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Bulletin of Volcanology

, Volume 67, Issue 8, pp 768–782 | Cite as

On the conditions of sheet injections and eruptions in stratovolcanoes

  • Agust GudmundssonEmail author
  • Sonja L. Brenner
Research Article

Abstract

Nearly all eruptions in stratovolcanoes (composite volcanoes, central volcanoes) are supplied with magma through fractures. Consequently, a primary physical condition for an eruption to occur in a stratovolcano is that a magma-driven fracture is able to propagate to the surface. Magma-filled fractures, frozen or fluid, are referred to as sheet intrusions. More specifically, they are named dykes when subvertical, and inclined (or cone) sheets when inclined. Field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths. For this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions. Whether a sheet intrusion becomes arrested or, alternatively, propagates to the surface depends primarily on the stress field in the stratovolcano. A stratovolcano normally consists of layers of contrasting mechanical properties, such as soft (low Young’s modulus) pyroclastic units and stiff (high Young’s modulus) lava flows. We present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest. The models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised. We propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress-field homogenisation. Field studies show that the sheet intrusions in individual stratovolcanoes have various dips: some are vertical dykes, others inclined sheets, and still others horizontal sills. Analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption. This effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high. We conclude that the shallower the dip of a sheet intrusion, the less will be its volumetric magma transport to the surface of a stratovolcano.

Keywords

Volcanic eruption Sheet arrest Crustal layering Crustal stress Magma transport Stratovolcano 

Notes

Acknowledgements

This work was supported through a grant from the European Commission (EVG1-CT-2002-00073 PREPARED), several grants from the Norway Research Council, as well as a PhD grant from Statoil (to A. Gudmundsson) for S. L. Brenner. We thank the Bulletin Volcanology reviewers for helpful comments

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  1. 1.Department of Structural Geology and GeodynamicsGeoscience Centre, University of GöttingenGöttingenGermany

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