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| Titel |
Strengths and strain energies of volcanic edifices: implications for eruptions, collapse calderas, and landslides |
| VerfasserIn |
A. Gudmundsson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1561-8633
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| Digitales Dokument |
URL |
| Erschienen |
In: Natural Hazards and Earth System Science ; 12, no. 7 ; Nr. 12, no. 7 (2012-07-19), S.2241-2258 |
| Datensatznummer |
250010990
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| Publikation (Nr.) |
 copernicus.org/nhess-12-2241-2012.pdf |
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| Zusammenfassung |
| Natural hazards associated with volcanic edifices depend partly on how
fracture resistant the edifices are, i.e. on their strengths.
Observations worldwide indicate that large fluid-driven extension fractures
(dikes, inclined sheets), shear fractures (landslides), and mixed-mode fractures (ring dikes and ring faults) normally propagate more easily in a
basaltic edifice (shield volcano) than in a stratovolcano. For example,
dike-fed eruptions occur once every few years in many basaltic edifices but
once every 102-3 yr in many stratovolcanoes. Large landslides and
caldera collapses also appear to be more common in a typical basaltic
edifice/shield volcano than in a typical stratovolcano. In contrast to a
basaltic edifice, a stratovolcano is composed of mechanically dissimilar
rock layers, i.e. layers with mismatching elastic properties (primarily
Young's modulus). Elastic mismatch encourages fracture deflection and arrest
at contacts and increases the amount of energy needed for a large-scale
edifice failure. Fracture-related hazards depend on the potential energy
available to propagate the fractures which, in turn, depends on the boundary
conditions during fracture propagation. Here there are two possible
scenarios: one in which the outer boundary of the volcanic edifice or rift
zone does not move during the fracture propagation (constant displacement);
the other in which the boundary moves (constant load). In the former, the
total potential energy is the strain energy stored in the volcano before
fracture formation; in the latter, the total potential energy is the strain
energy plus the work done by the forces moving the boundary.
Constant-displacement boundary conditions favor small eruptions, landslides,
and caldera collapses, whereas constant-load conditions favor comparatively
large eruptions, landslides, and collapses. For a typical magma chamber
(sill-like with a diameter of 8 km), the strain energy change due to
magma-chamber inflation is estimated at the order of 1014 J (0.1 PJ).
For comparison, the surface energy needed to form a typical feeder dike is
of the same order of magnitude, or 1014 J. There are several processes
besides magma-chamber inflation that may increase the strain energy in a
volcano before eruption. Thus, during a typical unrest period with
magma-chamber inflation, the added strain energy in the volcano is large
enough for a typical feeder dike to form. An injected dike, however, only
reaches the surface and becomes a feeder if it is able to propagate
through the numerous layers and contacts that tend to deflect or arrest
dikes. The strong elastic mismatch between layers that constitute
stratovolcanoes not only encourages fracture arrest, but also the storage of
more strain energy (than in a typical basaltic edifice/shield volcano)
before fracture formation and failure. It is thus through producing
materials of widely different mechanical properties that stratovolcanoes
become strong and resilient. |
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