An Experimental Investigation of the Scaling of Columnar Joints

Columnar jointing is a fracture pattern common in igneous rocks in which cracks self-organize into a roughly hexagonal arrangement, leaving behind an ordered colonnade. We report observations of columnar jointing in a laboratory analog system, desiccated corn starch slurries. Using measurements of moisture density, evaporation rates, and fracture advance rates as evidence, we suggest an advective-diffusive system is responsible for the rough scaling behavior of columnar joints. This theory explains the order of magnitude difference in scales between jointing in lavas and in starches. We investigated the scaling of average columnar cross-sectional areas due to the evaporation rate, the analog of the cooling rate of igneous columnar joints. We measured column areas in experiments where the evaporation rate depended on lamp height and time, in experiments where the evaporation rate was fixed using feedback methods, and in experiments where gelatin was added to vary the rheology of the starch. Our results suggest that the column area at a particular depth is related to both the current conditions, and hysteretically to the geometry of the pattern at previous depths. We argue that there exists a range of stable column scales allowed for any particular evaporation rate.Comment: 12 pages, 11 figures, for supporting online movies, go to http://www.physics.utoronto.ca/nonlinear/movies/starch_movies.htm

Thermal properties of vesicular rhyolite

Thermal diffusivity of rhyolite melt and rhyolite foam (70–80% porosity) has been measured using the radial heat transfer method. Cylindrical samples (length 50–55 mm, diameter 22 mm) of rhyolite melt and foam have been derived by heating samples of Little Glass Mountain obsidian. Using available data on heat capacity and density of rhyolite melt, the thermal conductivity of samples has been determined. The difference in thermal conductivity between rhyolite melt and foam at igneous temperatures ( 1000°C) is about one order of magnitude. The effect of thermal insulation of magmas due to vesiculation and foaming of the top layer is discussed in terms of the data obtained using a simple illustrative model of magma chamber convection

Petrology and Geochemical Comparation of Pumice and Scoria Rocks of Slamet Volcano, Central Java

Slamet Volcano is one of Indonesia Quaternary Stratovolcanoes in Central Java Province. Slamet volcano is divided into two parts, Old Slamet in the western part and Young Slamet. The author examined a comparation data of pyroclastic rock of Slamet Volcano, the pyroclastic rocks are pumice from Old Slamet, the scoria fall, and scoria cones are from Young Slamet. They have different geochemical and petrology features, pumice rock has higher SiO2 from 60 to 64 wt.%, scoria fall has SiO2 49.81 to 50.56 wt. %, and scoria cone has SiO2 49.26 wt. %. Petrographic observation showed that pumice is vesicular and contains of phenocryst pyroxene, plagioclase and biotite, scoria fall, and scoria cones have similar petrographic characteristic they have phenocryst of plagioclase, olivine, and pyroxene with hyalopilitic texture. The contrast of major element combined with petrographic features suggest that pyroclastic rock in Slamet Volcano formed by different magma and the magma has differentiation process of Slamet magma is generally caused by magma mixing

Rheology of magmas with bimodal crystal size and shape distributions: insights from analog experiments

Magmas in volcanic conduits commonly contain microlites in association with preexisting phenocrysts, as often indicated by volcanic rock textures. In this study, we present two different experiments that inves- tigate the flow behavior of these bidisperse systems. In the first experiments, rotational rheometric methods are used to determine the rheology of monodisperse and polydisperse suspensions consisting of smaller, prolate particles (microlites) and larger, equant particles (phenocrysts) in a bubble‐free Newtonian liquid (silicate melt). Our data show that increasing the relative proportion of prolate microlites to equant pheno- crysts in a magma at constant total particle content can increase the relative viscosity by up to three orders of magnitude. Consequently, the rheological effect of particles in magmas cannot be modeled by assuming a monodisperse population of particles. We propose a new model that uses interpolated parameters based on the relative proportions of small and large particles and produces a considerably improved fit to the data than earlier models. In a second series of experiments we investigate the textures produced by shearing bimodal suspensions in gradually solidifying epoxy resin in a concentric cylinder setup. The resulting textures show the prolate particles are aligned with the flow lines and spherical particles are found in well‐organized strings, with sphere‐depleted shear bands in high‐shear regions. These observations may explain the measured variation in the shear thinning and yield stress behavior with increasing solid fraction and particle aspect ratio. The implications for magma flow are discussed, and rheological results and tex- tural observations are compared with observations on natural samples

Scaling of columnar joints in basalt

We describe field work, analysis, and modeling of columnar joints from the Columbia River Basalt Group. This work is focused on the regions around the Grand Coulee, Snake River, and Columbia Gorge, which form parts of this unusually homogeneous and very large sample of columnar basalt. We examine in detail the scaling relationship between the column width and the size of the striae and relate these quantitatively to thermal and fracture models. We found that the column radius and stria size are proportional to each other and inversely proportional to the cooling rate of the lava. Near a flow margin, our results put observational constraints on diffusive thermal models of joint formation. Deeper than a few meters into a colonnade, our measurements are consistent with a simple advection-diffusion model of two-phase convective cooling within the joints, regardless of the direction of cooling. This model allows an accurate comparison of igneous columnar jointing and joints due to desiccation in laboratory analog systems. We also identify a new length scale in which wavy columns can appear in some colonnades. The mechanisms leading to the wavy columns are likely related to those underlying similar wavy cracks in 2-D analog systems

Clastic Polygonal Networks Around Lyot Crater, Mars: Possible Formation Mechanisms From Morphometric Analysis

Polygonal networks of patterned ground are a common feature in cold-climate environments. They can form through the thermal contraction of ice-cemented sediment (i.e. formed from fractures), or the freezing and thawing of ground ice (i.e. formed by patterns of clasts, or ground deformation). The characteristics of these landforms provide information about environmental conditions. Analogous polygonal forms have been observed on Mars leading to inferences about environmental conditions. We have identified clastic polygonal features located around Lyot crater, Mars (50°N, 30°E). These polygons are unusually large (> 100 m diameter) compared to terrestrial clastic polygons, and contain very large clasts, some of which are up to 15 metres in diameter. The polygons are distributed in a wide arc around the eastern side of Lyot crater, at a consistent distance from the crater rim. Using high-resolution imaging data, we digitised these features to extract morphological information. These data are compared to existing terrestrial and Martian polygon data to look for similarities and differences and to inform hypotheses concerning possible formation mechanisms. Our results show the clastic polygons do not have any morphometric features that indicate they are similar to terrestrial sorted, clastic polygons formed by freeze-thaw processes. They are too large, do not show the expected variation in form with slope, and have clasts that do not scale in size with polygon diameter. However, the clastic networks are similar in network morphology to thermal contraction cracks, and there is a potential direct Martian analogue in a sub-type of thermal contraction polygons located in Utopia Planitia. Based upon our observations, we reject the hypothesis that polygons located around Lyot formed as freeze-thaw polygons and instead an alternative mechanism is put forward: they result from the infilling of earlier thermal contraction cracks by wind-blown material, which then became compressed and/or cemented resulting in a resistant fill. Erosion then leads to preservation of these polygons in positive relief, while later weathering results in the fracturing of the fill material to form angular clasts. These results suggest that there was an extensive area of ice-rich terrain, the extent of which is linked to ejecta from Lyot crater

プリュ-ム発生の非線形力学

金沢大学理学部本研究では、簡単な反応拡散モデルに基づいて,核一マントル境界でのプリュ-ムの発生機構や内核成長に関連した周期的あるいは準周期的変動の物理機構について研究した。対象としたモデルは,二成分系溶液の固化反応で,熱と物質についての二つの反応拡散方程式から構成される。反応は、核形成と成長によって進行し,反応速度は,温度に対して指数関数的な依存性を持つ。数値実験に基づくと,反応の進行は,時間的に準周期的に進行し空間的に層状構造を作る。数値モデルの結果を参考にすると,縞のできる条件は次ぎのようになる。1.対流が起こらないような状態にある。対流が起こるか起こらないかは、Reighlay数によって決まる。そこで次のような場合が考えられる。(1)温度差が小さい。(2)スケ-ルが小さい。(3)粘性が小さい。(4)重力加速度が小さい。さらに次の条件も必要となる。2指数関数的な反応速度を持つ。内核の成長の初期段階では、これらの条件が満足されていると考えられるので、現在の内核には、その成長過程で出来た、結晶の粒子密度や平均粒径及び結晶相の割合いの変化の縞が,中心部から外側に向って一定の割合いで広がりながらある程度まで続いていると考えられる。核一マントル境界から発生するプリュ-ムは、シュ-バ-トらの数値実験によると、ソリトンの分裂と見なせる。この周期的なプリュ-ムの発生過程では、粘性率の指数関数的な温度依存性が重要になる。プリュ-ム発生における物質輸送と多成分系メルトの冷却における反応の効率の類似性に注目すれば,時間的空間的に(準)周期的な変動を作る機構には,場の変化に対して指数関数的に応答するという非線形性に共通点がある。研究課題/領域番号:02246206, 研究期間(年度):1990出典：研究課題「プリュ-ム発生の非線形力学」課題番号02246206（KAKEN：科学研究費助成事業データベース（国立情報学研究所）） （https://kaken.nii.ac.jp/ja/grant/KAKENHI-PROJECT-02246206/）を加工して作