Abstract
AbstractWe 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.
Highlights
Fissure eruptions have been documented to evolve with time, from initially elongated fissures to centralized vents (Jones et al, 2017; Keating et al, 2008; Wylie et al, 1999)
Gelatin is commonly used as an analogue for the Earth's crust because its Young's modulus can be controlled by varying the concentration during preparation and because it is transparent, so we can observe what occurs inside the medium
For experiments with a short time interval, the flow in the dike quickly channelized into a narrow conduit (Figure 2a), which tended to be elliptical in cross section, a few centimeters across and approximately half a centimeter thick
Summary
Fissure eruptions have been documented to evolve with time, from initially elongated fissures to centralized vents (Jones et al, 2017; Keating et al, 2008; Wylie et al, 1999). The change of geometry from planar to cylindrical reflects the easiest form of magma transport; a planar form is most efficient for propagation through a brittle, elastic medium; a cylindrical form is the most thermally efficient (Keating et al, 2008) Dikes that supply such eruptions may evolve with time at depth, as they have a high ratio of surface area to volume and are very vulnerable to cooling and, eventually, solidification. Cooling is heterogeneous along the dike surface, in that regions that become cool become more viscous, encouraging flow toward the hotter regions of the dike, which remain warm and active As they transition to cylindrical geometries, the surface area and heat loss drop, allowing thermally efficient magma transport to the surface (Fukushima et al, 2010).
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