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Travertine Terracing: Patterns and Mechanisms. In: Tufas and Speleothems: Unravelling the Microbial and Physical Controls

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Travertine Terracing: Patterns and Mechanisms. In: Tufas and Speleothems: Unravelling the Microbial and Physical Controls See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/236150787 Travertine terracing: patterns and mechanisms. In: Tufas and Speleothems: Unravelling the Microbial and Physical Controls Article in Geological Society London Special Publications · January 2010 DOI: 10.1144/SP336.18 CITATIONS READS 11 128 4 authors, including: Dag Kristian Dysthe Bjorn Jamtveit University of Oslo University of Oslo 98 PUBLICATIONS 1,166 CITATIONS 150 PUBLICATIONS 4,821 CITATIONS SEE PROFILE SEE PROFILE Some of the authors of this publication are also working on these related projects: Reaction-induced fracturing and fluid pathway generation View project Fluid transport through reaction-induced feldspar nanopores View project All content following this page was uploaded by Bjorn Jamtveit on 23 January 2017. The user has requested enhancement of the downloaded file. Geological Society, London, Special Publications Travertine terracing: patterns and mechanisms Øyvind Hammer, Dag K. Dysthe and Bjørn Jamtveit Geological Society, London, Special Publications 2010; v. 336; p. 345-355 doi:10.1144/SP336.18 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by Universitetet i Oslo on 22 June 2010 © 2010 Geological Society of London Travertine terracing: patterns and mechanisms ØYVIND HAMMER*, DAG K. DYSTHE & BJØRN JAMTVEIT Physics of Geological Processes, University of Oslo, PO Box 1048 Blindern, 0316 Oslo, Norway *Corresponding author (e-mail: [email protected]) Abstract: Travertine terracing is one of the most eye-catching phenomena in limestone caves and around hydrothermal springs, but remains fairly poorly understood. The interactions between water chemistry, precipitation kinetics, topography, hydrodynamics, carbon dioxide degassing, biology, erosion and sedimentation constitute a complex, dynamic pattern formation process. The processes can be described and modeled at a range of abstraction levels. At the detailed level concerning the physical and chemical mechanisms responsible for precipitation localization at rims, a single explanation is probably insufficient. Instead, a multitude of effects are likely to contribute, of varying importance depending on scale, flux and other parameters. Travertine terracing is undoubtedly among the precipitation and possibly particle transport and most spectacular geological phenomena on Earth. biology. Ranging in vertical scale from millimetres to tens of metres, travertine terraces form intricate, delicate patterns as well as imposing waterfalls. Such ter- Morphology and hydrodynamics races are common not only in limestone caves and around hot springs, but also in streams and rivers The terminology of travertine terrace morphology is in limestone terrain (Pentecost 2005, 59–66). In confusing. Speleologists often use the roughly addition, analogous patterns are found in completely equivalent terms ‘rimstone’ and ‘gours’, with different systems, such as silica sinter deposits and ‘microgours’ referring to cm-scale terraces. For water ice. Some of the more spectacular examples open-air travertine systems, morphological classifi- of travertine terracing are found in Yellowstone cation schemes have been proposed by several National Park (Bargar 1978; Fouke et al. 2000), authors. Pentecost & Viles (1994) define ‘barrages’ Pamukkale in Turkey (Altunel & Hancock 1993), as terraces that are filled with water, forming pools Huanglong Scenic District in China (Lu et al. and lakes. ‘Cascades’ are smoother forms on steep 2000) and northern Spitsbergen, Norway (Hammer slopes, often with smaller terrace-like structures et al. 2005). A list of travertine occurrences in superimposed on them (Fig. 1). Pentecost (2005) Europe and Asia Minor with notes on terracing uses ‘dams’ instead of barrages. Fouke et al. was given by Pentecost (1995). Travertine terraces (2000) and Bargar (1978) use three size categories formed an inspiration for Renaissance ornamental of barrages, namely ‘terraces’ with areas of tens of water cascades (Berger 1974). However, in spite square metres, ‘terracettes’ of a few square metres of their great interest, both aesthetically and scienti- and ‘microterracettes’ of a few square centimetres fically, the formation of travertine terraces has until or less (Fig. 2). Microterracettes are therefore analo- recently received only scattered scientific attention, gous to microgours. Pentecost (2005) suggests the especially on the theoretical side (Wooding 1991 term ‘minidam’ for a dam with an interdam distance provides one notable exception). (IDD) ranging from 1 cm to 1 m. When trying to understand this pattern formation In this paper we will use the classification of system, a series of fundamental questions arise. Fouke et al. (2000), extending it to the analogous What processes are responsible for the enhanced speleothems. In addition, we recognize that on precipitation at the terrace rim? On a higher level steep slopes, microterracettes will not form pools of abstraction, how do these local processes lead (and are therefore not barrages) but grade into to global self-organization? How do the terraces subdued ‘microridges’ normal to flow. We also evolve in time, and what are their statistical proper- use ‘terrace’ as a general term regardless of size. ties under different conditions? Answering such We use ‘rim’ for the top of the outer wall of the questions requires a cross-disciplinary approach, terrace, and ‘pool’ for the water body behind it. involving field work, laboratory experiments, com- The rim is usually convex outwards (downstream), puter modeling and mathematical theory in order or consists of a series of near-parabolic lobes point- to study the complicated feedbacks between hydro- ing in the downstream direction (Fig. 2). Inside dynamics, water chemistry, calcium carbonate pools, flow velocity is small due to the larger From:PEDLEY,H.M.&ROGERSON, M. (eds) Tufas and Speleothems: Unravelling the Microbial and Physical Controls. Geological Society, London, Special Publications, 336, 345–355. DOI: 10.1144/SP336.18 0305-8719/10/$15.00 # The Geological Society of London 2010. 346 Ø. HAMMER ET AL. Fig. 1. Travertine terracettes (left) and cascades (right). Mammoth Hot Springs, Yellowstone National Park, USA. Fig. 2. Terraces, terracettes and microterracettes at Mammoth Hot Springs, Yellowstone National Park, USA. TRAVERTINE TERRACING 347 depth, and the travertine morphologies are often and microterracettes is fairly constant, on the order botryoidal or ‘stromatolitic’ with porous textures, of 4–5 mm, regardless of slope, thus producing reflecting a diffusion-dominated regime often with pools of much larger area in regions of small substantial biological activity. These ‘shrubs’ are slope (Fig. 4). In fact, given a constant height h often assumed to have a bacterial origin (e.g. and a minimum inter-dam distance k, we can geo- Chafetz & Folk 1984) but can probably form from metrically predict the IDD as a function of slope purely inorganic processes (Jettestuen et al. 2006). angle a: Over the rim and on the steep outside wall of the h terrace, water flows in a thin sheet and flow velocity IDD ¼ þ k increases. Here, the travertine is usually more tan a compact. A basic observation is that travertine terraces Data given by Pentecost (2005) seem to fit seem to display scaling properties, with microterrac- this simple model well (Fig. 5). Conversely, if the ettes often having similar morphologies as the underlying slope is constant, the IDD will also be largest terraces. As a working hypothesis we nearly constant. This effect may contribute to the might postulate that a similar mechanism is respon- possible characteristic wavelength which will be sible for pattern formation at all scales, and that a discussed below. The ‘unit-step phenomenon’ can continuous coarsening process could lead from be explained by the fact that the surface-tension smaller to larger terraces. However, at the smallest meniscus lapping onto the rear wall of the pool scales surface tension exerts an important influence can only reach to a certain constant height, allowing on the hydrodynamics, producing ‘bags’ of water free flow from the upstream rim over the wetted overhanging the rim and a meniscus at the base of terrace wall. Given the temperature control on the exposed drop wall (Fig. 3). In most localities surface tension, terracette height might possibly it can be observed that the height of terracettes vary with temperature. At larger scales, surface Fig. 3. Due to surface tension, ‘bags’ of water hang over the rims of these microterracettes at the Jotun hydrothermal spring, northern Spitsbergen. Message board peg for scale. 348 Ø. HAMMER ET AL. offer spectacular exposures (Guo & Riding 1998; Hammer et al. 2007), demonstrating general increase in scale and steepness with time (upwards coarsening), higher precipitation rates at rims and terrace walls, and upstream or usually downstream migration (Fig. 7). Several authors have mapped travertine precipi- tation rates in natural terraced systems (Liu et al. 1995; Lu et al. 2000; Bono et al. 2001; Hammer et al. 2005). These studies demonstrate substantially higher precipitation rates in areas of high flow vel- ocity near terrace rims and walls, causing relative upwards and outwards growth
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