Geochemistry Geophysics Geosystems
3
G
AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society
Article Volume 4, Number 9 18 September 2003 1077, doi:10.1029/2002GC000483 ISSN: 1525-2027
A new model for submarine volcanic collapse formation Jennifer L. Engels Department of Geology and Geophysics, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, Hawaii 96822, USA (
[email protected])
Margo H. Edwards Hawaii Institute of Geophysics and Planetology, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, 1680 East-West Road, Honolulu, Hawaii 96822, USA (
[email protected])
Daniel J. Fornari Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, USA (
[email protected])
Michael R. Perfit Department of Geological Sciences, University of Florida, Gainesville, Florida 32611, USA (
[email protected])
Johnson R. Cann School of Earth Sciences, University of Leeds, Leeds LS2 9JT, United Kingdom (
[email protected])
[1] Collapse pits and an associated suite of collapse-related features that form in submarine lava flows are ubiquitous on the global mid-ocean ridge crest. Collapse pits, the lava tube systems they expose, and lenses of talus created by the collapse process combine to produce a permeable region in the shallow ocean crust and are thought to contribute significantly to the 100–300 m thick low velocity zone observed at intermediate to fast-spreading mid-ocean ridges. This horizon of low-density, high-porosity material is likely to be an important aquifer for the transfer of hydrothermal fluids in the upper ocean crust. In a recent survey of the East Pacific Rise at 9°370N, we used photographs, video and observations from the submersible Alvin, and DSL-120A side scan data to determine that 13% of the 720,000 m2 of seafloor imaged had foundered to form collapse pits. In 98% of the images collapse pits occurred in lobate flows, and the rest in sheet flows. On the basis of our observations and analyses of collapse features, and incorporating data from previous models for collapse formation plus laboratory and theoretical models of basalt lava behavior in the deep ocean, we develop a detailed multistage physical model for collapse formation in the deep ocean. In our model, lava extruded on the seafloor traps pockets of seawater beneath the flow that are instantly vaporized to a briny steam. The seawater is transformed to vapor at temperatures above 480°C with a 20 times expansion in volume. Bubbles of vapor rise through the lava and concentrate below the chilled upper crust of the lava flow, creating gasfilled cavities at magmatic temperatures. Fluid lava from the cavity roofs drips into the vapor pockets to create delicate drip and septa structures, a process that may be enhanced by water vapor diffusing into the magma and reducing its melting point. As the vapor pocket cools, the pressure within it drops, causing a pressure gradient to develop across the upper crust. The pressure gradient often causes the roof crust to collapse during cooling, though vapor pocket geometry may be such that the roof remains intact during subsidence of the underlying lava. Alternatively, drainaway of the molten lava may cause collapse in locations where inflated lava roof crusts are not supported from below by bounding walls or lava pillars. Post-eruption seismicity, lava movement, or hydrovolcanic explosions may cause continued collapse of the lava carapace after the eruption.
Copyright 2003 by the American Geophysical Union
1 of 22
Geochemistry Geophysics Geosystems
3
G
engels et al.: submarine volcanic collapse
10.1029/2002GC000483
Components: 12,414 words, 6 figures. Keywords: Collapse; vapor; mid-ocean ridge; volcanism. Index Terms: 3035 Marine Geology and Geophysics: Midocean ridge processes; 3045 Marine Geology and Geophysics: Seafloor morphology and bottom photography; 3015 Marine Geology and Geophysics: Heat flow (benthic) and hydrothermal processes. Received 28 November 2002; Revised 19 May 2003; Accepted 1 July 2003; Published 18 September 2003. Engels, J. L., M. H. Edwards, D. J. Fornari, M. R. Perfit, and J. R. Cann, A new model for submarine volcanic collapse formation, Geochem. Geophys. Geosyst., 4(9), 1077, doi:10.1029/2002GC000483, 2003.
1. Introduction [2] The term ‘‘submarine collapse features’’ refers to a suite of volcanic structures that are related to the inflation and subsequent draining of fluid lavas erupted at the seafloor (Figure 1). Collapse features include pits within the volcanic carapace or crust (Figure 1), and associated features such as: cylindrical lava pillars, overhanging lava roofs, piles of talus from foundered roof material, and delicate lava drips or septa located on the undersides of roofs and attached to lava pillars (Figure 2). The dimensions of lava pits range from a few centimeters deep and 10 cm thick. (g) 6 m 4 m. Plan view of multistory collapse. Collapse surfaces are separated by 0.5 m. Total collapse depth 3 m. (h) 6 m 4 m. Plan view of lava pond collapse showing two remnant lava pillars capped by lobate crusts. Depth of collapse 3 m.
as a reservoir of hydrothermal fluids in both axial and off-axis zones. [4] In this study we develop the first detailed physical model of the formation of the whole range of collapse features observed on the EPR,
including pits, pillars, roofs, talus, and drips. Many other authors have presented qualitative models for the formation of collapse features at the large end of the collapse spectrum [e.g., Ballard et al., 1979; Francheteau et al., 1979; Gregg and Chadwick, 1996; Chadwick et al., 3 of 22
Geochemistry Geophysics Geosystems
3
G
engels et al.: submarine volcanic collapse
10.1029/2002GC000483
Figure 2. Photographs showing physical features on the interior of lava roof crusts that may be evidence for the interaction of lava with a vapor phase. (a) Interior of a lobate roof crust (sample 2357-1). Sample is 2 cm thick with a microvesicular surface texture and has cuspate and drip structures up to 2 cm in length. (b) 8 cm thick collapse roof (2497-6) showing drip structures on the interior surface up to 3 cm in length. Note the massive interior, cooling fractures, and vesicular zone below the drips. (c) 10 cm 7 cm. Fragment of top of a lava pillar (sample 2354-V) with >3 cm cuspate structures that appear to be the walls of lava bubbles. The interior surfaces have a dull residue indicative of interaction with a high temperature vapor [Perfit et al., 2003]. (d) 20 cm 15 cm. Jumbled sheet flow (sample 2759-7) containing a 3 cm diameter elongate vesicle of unknown orientation, and a water-filled bubble.
1999]. Some authors have numerically addressed the related issues of submarine lava flow inflation [e.g., Gregg and Chadwick, 1996; Chadwick et al., 1999; Gregg et al., 2000; Gregg and Fink, 2000; Fox et al., 2001; Chadwick, 2003], formation of pillars [Gregg and Chadwick, 1996; Gregg et al., 2000], formation of lineated sheet flows in collapse pits [Chadwick et al., 1999], and formation of deep-sea limu o pele (bubble wall fragments) [Clague et al., 2000; Maicher and White, 2001; Clague and Davis, 2002]. However, no one model conceptually incorporates all time-sequenced elements of the growth and foundering of a lava roof, and the resultant pillars, drips, and talus that provide insight into the specific eruption that created them. Using elements of the models
described above, analog examples of Hawaiian collapse, laboratory and theoretical modeling and observations of lava flow behavior in the deep ocean, and the results of an Alvin and towed camera investigation of the neovolcanic zone of the EPR at 9°370N in 2000, we describe in detail each stage in the genesis of collapse features. We have quantified our conceptual model by estimating the cooling behavior of lava, the rise rate of gaseous bubbles, and the density contrasts between lava and gaseous seawater. Central to our hypotheses about collapse formation is the presence of vaporized seawater that becomes trapped between the crust of a lava roof and the molten lava interior of a flow [see also Perfit et al., 2003]. We demonstrate that it would not be possible to form 4 of 22
Geochemistry Geophysics Geosystems
3
G
engels et al.: submarine volcanic collapse
the characteristic suite of observed collapsed features without this vapor phase. The specific purpose of our paper is to lay down the first conceptual framework for the origin of collapse features and their implications for a whole range of submarine lava flow processes.
2. Background [5] Submarine volcanic collapse has been documented on the slow-spreading Mid-Atlantic Ridge (MAR) [Scheirer et al., 2000], the intermediatespreading Juan de Fuca Ridge (JdFR) and Galapagos Rift [e.g., Ballard et al., 1979; Embley and Chadwick, 1994], the fast-spreading EPR [Haymon et al., 1991; Fornari et al., 1998a], the superfastspreading southern EPR (SEPR) [Sinton et al., 2000; White et al., 2000], on seamounts [Scheirer et al., 2000; Fornari et al., 1988], and in shallow water (
