Raquel Herrera

Craters of elevation revisited: forced-folds, bulging and uplift of volcanoes

The eighteenth/nineteenth century ‘craters of elevation’ theory required magma to uplift strata, doming the surface and creating a central down-fallen ‘crater’ or graben. Exponents of craters of elevation attempted to apply it to explain the origin of all volcanoes, and rapidly the contemporary competing ‘craters of eruption’ theory replaced it as the paradigm for volcano construction. Several historic examples have shown that intrusions can cause uplift, termed bulges and can create features like those proposed for craters of elevation (e.g. at Usu 1944, Bezymianny 1955 and Mt. St. Helens 1980). Work on sedimentary basins that have had igneous activity has shown that intrusions create ‘forced folds’ that uplift and deform strata in a similar way to that originally proposed for craters of elevation. In view of the above, we investigate large-scale intrusion-related topographic changes at two sites where the craters of elevation theory was developed: the monogenetic volcanoes of the Chaîne des Puys, France and the Teide stratovolcano, Tenerife. We combine observations of such features with examples of forced folding to integrate the two fields of research. Our observations in the Chaîne des Puys show that: (1) the Petit Puy de Dôme has a bulge of up to 150-m uplift. The uplift has a central depressed area (a graben), a dense network of normal faults, basal thrusts and an aborted landslide. (2) The Grosmanaux volcano is a forced fold created by uplift of a previously flat-lying area, and has dense faulting and a graben on the resultant topographic bulge. It was the site also of a major vulcanian eruption from the associated Kilian crater. (3) The Gouttes volcano was uplifted by an intrusion like the Petit Puy de Dôme, but then collapsed to generate a landslide and lateral blast. (4) Excavation in the Lemptégy Volcano exposes intra-eruption intrusions with associated uplift, providing examples in cross-section of the internal deformation likely to be found inside other Chaîne des Puys uplifted bulges. On Teide, a bulge near the summit shows similar structures and surface tilting as seen on the Petit Puy de Dôme and this bulging may have formed during the eruption of the Lavas Negras, the most recent activity on the summit area. Fault scarps on Teide also expose small cryptodomes, like those seen at Lemptégy. These examples, coupled with field studies on eroded intrusions, data on forced folds in basins and analogue models, show how large-scale topographic remodelling and structural change can be created by intrusions. These can rapidly and significantly change the volcanic edifice. A crater of elevation bulge, or forced fold that is stabilised by the cooling of the intrusion, will remain an important structural element in a volcano. This process starts even at the small scale of monogenetic volcanoes, and could occur through the lifetime of any growing stratovolcano. Such activity may be commonplace, but may be masked by concomitant eruption or removed by subsequent collapse. Monitoring and hazard strategies should be ready to deal with such large-scale events that will seriously modify the eruptive activity and stability of a volcano within days or weeks.

Contrasting catastrophic eruptions predicted by different intrusion and collapse scenarios

Catastrophic volcanic eruptions triggered by landslide collapses can jet upwards or blast sideways. Magma intrusion is related to both landslide-triggered eruptive scenarios (lateral or vertical), but it is not clear how such different responses are produced, nor if any precursor can be used for forecasting them. We approach this problem with physical analogue modelling enhanced with X-ray Multiple Detector Computed Tomography scanning, used to track evolution of internal intrusion, and its related faulting and surface deformation. We find that intrusions produce three different volcano deformation patterns, one of them involving asymmetric intrusion and deformation, with the early development of a listric slump fault producing pronounced slippage of one sector. This previously undescribed early deep potential slip surface provides a unified explanation for the two different eruptive scenarios (lateral vs. vertical). Lateral blast only occurs in flank collapse when the intrusion has risen into the sliding block. Otherwise, vertical rather than lateral expansion of magma is promoted by summit dilatation and flank buttressing. The distinctive surface deformation evolution detected opens the possibility to forecast the possible eruptive scenarios: laterally directed blast should only be expected when surface deformation begins to develop oblique to the first major fault.