Rock Fracture Project Workshop

FROM DIKE TO BUD TO PLUG: CRITICAL PHYSICAL TRANSITIONS IN THE EVOLUTION OF SUSTAINABLE BASALTIC VOLCANISM Meredith Townsend and David D. Pollard Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 e-mail: [email protected] this type have been well documented in places such as Abstract Kilauea in Hawaii (Richter et al. 1970), Krafla in Iceland (Dauteuil et al. 2001), the Galapagos This proposal suggests the use of field work, (Chadwick and Dieterich 1995), and even on Io, a laboratory work, and modeling to investigate thermal moon of Jupiter (Wilson and Head 2001). Additionally, pore pressurization around recently emplaced dikes as these types of eruptions are inferred to have operated in it relates to the formation of breccias and dike- ancient systems, like Ship Rock, New Mexico (Delaney perpendicular joint sets in the host rock adjacent to and Pollard 1981), and in ancient systems on both dikes that intrude fluid-saturated sedimentary rocks. Venus and Mars (Ernst et al. 2001). The formation of dike-perpendicular joints and breccias Although direct observation supports the hypothesis is a necessary precursor to erosion and sustained flow that volcanic plugs form by this sequence of events, to through a conduit; thus, understanding their formation date there is no conclusive theoretical model that is critical to the outstanding question of how dikes and integrates the primary physical phenomena to explain fissures evolve into larger volcanic plugs. Field work how this happens and under what conditions this will be carried out at Ship Rock, New Mexico, an transition from dike to plug is likely to occur. Some Oligocene-aged diatreme surrounded radially by proposed theories for conduit widening and flow minette dikes and smaller plugs. Ship Rock and other localization include progressive melting of the host intrusions of the Navajo Volcanic Field cut through a rock (Fialko and Rubin 1999; Bruce and Huppert 1989; sequence of Cretaceous sediments of the San Juan Quareni et al. 2001), pore-pressure and conduit- Basin, and hence are an excellent natural laboratory pressure buildup from water-magma interaction for studying the effect of lithology on thermal pore (Delaney and Pollard 1981; Dobran and Papale 1993), pressurization. Three projects will seek to characterize variation in magmatic pressure inducing wall collapse the dike-perpendicular joints, examine the role of (Dobran and Papale 1993; Macedonio et al. 1994), and thermal pore pressurization in their formation, and erosion from particle collision and/or shear stress of understand how their formation in turn affects the pore flowing magma (Dobran and Papale 1993). Although pressure field. Completion of these projects should these are all plausible mechanisms for changing conduit provide the basis for future work on the subsequent geometry, there are major unresolved issues within each stages in conduit geometry evolution, for example host theory that render them generally inconclusive. For rock erosion and magma flow problems. example, the numerical model of Quareni et al. (2001) shows that even when maximizing heat transfer, Keywords: significant melting of the host rock only occurs basaltic volcanism, thermal pore pressurization, hundreds of years after the onset of eruption, which physics-based models, heat and mass transport, rock conflicts with observations of timing at real eruptions. mechanics, Ship Rock Furthermore, while these studies provide templates for addressing the various physical phenomena Introduction independently, none have considered how the processes are coupled. A conclusive theoretical treatment that Basaltic volcanic eruptions commonly are integrates heat transfer, deformation, erosion, and fluid characterized by three stages. In the upper few flow would greatly benefit our predictive capabilities, kilometers of the crust, magma rises in planar dikes, which has implications for societies where this type of which result in fissure eruptions when they intersect the volcanism poses a hazard. For this project I propose to surface. In some cases, the fissures do not feed use a combination of field work, laboratory work, and significant eruptions and quickly die out. In other cases, modeling to test the hypothesis that thermal pore magma stops flowing in much of the fissure but pressurization causes brecciation of the host rock, and continues to erupt from discrete vents until finally the that this is a necessary precursor to erosion and flow is sustained through a single central vent underlain sustained magmatic flow through a central vent by a volcanic plug that is able to carry much greater volumes of magma to the surface. Active eruptions of Stanford Rock Fracture Project Vol. 24, 2013 I-1 Field Site: Ship Rock, New Mexico referred to as “buds” that appear to be juvenile plugs (figure 2). The authors also note that breccias can be Ship Rock is part of the Navajo Volcanic Field found along the margins of the dikes and plugs, which (NVF), a group of ultrapotassic diatremes, plugs, and lead them to hypothesize that fracturing and erosion of dikes located in the central Colorado Plateau (Laughlin host rock was responsible for conduit widening. et al. 1985). Volcanism in this region began around 28 m.y.a and continued to at least 19 m.y.a. and was likely contemporaneous with the uplift of the Plateau (Laughlin et al. 1985). Petrographically, the NVF consists primarily of basaltic tuff and tuff breccia cut by mafic minette dikes and plugs (Delaney and Pollard 1981). The land surface in the vicinity of Ship Rock is the Mancos Shale, a basal transgressive mudstone that stratigraphically underlies the Mesaverde group, a sequence of upper Cretaceous sandstones (Beaumont 1955). Sandstones of the Mesaverde group crop out ~30 km south of Ship Rock and form the land surface through which dikes near the Bennett Peak plug intrude Figure 2. Left: A bud around a segment of the (O’Sullivan and Strobell 1988). Based on the northeastern dike; Right: Geologic map of the stratigraphy, Delaney and Pollard (1981) estimate that same segment, showing minette dike material in the current land surface at Ship Rock was at about 1 km pink, “monobreccia” (brecciated host rock) in light grey, and “heterobreccia” (brecciated host depth at the time of eruption. Ship Rock itself is a rock and minette) in dark grey. minette diatreme surrounded radially by minette dikes and smaller plugs (figure 1). Because Ship Rock is petrographically similar and only ~65 km north of the Preliminary Work Narbona Pass Crater (Brande et al. 2008), we infer that With help from Dave Pollard, a field trip to the like Narbona Pass, Ship Rock grew to its current size Navajo Volcanic Field in July 2012 produced a careful during a phreatomagmatic eruption. investigation of the breccias and other deformation Ancient systems like Ship Rock that have been around the dikes at Ship Rock, as well as field maps of exposed by erosion provide a unique opportunity to other dike-plug systems in the NVF. Samples of breccias, minette, and Mancos shale were collected from the northeastern dike at Ship Rock, as well as a few samples of the Menefee sandstone adjacent to a dike ~35 km to the south of Ship Rock. Many of these samples were made into thin sections, and through petrographic analysis a conceptual model was formulated for the processes contributing to conduit geometry evolution and a chronological order in which these processes take place. Figure 3 shows a cartoon of this conceptual model: Stage 1: Dike-parallel fractures (figure 3a) Around all of the dikes at Ship Rock are fractures in the host rock subparallel to the dikes which often extend beyond the dike tip (figure 4). During fissure eruptions Figure 1. Ship Rock, a volcanic neck with dikes in Hawaii, ground cracks are commonly observed to extending radially away (from Pollard and Fletcher form ahead of the propagating fissure subparallel to the 2005 – photo by D.L. Baars). propagation path (Pollard et al. 1983). The classic elasticity problem of a pressurized crack predicts a directly investigate a volcanic plumbing system. bimodal distribution of the maximum tensile stress Analysis of the structures and field relations at Ship positioned to either side of the crack tip, which suggests Rock lead Delaney and Pollard (1981) to conclude that that the ground cracks at Kilauea and the subparallel the main diatreme initiated as a planar dike. The authors fractures around the dikes at Ship Rock formed in found that there are many dikes not connected to plugs, response to intrusion of magma and dike opening. but nowhere are there plugs without dikes. Along many of the dikes, there are places of anomalous widening Stanford Rock Fracture Project Vol. 24, 2013 I-2 Figure 3. Map view cartoon of the conceptual model for processes within dikes/fissures leading up to bud and plug formation. Note the progressive appearance of oxides, represented as black dots, in the host rock. a: fractures form ahead of the propagating dike tip; b: magma intrudes, joints form in the host rock perpendicular to the contact; c: magma at the margins fractures, host rock partially fluidizes and mingles with magma clasts to form peperite; d: peperites are eroded; e: host rock is eroded. Stage 2: Intrusion, dike-perpendicular fractures (figure 3b) In the host rock around many of the dikes at Ship Rock are vertical fractures (figure 5a), which are perpendicular to the dikes (figure 5b). Inspection of fracture surfaces reveals hackle marks, suggesting these fractures are mode-I joints. Stage 3: Formation of peperite (figure 3c) Observations at Ship Rock and in thin sections of breccias show some undulating fingers of shale mingling with minette (figure 6). Occasionally shale forms the matrix of brecciated minette (figure 7), which is a rock called peperite (Skilling et al. 2002). In order for the shale to form these finger-like textures and form Figure 4.

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