Towards a new model of tumuli growth: Incorporating bending models and observations of active lava flows
Abstract
Tumuli are the morphologic expression of pressure concentrating within an inflating lava flow. An existing model of tumuli growth (Rossi and Gudmundsson, 1996), suggests that approximately 40m of magmastic overpressure is needed to bend the surface crust of an active lava flow into the characteristic whale-back shape of a tumulus. This model assumes a small-deflection bending of a broken, rigid crust overlying a stronger viscoelastic layer, and uses reasonable values of tumuli dimensions and crustal thickness as boundary conditions. We measured the dimensions and crustal thicknesses of more than 100 tumuli on flows at Mount Etna and Kilauea volcanoes, and used the model to generate pressure estimates for each tumuli in an attempt to discover the nature and magnitude of pressure variations with active lava flow interiors. Although the model gives reasonable values of magmastic pressure for many of our measured tumuli, some values were unreasonably high (greater than 10m magmastic pressure) or low (less than 0.1m magmastic pressure). For those tumuli that have unreasonably low pressure estimates with the existing model, we find that more reasonable values are calculated if we consider whether the edges of the tumuli are clamped. Clamping requires greater magmastic overpressure to bend and break the edges than predicted by the current model. We also find that shape affects the pressure estimates. Tumuli are typically elliptical in plan view, and require greater pressure for bending than for circular features. For those tumuli that yield unreasonably large values of magmastic overpressure, a large-deflection bending model yields more reasonable values. Also, allowing the brittle crust to contribute to the strength of the bending layer improves the calculated values. We also incorporate observations of actively growing tumuli, and find that some are as wide as the entire flow lobe. Many also form at near constrictions in the flow lobe. Cracking of the flow surface may occur very early in the growth of tumuli, and thus their final height reflects mostly lifting, rather than bending. In these cases, bending is only considered during the initial stages of crack formation, with lifting the dominant process responsible for tumulus height.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.V53C1759A
- Keywords:
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- 8414 Eruption mechanisms and flow emplacement;
- 8429 Lava rheology and morphology;
- 8486 Field relationships (1090;
- 3690);
- 8499 General or miscellaneous