What Controls Sill Formation: An Overview From Analogue Models

Abstract

Understanding the mechanisms of shallow magma storage (less than ~10‐km depth) is crucial to defining volcanic plumbing systems and their effect on volcano unrest. Sills are common structures to emplace shallow magma. Many factors may control sill emplacement, but their relative importance has not been evaluated so far. To define a hierarchy among a selection of the proposed factors, we performed analogue models by injecting water at constant flux (magma analogue) within gelatin (crust analogue), investigating the effects of: interface strength and rigidity contrast between layers, density layering, ratio of layer thickness, compressive stresses, magma flow rate, and buoyancy pressure. Our results show that a strong rigidity contrast (i.e., a stiff layer overlying a weak one) and a higher buoyancy pressure are necessary and sufficient conditions for sill emplacement. Low rigidity contrasts require other contributions (weak interface and compressive stress) to develop sills; however, without rigidity contrast sill formation is always inhibited. These experiments suggest that sill emplacement is primarily controlled by rigidity contrasts and magma buoyancy pressure; the second‐order parameters are lateral compression and weak interfaces; the third‐order parameters are the ratio of layer thicknesses, magma flow rate, and density layering. These results are in agreement with field observations in Iceland, Tenerife, Utah, and La Réunion, supporting our proposed hierarchy.

Shear Wave Measurements of a Gelatin’s Young’s Modulus

“…We also poured slowly to avoid the melting much of the lower layer. This produced an ∼2 cm thick, welded transitional region between the layers (marked in Figure 2), which matched the "strong interface" type described by Urbani et al (2018) and Sili et al (2019), for studies of sill emplacement. We did not make an experiment of a "weak interface, " such that the interface is sharp and the layers are disconnected, because we expected the wave would not effectively transfer from one layer to the next.…”

“…The Poisson's ratio is therefore expected to be extremely close to 0.5, the value that is commonly cited in published literature (Menand and Tait, 2002;Rivalta et al, 2005;Kavanagh et al, 2013;Urbani et al, 2018;Sili et al, 2019). Assuming that pressure waves are even visible using our polarizing filters, we would not be able to record them given our tank dimensions of <1 m and camera recording frequency of 50 Hz, which correspond to a maximum detectable velocity of 50 m/s.…”

“…Gelatin is commonly used in experiments as an analog for the Earth's crust, typically when studying magmatic dike propagation (e.g., Fiske and Jackson, 1972;Takada, 1990;Menand and Tait, 2002;Taisne and Tait, 2009;Kavanagh et al, 2018;Urbani et al, 2018;Derrien and Taisne, 2019;Sili et al, 2019) or modeling fault slip behaviors (Corbi et al, 2011;Rosenau et al, 2017). It has been shown to scale the upper crust when prepared with a concentration between 2.5 and 5.0 wt% and cured at 5 • C (Di Giuseppe et al, 2009;Kavanagh et al, 2013;van Otterloo and Cruden, 2016).…”

Relating Dike Geometry and Injection Rate in Analogue Flux-Driven Experiments

“…Where K c is the fracture toughness of the host material, Δρ is the density difference between the intruded fluid and the host Kavanagh et al, 2013) and the host rock (nature; Heap et al, 2009). K c is the fracture toughness: for the gelatin, K c has been calculated from the Young modulus (E), using the equation (Kavanagh et al, 2013): K c (1.4 ± 0.1) E √ (E 2.5 kPa; Sili et al, 2019). Values of K c in nature are from Gudmundsson, (2009); Rivalta et al, 2015.…”

Intrusion Induced Global Warming Preceding Continental Flood Basalt Volcanism

“…Some previous workers focus on the effects of mechanical strength changes across layers which deflect a dike into sills and hence the sill intrusion depth is determined by the location of the layer boundary 61,64 . A recent analogue model study summarizes that buoyancy pressure from density contrast between host rock and the injecting fluid, rigidity contrast and lateral compression are the major controls on formation of sills 65 . Menand 61 reviewed existing models for sill emplacement depths as controlled by four major factors: (1) the buoyancy pressure due to the density contrast between host rock and injecting fluid, (2) the rigidity contrast between strata,…”