Analogue models of the effect of long-term basement fault movement on volcanic edifices

Long-term fault movement under volcanoes can control the edifice structure and can generate collapse events. To study faulting effects, we explore a wide range of fault geometries and motions, from normal, through vertical to reverse and dip-slip to strike-slip, using simple analogue models. We explore the effect of cumulative sub-volcanic fault motions and find that there is a strong influence on the structural evolution and potential instability of volcanoes. The variety of fault types and geometries are tested with realistically scaled displacements, demonstrating a general tendency to produce regions of instability parallel to fault strike, whatever the fault motion. Where there is oblique-slip faulting, the instability is always on the downthrown side and usually in the volcano flank sector facing the strike-slip sense of motion. Different positions of the fault beneath the volcano change the location, type and magnitude of the instability produced. For example, the further the fault is from the central axis, the larger the destabilised sector. Also, with greater fault offset from the central axis larger unstable volumes are generated. Such failures are normal to fault strike. Using simple geometric dimensionless numbers, such as the fault dip, degree of oblique motion (angle of obliquity), and the fault position, we graphically display the geometry of structures produced. The models are applied to volcanoes with known underlying faults, and we demonstrate the importance of these faults in determining volcanic structures and slope instability. Using the knowledge of fault patterns gained from these experiments, geological mapping on volcanoes can locate fault influence and unstable zones, and hence monitoring of unstable flanks could be carried out to determine the actual response to faulting in specific cases

Sedimentology of Volcanic Debris Avalanche Deposits

The deposits of volcanic debris avalanches (VDAs) contain diagnostic features that distinguish them from those of other landslides. In this chapter, we summarize the sedimentary characteristics and the different (litho-)facies described over the past four decades, and how findings from individual case studies can be adapted as globally applicable sedimentological tools. A plethora of descriptive terms and partially conflicting definitions emerged in the ever-growing literature on VDA deposits (VDADs). These we summarize and make recommendations for future use. Different facies models that were developed at different volcanoes might point to unique emplacement conditions (e.g. dry versus wet; confined versus unconfined) and, if confirmed, the apparent ‘conflict' of terminology might help identify the paleo-settings of ancient VDAs. General observations of large unsaturated landslides of different origin show that preservation of source stratigraphy, (mega-)clasts, jigsaw-fractured clasts, and incorporation of runout path material are common features. Their unique composition, grain sizes, and abundance of matrix sets VDADs apart from deposits of large rockslides and debris flows. The latter can be associated with VDAs, and whether they formed syn- or post-VDAD emplacement is reflected in forensic evidence within the depositional sequences. Recent case studies illustrate the advances in analytical techniques and in understanding the processes of debris avalanche transport and deposition forty years after the eruption and lateral collapse of Mount St. Helens volcano

A Historical Perspective on Lateral Collapse and Volcanic Debris Avalanches

In the four decades since the 1980 eruption of Mount St. Helens, debris-avalanche deposits generated by gravitational lateral collapse of volcanoes have become widely recognized. Selected regionally sequenced case studies highlight the evolution of thought regarding these events prior to 1980 in contrast to subsequent research with benefit of insights from the events of May 18, 1980. These typically hummocky deposits, of volcanic materials but lying far beyond volcanoes, had puzzled geologists for more than a century and been interpreted as a wide range of primary and secondary volcanic or non-volcanic features. Contrary to general perception, however, the volcanological literature contained multiple accounts prior to 1980 that recognized the landslide origin of some of these deposits, albeit mostly in regional publications not widely known. The burst of interest in lateral-collapse events after 1980 has led to an average of one regional or global debris-avalanche inventory annually in terrestrial or submarine settings and the recognition of a thousand events from nearly 600 volcanoes. The last major volcaniclastic process to be widely recognized and understood, large-volume debris avalanches originating from lateral collapse of volcanic edifices have been found to be a relatively common occurrence across a wide spectrum of volcanic features and settings