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| Titel |
Basaltic feeder dykes in rift zones: geometry, emplacement, and effusion rates |
| VerfasserIn |
I. Galindo, 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. 12 ; Nr. 12, no. 12 (2012-12-18), S.3683-3700 |
| Datensatznummer |
250011259
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| Publikation (Nr.) |
 copernicus.org/nhess-12-3683-2012.pdf |
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| Zusammenfassung |
| Most volcanic hazards depend on an injected dyke reaching
the surface to form a feeder. Assessing the volcanic hazard in an area is
thus related to understanding the condition for the formation of a feeder
dyke in that area. For this latter, we need good field data on feeder dykes,
their geometries, internal structures, and other characteristics that
distinguish them from non-feeders. Unfortunately, feeder dykes are rarely
observed, partly because they are commonly covered by their own products. For
this reason, outcrops are scarce and usually restricted to cliffs, ravines,
and man-made outcrops. Here we report the results of a study of feeder dykes
in Tenerife (Canary Islands, Spain) and Iceland, focusing on their field
characteristics and how their propagation is affected by existing
structures. Although Holocene fissure eruptions have been common in both
islands, only eleven basaltic feeder dykes have been identified: eight in
Tenerife and three in Iceland. They are all well preserved and the relation
with the eruptive fissure and/or the deposits is well exposed. While the
eruptive fissures are generally longer in Iceland than in Tenerife, their
feeders show many similarities, the main ones being that the feeder dykes
(1) are generally sheet-shaped; (2) are segmented (as are the associated
volcanic fissures); (3) normally contain elongated (prolate ellipsoidal)
cavities in their central, topmost parts, that is, 2–3 m below the surface
(with solidified magma drops on the cavity walls); (4) contain vesicles
which increase in size and number close to the surface; (5) sometimes inject
oblique dyke fingers into the planes of existing faults that cross the dyke
paths; and (6) may reactivate, that is, trigger slip on existing faults. We
analyse theoretically the feeder dyke of the 1991 Hekla eruption in Iceland.
Our results indicate that during the initial peak in the effusion rate the
opening (aperture) of the feeder dyke was as wide as 0.77 m, but quickly
decreased to about 0.56 m. During the subsequent decline in the effusion
rate to a minimum, the aperture decreased to about 0.19 m. At a later abrupt
increase in the effusion rate, the feeder-dyke opening may have increased to
about 0.34 m, and then decreased again as the effusion rate gradually
declined during the end stages of the eruption. These thickness estimates
fit well with those of many feeders in Iceland and Tenerife, and with the
general dyke thickness within fossil central volcanoes in Iceland. |
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