Uddalak Biswas

• Biswas, Uddalak; Galland, Olivier & Carlson, Andreas (2026). Laponite® RD and hydroxyethyl cellulose as host rock and magma analogue to simulate a wide range of rheological combinations in the volcanic plumbing system. Tectonophysics. ISSN 0040-1951. 937. doi: 10.1016/j.tecto.2026.231351. Fulltekst i vitenarkiv Vis sammendrag

  Analogue modelling is essential for understanding magma emplacement in the Earth’s crust and requires strict geometric, kinematic and dynamic scaling. Therefore, material selection must satisfy the scaling laws to reproduce the natural rheological variability. Although often represented by end-member rheologies, crustal rocks in nature exhibit complex rheological behaviour. In this study, we analysed two complex materials, Laponite® RD (LRD) and hydroxyethyl cellulose (HEC), to assess their effectiveness as geological analogues for country rock and magma in dyke emplacement experiments. We provide a detailed characterisation of LRD, highlighting its tuneable rheological properties via curing time (t_c). To characterise the complex rheology of the LRD gels, rotational and oscillatory tests, including flow, amplitude, frequency sweep, and creep and recovery tests, were performed at curing times (t_c) ranging from 30 to 240 min. We tested the rheology of aqueous HEC solutions at concentrations of 0.50, 0.75, and 1.00 wt%. Additionally, this study highlights the significant role of surface tension in laboratory analogue experiments, demonstrating the need to minimise interfacial tension by selecting an appropriate magma analogue fluid when using a viscoelastic host. The rheological test results showed that the storage modulus (G^') and yield stress (τ_y) increased systematically with increasing t_c. The flow and surface tension test results indicate that HEC hydrous solutions serve as robust magma analogues, offering variable viscosities with minimal alterations in surface tension and density. Combining the rheological analysis results of LRD and HEC, we propose a new methodology for simulating a wide range of rheological conditions by systematically varying the LRD curing time and employing different concentrations of HEC as a magma analogue. Our approach emphasises precise rheological characterisation together with control of magma-analogue density and surface tension, minimising the interfacial tension and buoyancy contrast with the host for accurate scaling in analogue models of volcanic plumbing systems.

• Biswas, Uddalak; Mitra, Atin Kumar & Mandal, Nibir (2023). Wall Fracturing Versus Mechanical Instability as Competing Intrusion Mechanisms of Dikes: Insights From Laboratory Experiments. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 128(9). doi: 10.1029/2023jb026921. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Igneous dike intrusion is a primary crust‐forming process at the Earth's plate boundaries. Understanding its mechanism is thus crucially important in lithospheric studies. Our present article combines experimental and field observations to investigate the problem of dike emplacement from a mechanical perspective. Scaled laboratory experiments were conducted by injecting immiscible liquids into visco‐elastic and visco‐elasto‐plastic host materials at varying volumetric flow rates (VFR = 0.100 to 1.670 ml s −1 ). Another set of experiments used different injecting liquid–host combinations to set their viscosity ratios ( η *) at low (10 5 ), moderate (10 6 ), and high (10 9 ) values. These two lines of experiments allow us to recognize three principal mechanisms of liquid pathways: tensile fracturing of the host, wave instability at the liquid‐host interface, and coupled fracturing‐wave instability process. The three mechanisms give rise to a wide variation in intrusion geometry, ranging from planar structures with elliptical outlines to typical bulbous geometry, with intermediate patterns characterized by in‐plane and off‐plane wavy interfaces with the host. This study uses the experimental data to constrain the VFR conditions that determine the fracturing versus wave instability‐controlled mechanisms. It is also shown from the experiments that η * can significantly influence the evolution of three‐dimensional intrusive geometry. The Chotonagpur Granite Gneiss Complex in eastern India is chosen as a field study area to validate our laboratory findings using a 2D shape analysis of the intrusive boundaries in terms of their fractal dimensions ( D ) and skewness‐kurtosis estimates.

• Biswas, Uddalak; Mukhopadhyay, Manaska & Mandal, Nibir (2023). Fractal analysis of anisotropic shear-fracture roughness from rocks and analogue laboratory models: A new approach for heterogeneous-slip characterization. International Journal of Rock Mechanics And Mining Sciences. ISSN 1365-1609. 169, s. 105432–105432. doi: 10.1016/j.ijrmms.2023.105432. Fulltekst i vitenarkiv Vis sammendrag

  Shear fracture surfaces are commonly rough on varied scales, and understanding the roughness properties is crucial to predict their slip behaviour. Using natural rock samples from the Singhbhum Shear Zone, eastern India, this study develops an approach to heterogeneous surface roughness analysis of shear fractures, forming two distinct domains: striated, anisotropic zone and non-striated, isotropic zone. Analogue experiments run on brittle-ductile models under a pure shear condition produce similar heterogeneous roughness characteristics of shear fractures. Using a laser profilometer, we constructed 3D topologies of both natural and experimental shear surfaces, which are presented to show the distinctive roughness characteristics of the striated and non-striated zones in terms of their fractal properties. ΔD, which stands for the difference between across- (D⊥) and along- (D∥) the slip direction, is calculated to evaluate the degree of their roughness anisotropy. This fractal parameter indicates strong anisotropy with ΔD = 0.0787–0.2118 in striated zones, which is significantly weak, where ΔD = 0.0024–0.0603 in non-striated zones. Finally, the article presents an in-depth discussion of this roughness study from the perspective of slip-stuck kinematics on shear surfaces from laboratory experiments and their geophysical implications, e.g., interpretation of earthquake event patterns on seismic faults and permeability of fractured rocks.

• Mukhopadhyay, Manaska; Biswas, Uddalak; Mandal, Nibir & Misra, Santanu (2019). On the Development of Shear Surface Roughness. Journal of Geophysical Research (JGR): Solid Earth. ISSN 2169-9313. 124(2), s. 1293–1293. doi: 10.1029/2018jb016677. Fulltekst i vitenarkiv Vis sammendrag

  Abstract From deformed quartzites in the Singhbhum Shear Zone, eastern India, we report shear fractures of varying surface roughness: very smooth, containing no lineation to strongly rough with prominent slickenlines. We reproduced them in analogue laboratory experiments, which suggest that the modes of shear failure (brittle versus ductile) and the fracture orientation are potential factors to control the fracture roughness. The experiments were conducted on cohesive sand‐talc models with varying sand:talc volume ratio. Pure sand models underwent Coulomb failure in the brittle regime; this failure mode switched to plastic yielding in the ductile regime with increasing talc content. Such a transition in failure behavior resulted in a remarkable variation in the fracture roughness characteristics. Shear fractures produced by Coulomb failure are smooth, and devoid of any slickenlines, whereas those produced by plastic yielding display strongly linear roughness, defined by cylindrical ridges along the slip direction. Such linear irregularities become more prominent with increasing fracture orientation ( θ ) to the compression direction ( θ  = 30 to 60°). We develop a new computational technique, based on controlled optical images to map the shear surface geometry from field casts and laboratory samples. Binarization of the irregular surface images (cantor set) provides 1‐D fractal dimension ( D ), which is used to quantify the roughness variability, and the degree of their anisotropy in terms of Δ D (difference in D across and along the slip direction). From numerical models, we finally show onset of wave instability in the mechanically distinct rupture zone as an alternative mechanism for slickenline formation.

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• Biswas, Uddalak; Galland, Olivier & Carlson, Andreas (2026). Formation of various geometry by viscous magma intruding into visco-elasto-plastic deep crust: a 2D analogue experiment. Fulltekst i vitenarkiv Vis sammendrag

  Magma ascends to the Earth's surface for eruption by traversing the crust, which is the uppermost layer above the mantle, where the magma originates. As this substantial volume of magma moves through the crust, it deforms the crustal rocks, facilitating its progression toward the surface and subsurface. The resulting geometry, which may manifest as thin sheets, branching fingers, or blunt lobes, is set by the competition between the intruding fluid and the mechanical response of the host material. These deformation mechanisms, which control the geometry of the magmatic bodies, are essential for assessing hazards related to volcanic eruptions, seismic activity due to volcanism, and the localisation of economically valuable ore deposits. As the deformation of the crustal rocks by the intruding magma occurs at a certain depth and natural exposures record only the end state of this strongly dynamic process, the mechanics behind this process can’t be inferred directly. To address this gap, researchers have employed analogue modelling as a tool to simulate a simplified analogous intrusion process of magma at a laboratory scale for decades. Conventionally, in these analogue experiments, crustal rocks have been modelled using elastic or plastic Coulomb materials, whereas magma has been represented as a viscous fluid by previous researchers. However, recent studies have shown that deep crustal rocks exhibit a complex visco-elastic to visco-elasto-plastic rheology in nature. To simulate the crustal rocks more accurately, in this study, we approached the problem by injecting a viscous fluid as a magma analogue into an aqueous Laponite® RD (LRD) gel, which acted as a model host rock, confined in a two-dimensional Hele-Shaw cell. The LRD gel exhibits diverse rheological properties, transitioning progressively from a viscous liquid to a viscoelastic state, and ultimately to an elastico-viscous solid, as its curing time (tc) increases. Considering this property of the LRD gel, we used tc = 30, 60, 90, 150, and 240 min to represent crustal rock under different physical and chemical conditions. On the other hand, magma in nature exhibits variable viscosity depending on its composition, temperature and volatile content. In parallel, we varied the injected fluid using water and hydroxyethyl cellulose solutions at 0.50 and 0.75wt%, to span the range of viscosity. Injection was carried out at a fixed volumetric flow rate of 10 ml/min up to a maximum injected volume of 50 ml, so that the flow rate is held constant while the rheology of the host and viscosity of the fluid are varied. Additionally, polyamide tracer particles with a diameter of 60 μm were embedded in the gel, enabling particle image velocimetry (PIV) to simultaneously resolve the deformation field of the host as the intrusion expanded. The experiments produced distinct intrusion geometries that systematically depended on host rheology and the viscosity of the injected fluid. The velocimetry fields from the PIV analysis further show that intrusions of broadly similar shapes can be accommodated by clearly different velocity and strain distributions within the host. Combining the evolving geometry with the accompanying strain pattern therefore offers a route toward identifying which deformation mechanism accommodated a given volume of magma, and this diagnostic can in principle be applied to interpret the intrusion patterns observed in the field.

• Biswas, Uddalak; Galland, Olivier & Carlson, Andreas (2026). Magma emplacement mechanism in visco-elasto-plastic crustal rock: an insight from quantitative 2D analogue experiments. doi: 10.5194/egusphere-egu26-18980. Fulltekst i vitenarkiv Vis sammendrag

  Magma generated at depth ascends through the crust and erupts at the surface. In this process, the magma volume creates space by deforming the crustal rock. To study this process, researchers have employed analogue experiments for decades. Conventionally, most analogue models assume the crustal rock as a purely elastic, brittle (Coulomb), or viscous material. However, natural rocks exhibit more complex visco-elasto-plastic behaviours, with rheological properties transitioning from elastic-plastic to visco-elasto-plastic with increasing depth. Conversely, magma itself behaves as a Newtonian fluid, exhibiting a range of viscosities influenced by factors such as silica and volatile content. In this study, we conducted scaled 2D analogue magma emplacement experiments using Laponite® RD (LRD), a complex visco-elasto-plastic rheological material, as a host rock analogue. The rheological properties of LRD were systematically varied by changing the curing time (tc) from tc = 30 min to tc = 240 min to simulate a broad range of crustal rheological behaviours, corresponding to different depths from viscous to visco-elasto-plastic regimes. Food-coloured water and hydroxyethyl cellulose (HEC) aqueous solutions, with concentrations of 0.50 wt% and 0.75 wt%, are used to simulate magma with varying viscosities. Polyamide seeding particles (PSPs) of 60µm diameter were incorporated inside the LRD solution to quantify host-rock deformation during magma emplacement via particle image velocimetry (PIV). Our experimental results show that magma intrusions exhibit various shapes, ranging from straight-edged fractures in the case of low-viscosity water and viscoelastic LRD of high tc to bulbous, rounded forms in high-viscosity HEC and viscous LRD of low tc. Furthermore, PIV data enabled the identification of domains exhibiting distinct deformation types, thereby delineating changes in deformation regimes attributable to variations in the rheological properties of the host rock. Finally, by integrating geometric and PIV analyses, we established a relationship between the intrusion morphology and host rock rheology, and propose a mechanical model that elucidates the deformation mechanisms operative within the host during magma emplacement across different rheological combinations of host and magma materials.

• Biswas, Uddalak; Mitra, Atin Kumar & Mandal, Nibir (2025). Gel wax and Ultrasound Transmission Gel as upper and lower crustal rheology analogue. doi: 10.5194/egusphere-egu24-744. Fulltekst i vitenarkiv Vis sammendrag

  The laboratory modelling of upper and lower crustal rocks has been challenging in terms of rheological scaling for geologists. Upper and lower crusts have been considered as coulomb-brittle or elastic and viscous rheological material, respectively, by the analogue modeller in simulating several deformation related phenomena. However, in nature, they are not purely viscous or elastic, or plastic. Rather, they behave visco-elasto-plastically. This work introduces two new materials of complex visco-elastic and visco-elastoplastic rheology i) Gel wax and, ii) Ultrasound Transmission Gel (USTG). Gel wax is a commercial wax which is composed of mineral oil and hydrocarbon-based polymer. It is used for making transparent, long-lasting candles with melting temperatures of 70-80°C. On the other hand, USTG is a gel-like substance made mainly of Carbopol powder and water.In this present work, we performed amplitude and frequency sweep tests in a rheometer for both the Gel wax and USTG to understand their complex rheology. The results suggest that Gel wax can be an appropriate analogue material to simulate upper crustal experiments. Similarly, visco-elastoplastic rheology of USTG is analogous to lower crustal rocks.Considering these materials as crustal analogues, we conducted a few dike emplacement experiments following proper geometric, kinematic and dynamic scaling. The experimental results revealed remarkable 3D dike geometry as both the materials are transparent. Finally, we matched these patterns with a few exposed dikes observed in the field, which supports the applicability of Gel wax and USTG as crustal rock analogues.

• Biswas, Uddalak; Galland, Olivier & Carlson, Andreas (2025). Variable curing time of Laponite RD: a convenient way to simulate a wide range of crustal rheology. Fulltekst i vitenarkiv Vis sammendrag

  Crustal rocks exhibit complex and highly variable rheological behaviors, which are challenging to account for in laboratory simulations of geological processes. The choice of relevant analogue materials is crucial for a successful laboratory experiment that can address such processes more accurately. The advent of new technologies and complex synthetic materials widens the window for the analogue experimentalist to revisit some of the problems for deeper understanding. Most of standard analogue materials used for simulating crustal rocks exhibit endmember rheology, i.e., viscous, elastic or coulomb (plastic). However,crustal rocks can exhibit a wide range of complex rheological properties, such as visco-elastic, visco-plastic, elasto-plastic and viscoelasto-plastic, during deformation under stress. In the last decade, a few researchers started using complex rheological materials, such as Carbopol, to simulate those processes more accurately. Laponite RD is one of such advanced complex rheological materials. Laponite RD is a synthetic sheet silicate mineral that resembles a natural clay mineral called hectorite. Laponite RD is used mostly as a rheological modifier in cosmetics, paint and the pharmaceutical industry. This material comes in powder form and transforms into gel when mixed with deionised water. This gel material was investigated previously by a few analogue modellers. However, these researches consist of rheological variation, either depending upon concentration (in terms of weight%, wt.%) or very long curing time. However, during the investigation of the rheological properties of Laponite RD materials, we identified that a variation in short (< 4 hours) curing time (tc) can simulate a wide range of complex rheological behaviour of crustal rock from viscous to visco-elastic to elastic. We ran rheometrical measurements (amplitude sweep test and frequency sweep test) on 3 wt.% Laponite RD gels with variable curing times from tc=30 min to tc=240 min. Note that the gels were cured within the rheometer. The test results show a consistent increase in both the storage modulus (G′) and Loss modulus (G″) of the samples with increasing tc (COULD BE GREAT TO SHOW A FIGURE). To test the reproducibility of these analyses, we ran the same analysis with fixed tc several times and found that each run yielded similar values for G′ and G″ for a specific tc. Our tests highlighted the effect of the thixotropic property of Laponite gel on its rheology: handling of the laponite to place it in the rheometer dramatically affects its rheology. Hence, our results show that it is crucial to cure the sample at the place of the experiment to simulate accurate rheological behaviour. Finally, taking an example of Laponite RD gel with variable tc as host in a magma emplacement experiment, we demonstrated the use of these gels to simulate a wide range of complex rheological behaviour of crustal rocks (refer to figure). Our study shows that Laponite RD gels are robust and easy-to-handle alternatives to Carbopol gels commonly used in analogue modelling.

• Biswas, Uddalak; Galland, Olivier & Carlson, Andreas (2025). 2D analogue models of magma emplacement in the visco-elastic crust. Fulltekst i vitenarkiv Vis sammendrag

  The Earth’s crust exhibits very complex rheology depending upon several physio-chemical conditions like temperature, pressure and lithology. This complex rheology of the crustal rocks is challenging to model. Thus, most researchers model magma propagation through crustal rocks of end-member rheologies such as viscous, coulomb (plastic) and elastic materials. These models acquire crucial information regarding the magma emplacement mechanisms. However, they are not sufficient to capture the whole spectrum of rheological behaviours of the host rock during the emplacement process. In this work, we performed a series of 2D magma injection experiments in the laboratory considering laponite RD gels as crustal rock analogues, showing complex rheological behaviour during deformation. According to the rheological analysis using a rheometer (Anton Paar MCR 702), laponite in deionised water solution exhibits a range of rheological behaviours from purely viscous to visco-elastic to purely elastic depending upon varying concentration (weight percentage, wt%) in the water. We used laponite gels of 2 wt% to 3.5 wt% to test the effects of varying rheology (yield stress = 0.597 Pa to 10.6 Pa) of the crustal rock analogue in the laboratory. These experiments used dyed oil (viscosity= 65 mPa.s) and water (viscosity= 1 mPa.s) as magma equivalents. Our 2D experiments revealed a range of intrusion shapes, from thin liquid-filled cracks of simple shapes to more massive fluid intrusions of complex shapes, indicating their varied mechanism of formation from brittle fracturing to ductile flow. To reveal the gel deformation accommodating the emplacement of the magma analogue, we implemented image analysis combining the photoelastic properties of the laponite gel and Digital Image Correlation. The resulting quantitative results allow us correlating the intrusion morphologies with the deformation mechanisms within the gel. Finally, from the experimental observations, we propose a model that connects the host rheology with the magma emplacement mechanism.

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