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首页> 外文期刊>Earth and Planetary Science Letters: A Letter Journal Devoted to the Development in Time of the Earth and Planetary System >Modelling the emplacement of compound lava flows
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Modelling the emplacement of compound lava flows

机译：模拟复合熔岩流的位置

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The physical variables controlling crust-dominated lava flow have been investigated using laboratory experiments in which molten polyglycol wax was extruded from a point source on to a horizontal plane under cold water. The wax initially spread axisymmpetrically and a crust of solid wax grew. Eventually wax broke out from the flow's periphery, sending out a flow lobe which in turn cooled and produced another breakout. The process repeated itself many times, building a 'compound lava'. The time for the first breakout to form correlates well with the theoretically predicted time (t_c) required for cooling to form a crust thick enough for its strength to limit the flow's spreading rate. This time is proportional to the product of effusion rate (Q) and initial magma viscosity (μ) and inversely proportional to the square of the crust strength at the flow front. The number of flow units and the apparent fractal dimension of the flow perimeter increase with time normalised by t_c. Our model illuminates the physical basis for the observation by Walker [G.P.L. Walker, Bull. Volcanol. 35 (1972) 579-590] that compound lava flows form by slow effusion of low viscosity magma, whereas faster effusion and higher viscosity favour lavas with fewer flow units. Because compound flows require t t_c, and given that t_c ∝ Qμ and the relationship between volume and effusion rate is V = Qt, simple and compound lava flows are predicted to fall in separate fields on a graph of μ against V/Q~2, all else being equal. Compound flows plot at small values of μ and large values of V/Q~2, with the position of the simple/compound boundary defined by field data implying a crust strength of order 10~4 Pa for basaltic to intermediate lavas. Whether a flow remains as a simple flow or matures into a compound flow field depends on the combined effect of viscosity, eruption rate and eruption duration (and hence volume) and these parameters need to be taken in to account when using morphology to infer eruption conditions. In particular, compound flows form from terrestrial subaerial point source eruptions on horizontal ground when μ ＜ 0.002V/Q~2.
机译：使用实验室实验已经研究了控制地壳为主的熔岩流的物理变量，其中将熔融的聚乙二醇蜡从点源挤出到冷水下的水平面上。蜡最初沿轴方向散布，并且固体蜡的外皮生长。最终，蜡从流的外围破裂，散发出流瓣，该流瓣又被冷却并产生另一个破裂。这个过程重复了很多次，形成了“复合熔岩”。第一次破裂形成的时间与冷却形成硬壳的厚度所需的理论预测时间（t_c）紧密相关，硬壳的强度足以限制流的扩散速度。此时间与积水率（Q）和初始岩浆粘度（μ）的乘积成正比，与流动前沿的地壳强度的平方成反比。流量单位的数量和流量周长的表观分形维数随时间以t_c标准化。我们的模型为沃克[G.P.L.沃克，公牛。火山[35（1972）579-590]认为，低粘度岩浆的缓慢喷出形成了复合熔岩流，而更快的喷出和较高的粘度有利于流动单元较少的熔岩。由于混合流需要t t_c，并且假设t_c ∝Qμ且体积与流出速率之间的关系为V = Qt，因此简单熔岩流和复合熔岩流预计将落在μ相对于V / Q〜的图上的不同字段中。 2，其他所有条件都一样。复合流以μ的小值和V / Q〜2的大值绘制，由现场数据定义的简单/复合边界位置暗示了从玄武岩到中层熔岩的地壳强度为10〜4 Pa。流动是保持为简单流动还是成熟到复合流场中取决于粘度，喷发速率和喷发持续时间（进而是体积）的综合影响，在使用形态学推断喷发条件时需要考虑这些参数。特别是，当μ＜0.002V / Q〜2时，地面地面点源喷发形成复合流。

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