Hydrothermal calderas |
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| Authors: | Olivier Merle Stéphanie Barde-Cabusson Benjamin van Wyk de Vries |
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| Institution: | 1. Laboratoire Magmas et Volcans, CNRS-IRD-Université Blaise Pascal, 5 rue Kessler, 63 038, Clermont-Ferrand, France
2. Dipartimento di Scienze della Terra, Università di Firenze, Via La Pira 4, 50121, Florence, Italy
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| Abstract: | The standard model of caldera formation is related to the emptying of a magma chamber and ensuing roof collapse during large
eruptions or subsurface withdrawal. Although this model works well for numerous volcanoes, it is inappropriate for many basaltic
volcanoes (with the notable exception of Hawaii), as these have eruptions that involve volumes of magma that are small compared
to the collapse. Many arc volcanoes also have similar oversized depressions, such as Poas (Costa Rica) and Aoba (Vanuatu).
In this article, we propose an alternative caldera model based on deep hydrothermal alteration of volcanic rocks in the central
part of the edifice. Under certain conditions, the clay-rich altered and pressurized core may flow under its own weight, spread
laterally, and trigger very large caldera-like collapse. Several specific mechanisms can generate the formation of such hydrothermal
calderas. Among them, we identify two principal modes: mode 1: ripening with summit loading and flank spreading and mode II:
unbuttressing with flank subsidence and flank sliding. Processes such as summit loading or flank subsidence may act simultaneously
in hybrid mechanisms. Natural examples are shown to illustrate the different modes of formation. For ripening, we give Aoba
(Vanuatu) as an example of probable summit loading, while Casita (Nicaragua) is the type example of flank spreading. For unbuttressing,
Nuku Hiva Island (Marquesas) is our example for flank subsidence and Piton de la Fournaise (La Réunion) is our example of
flank sliding. The whole process is slow and probably needs (a) at least a few tens of thousands of years to deeply alter
the edifice and reach conditions suitable for ductile flow and (b) a few hundred years to achieve the caldera collapse. The
size and the shape of the caldera strictly mimic that of the underlying weak core. Thus, the size of the caldera is not controlled
by the dimensions of the underlying magma reservoir. A collapsing hydrothermal caldera could generate significant phreatic
activity and trigger major eruptions from a coexisting magmatic complex. As the buildup to collapse is slow, such caldera-forming
events could be detected long before their onset. |
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