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Persistent activity at Masaya Volcano, Nicaragua, is characterised by cycles of intense degassing, lava lake development and pit crater formation. It provides a useful site to study the processes which govern such activity, because of its easy accessibility and relatively short cycles (years to decades). An understanding of the present activity is important because Masaya is visited by large numbers of tourists, is located close to major cities and has produced voluminous lavas, plinian eruptions and ignimbrites in the recent past. We provide structural and geophysical data that characterise the "normal" present state of activity. These indicate that the ongoing degassing phase (1993 to present) was not caused by fresh magma intrusion. It was associated with shallow density changes within the active Santiago pit crater. The activity appears to be associated predominantly with shallow changes in the pit crater structure. More hazardous activity will occur only if there are significant departures from the present gravity, deformation and seismic signatures.

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International audienceGravitational deformation strongly infl uences the structure and eruptive behavior of large volcanoes. Using scaled analog models, we characterize a range of structural architectures produced by volcano sagging and volcano spreading. These arise from the interplay of variable basement rigidity and volcano-basement (de-)coupling. From comparison to volcanoes on Earth (La Réunion and Hawaii) and Mars (Elysium and Olympus Montes), the models highlight a structural continuum in which large volcanoes throughout the Solar System lie

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Socompa Volcano arguably provides the world's best-exposed example of a sector collapse-derived debris avalanche deposit. New observations lead us to re-interpret the origin of the sector collapse. We show that it was triggered by failure of active thrust-anticlines in sediments and ignimbrites underlying the volcano. The thrust-anticlines were a result of gravitational spreading of substrata under the volcano load. About 80% of the resulting avalanche deposit is composed of substrata formerly residing under the volcano and in the anticlines. The collapse scar can be traced up to 5 km from the edi®ce, truncating two spreading-related anticlines, which collapsed in the event. Outcrops near the volcano preserve evidence of edi®ce material being carried along on top of mobilised substrata. On the north side of the scar, the avalanche motion was initially at right angles to the failure edge. Structural relations indicate that immediately prior to collapse the substrata disintegrated, became effectively liquidised, and were ejected from beneath the edi®ce. Catastrophic mobilisation of substrata probably resulted from breakdown of ignimbrite clasts and cement. It may have occurred through progressive rock fracture by high shear strain during spreading. Material ejected from under Socompa formed a layer on which volcanic edi®ce debris was transported. This interpretation of events explains the puzzling observation that avalanche units with the lowest gravitational potential energy moved the furthest. It can also account for avalanche motion normal to the collapse scar walls. Ignimbrites and other rock types probably capable of similar behaviour underlie many other volcanoes. Identi®cation of spreading at other sites could therefore be a ®rst step towards assessment of the potential for this style of catastrophic sector collapse. q

We present a global database on the subaerial morphometry of composite volcanoes. Data was extracted from the 90-m resolution Shuttle Radar Topography Mission (SRTM) digital elevation model (DEM). The 759 volcanoes included in the database are the composite (i.e., polygenetic) volcanoes listed in the Smithsonian Institution Global Volcanism Program (GVP) database that are covered by the SRTM DEM, have a constructional topography and a basal width larger than 2 km. The extent of each volcano edifice was defined using the NETVOLC algorithm, which computes outlines by minimizing a cost function based on breaks in slope around the edifices. Morphometric parameters were then calculated using the MORVOLC algorithm. The parameters characterize and quantify volcano size (basal width, summit width, height, and volume), profile shape (height/basal width and summit width/basal width ratios), plan shape (ellipticity and irregularity indexes), and slopes. In addition, 104 welldefined and relatively large summit craters/calderas were manually delineated and specific parameters were computed. Most parameters show large variation without clear separations, indicating a continuum of volcano morphologies. Large overlap between the main GVP morphologic types highlights the need for a more rigorous quantitative classification of volcano morphology. The database will be maintained and updated through a website under construction.

International audienc