Understanding the propagation and geometry of magmatic intrusions and the related surface displacement is critical for hazard and risk assessment at volcanoes. Magma ascends through the Earth’s crust towards its final subsurface position, or eventually towards its eruption site at the surface. Magma intrusions propagate and grow by deforming their crustal host rocks and by displacing the Earth’s surface. Because directly observing this process is impossible, accurate volcanic eruption forecasts depend on indirect geophysical monitoring of seismicity and surface deformation (e.g. Sigmundsson et al. 2018).
The lack of direct observations makes the interpretation of geophysical monitoring data a challenge, and results in heavily debated questions such as: What are the physical mechanisms that control magma-induced deformation? How do magma intrusions grow to their final geometries? How can surface deformation patterns be interpreted in terms of the intrusive processes in the subsurface?
To offer a new perspective on these questions, this presentation summarizes the observations and conclusions from the recently completed doctoral dissertation of the first author. This work compares observations from exposed volcanic plumbing systems and geophysical records of recent intrusions with novel 4D laboratory experiments.
First, magma-host rock interactions were investigated at a small-scale outcrop in the Oslo Rift System, Norway (Figure 1). We infer from structural and geochemical analyses that the propagation of mafic dyklets there was controlled by regional, pre-existing structural weaknesses in the host rock and thermo-chemical interactions between the sedimentary host rock and the magma (Poppe et al., in prep. (A)). Thermal contact metamorphism produced a secondary fluid, comprising a pore-fluid-magma mixture, that likely propagated along and ahead of the magma body. The magma propagated by opening-mode fracturing and bending of the host rock. At locations in e.g. Australia, Argentina, on the contrary, mixed-mode or shear-mode fracturing has been inferred (e.g. Dering et al. 2019; Spacapan et al. 2017).

Fig. 1 – Two overlapping dyklet tips intruded into sedimentary host rock on Hovedøya Island, Oslo Rift, Norway. Note the tapering geometry of the magma-filled fractures as well as the shear displacement along pre-existing fractures in the central, competent limestone layer (from Poppe et al., in prep. a). The ruler scales 30 cm.

Second, a novel 4D laboratory modeling approach was developed to overcome limitations of past laboratory models regarding (1) mechanically relevant rheology of model rock materials, and (2) imaging subsurface magma-induced deformation. As for (1), a set of detailed material tests showed that dry sand-plaster mixtures display more complex mechanical behavior than hitherto appreciated, and that these mixtures are suitable as analogues for the Earth’s upper crust (Poppe et al., in prep.(B)). As for (2), 3D displacement and strain fields within sand-plaster host material, induced by analogue golden syrup intrusions, were quantified in laboratory models over time by using medical wide beam X-ray Computed Tomography (CT) and Digital Volume Correlation (DVC).
The obtained displacement and strain fields show that analogue intrusions propagate by mixed-mode deformation of their sand-plaster host material (Figure 2).


Fig. 2 – Cumulative maximum shear strains in the central vertical X-Y cross-section through the model domain, final 3D rendering of the analogue intrusion, and final model surface topography, extracted from the reconstructed imagery of laboratory models of golden syrup intrusion in sand-plaster host material, by using medical X-ray Computed Tomography (adapted from Poppe et al., 2019).

The dominance of the fracturing mode is largely controlled by the host rock rheology, among other physical parameters, and leads to a spectrum of intrusion geometries as observed in nature. In low- to medium-strength host material, shear-mode fracturing dominates and thick cryptodomes or cup shapes form. In medium- to high-strength host material, opening-mode fracturing dominates and thin cone sheets, inclined sheets and dykes form (Figure 2; Poppe et al., 2019).
Finally, we extracted surface displacements from the reconstructed CT imagery by using dedicated Matlab codes. Unlike previous models assuming linear elastic rheology and tensile fracturing, our models of golden syrup dykes and steeply inclined sheet intrusions induce dome- or bulges-and-through-shaped uplift of the model surface without net subsidence. Comparable surface displacement patterns were observed during recent sheet intrusion events. Our results suggest that mixed-mode magma-induced fracturing may occur in the weak, highly fractured upper portion of some volcanoes and may affect surface displacement patterns. Consequently, we propose that geodetic inversion models for such settings should incorporate more complex non-elastic behavior.
Original languageEnglish
Publication statusPublished - Nov 2019
EventLASI VI: The physical geology of subvolcanic systems: laccoliths, sills and dykes - Malargue, Argentina
Duration: 25 Nov 201930 Nov 2019
http://lasi6.org/

Conference

ConferenceLASI VI
CountryArgentina
CityMalargue
Period25/11/1930/11/19
Internet address

    Research areas

  • intrusion, magma, analogue modeling, Digital volume correlation, Laboratory modeling

ID: 47159206