Gelatin is a commonly used material for analog experiments in geophysics, investigating fluid-filled fracture propagation (e.g., magmatic dikes), as well as fault slip. Quantification of its physical properties, such as the Young's modulus, is important for scaling experimental results to nature. Traditional methods to do so are either time consuming or destructive and cannot be performed in situ. We present an optical measurement technique, using shear waves. Polarizing filters enable visualization of the deviatoric stresses in a block of gelatin, so shear wave propagation can be observed. We demonstrate how the wave velocity can be measured and related to the Young's modulus, show how the results are comparable to another methodology and discuss processing techniques that maximize the measurement precision. This methodology is useful for experimentalist, as it is simple to implement into a laboratory setting, can make precise, time-efficient estimates of the material strength and additionally is non-destructive and can be performed in situ.
We investigate the conditions under which magma prefers to migrate through the crust via a dike or a conduit geometry. We performed a series of analogue experiments, repeatedly injecting warm, liquid gelatin, into a cold, solid gelatin medium and allowing the structure to evolve with time. We varied the liquid flux and the time interval between discrete injections of gelatin. The time interval controls the geometry of the migration, in that long intervals allow the intrusions to solidify, favoring the propagation of new dikes. Short time intervals allow the magma to channelize into a conduit. These times are characterized by the Fourier number (Fo), a ratio of time and thermal diffusion to dike thickness, so that long times scales have Fo > 102 and short time scales have Fo < 100. Between these time scales, a transitional behavior exists, in which new dikes nest inside of previous dikes. The flux controls the distance a dike can propagate before solidifying, in that high fluxes favor continual propagation, whereas low fluxes favor dike arrest due to solidification. For vertically propagating dikes, this indicates whether or not a dike can erupt. A transitional behavior exist, in which dikes may erupt at the surface in an unstable, on‐and‐off fashion. We supplemented the experimental findings with a 2‐D numerical model of thermal conduction to characterize the temperature gradient in the crust as a function of intrusion recurrence frequency. For very infrequent intrusions (Fo > 104 to 105) all thermal energy is lost, while more frequent intrusions allow heat to build up nearby.
We investigate the effect of magmatic reservoir pressure on the propagation of dikes that approach from below, using analogue experiments. We injected oil into gelatin and observed how dike propagation responded to the stress field around a pressurized, spherical reservoir, filled with water. The reservoir was modeled using two different setups: one simply using an inflatable rubber balloon and the other by constructing a liquid-filled cavity. We find that the dike's response is dependent on the sign of the reservoir pressure (i.e., inflated/overpressurized and deflated/underpressurized) as well as on the dike's initial orientation (i.e., if its strike is radially, circumferentially, or obliquely oriented to the reservoir). Dikes that are initially strike radial respond, respectively, by propagating toward or away from overpressurized or underpressurized reservoirs, taking advantage of the reservoir's hoop stresses. Otherwise-oriented dikes respond by changing orientation, twisting and curling into a form dictated by the principal stresses in the medium. For overpressurized reservoirs, they are coaxed to propagate radially to, and therefore approach, the reservoir. For underpressurized reservoirs, they generally reorient to propagate tangentially, which causes them to avoid the reservoir. The magnitude of reservoir pressure controls at which distance dikes can be affected, and, at natural scales, we estimate that this occurs within a radius of a few tens of kilometers. This diminishes with time, due to viscous stress relaxation of the crust, which will occur on a timescale of hundreds of years.Plain Language Summary Magma commonly moves up toward the surface by creating cracks in the crust. It flows inside of the cracks and propagates by applying pressure that drives the flow and damages surrounding rocks. Nature always finds the easiest path for the crack, so if it takes less pressure to push apart the ground vertically or horizontally, the crack will grow accordingly. As it makes its way to the surface, it may encounter local stress variations that change its propagating direction. This applies near magma storage regions, below volcanoes. If such a region is highly pressurized or deflated, then nearby cracks will "feel" the change in their surrounding conditions and react by aligning in a direction of favorable stress. This makes it look like they are growing toward or circling around the storage region, respectively. We studied this behavior using scaled model experiments in laboratory conditions. We use different types of materials to represent nature, such as gelatin as rock and oil as magma. We were able to show how these cracks change shape for different reservoir pressures. We found that after a large eruption, subsequent eruptions are more likely to occur farther from the summit of a volcano.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.