distribution of surface features. Much like in Note the lobate deposit shapes nature, the bulk of the sand mass deforms in the fric- in both, and the complex surface tional regime and responds to deformation by fault- structure relationships on the ing. When the material is homogenous in size and ridged deposit. mostly non-cohesive, analogue rockslides are always ridged, and when more cohesive materials (plaster or humid sand) are sandwiched in between non-cohe- Mombacho: a collapsing volcano sive sand, hummocks dominate the surface (Fig. 2). Volcan Mombacho, Nicaragua, forms part of Central Because these experiments are filmed, details on the American volcanic front (CAVF, Fig. 3), a volcanic arc chronology of deformation (i.e. kinematics) provide created by subduction of the Cocos Plate under the the missing link between deposits seen in nature and Carribean Plate. It is a fairly typical basaltic–andesitic their transport mode: extension dominates the first stratovolcano composed of alternating lava flows and stages of collapse and normal faults affect the whole tephra layers. The early Mombacho was most likely mass; then, as the slide travels further and reaches built prior to a huge ignimbritic eruption 23 000 gentler slopes, compression is active at the avalanche years ago involving a large volume of pumice that front and thrust structures develop. From initiation to mantled surrounding topographies creating deposits emplacement, systems of strike-slip faults observed on several metres thick (Fig. 4), and during which emp- both sides serve to accommodate differences in veloc- tying of the magma chamber resulted in the collapse ity within the moving mass. These various structures and formation of the nearby Apoyo Caldera (~5 km are all observed in nature at the same locations (thrust wide, see Fig. 3 for location). Hence, most of the cur- structures are more typically at the front and normal rent Mombacho edifice was constructed subsequently, faults at the back) although their respective propor- and the presence of a soft, easily compressible pumice tions may vary. Experimentally, variations in normal layer underneath substantially affected its structure versus thrust fault densities have been obtained by as it was being built. Indeed, as magma is erupted/in- simply changing
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