A model for submarine rhyolite dome growth: Ponza island (central Italy)

Citation
D. Derita et al., A model for submarine rhyolite dome growth: Ponza island (central Italy), J VOLCANOL, 107(4), 2001, pp. 221-239
Citations number
49
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Earth Sciences
Journal title
JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH
ISSN journal
0377-0273 → ACNP
Volume
107
Issue
4
Year of publication
2001
Pages
221 - 239
Database
ISI
SICI code
0377-0273(200107)107:4<221:AMFSRD>2.0.ZU;2-Y
Abstract
The Late Pliocene rhyolitic submarine volcanic rocks of Ponza island (Italy ) can be interpreted as the subaqueous equivalent of subaerial dome complex es in terms of geometry and structure. Three coalescing domes of about 1 km radius and aligned along a NE-trending regional fracture have been identif ied. The main difference between subaqueous and subaerial lava domes is tha t in a subaqueous environment, lava is likely to undergo pervasive hyalocla stic brecciation, so that domes are mainly composed of variously brecciated , in situ to clast-rotated hyaloclastite rather than coherent lava. We sugg est that the shape and rheologic behaviour through time of submarine domes are closely controlled by the development and thickness of the particulate hyaloclastic carapace, which assumes the role of the solid crust of domes i n subaerial environment. The thickness of the hyaloclastic carapace at Ponz a is greater than 150 m and emplaced during several different pulses (or er uptions). In the earliest pulses, lava is directly extruded on the seafloor and produces hyaloclastite, the degree of brecciation of which decreases i nward to the coherent flow-banded rhyolite lava of the feeder dike. Once th e hyaloclastic carapace is formed, further pulses of magma, combined with i ncrease in height of the dome result in a local stress pattern characterise d by a vertical sigma (1) much greater than sigma (2) = sigma (3), producin g concentric and radial fractures and normal faults. The newly rising magma , shielded by the hyaloclastic carapace, can intrude along these fault and fracture systems and invade previously emplaced but still water-saturated h yaloclastite. This produces the characteristic pattern of dikes observed at Ponza as a series of concentric dikes that are progressively less inclined outward with respect to the dome centre. These late stage dikes in turn pr oduce hyaloclastite at their margins, but generally less fragmented than th e embedding hyaloclastite, probably because the ascending magma is better s hielded from direct contact with sea water. Periodic gravity collapses of t he dome maintain the equilibrium between height and radius of the dome, as suggested at Ponza by the presence of mass flow deposits and the developmen t of small topographic basins on top of the domes, probably related to debr is removal. Some of these topographic depressions host subaqueous pyroclast ic deposits, suggesting that, if due to vertical growth of the dome, water depth above the dome becomes shallower, hydromagmatic explosions may occur. (C) 2001 Elsevier Science B.V. All rights reserved.