Secondary and tertiary creep of glacier ice as measured by borehole closure rates
Secondary and tertiary creep of glacier ice as measured by borehole closure rates
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DOI：
10.1029/rg015i001p00047
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发表时间：
1977-02
影响因子：
25.2
通讯作者：
W. Paterson
中科院分区：
文献类型：
--
作者：
W. Paterson
已发表的和以前未发表的测量封闭率在极地冰盖的五个钻孔进行了审查。数据涵盖0.15至1.0 MN m-2之间的有效剪应力、-16 °C至-28 °C之间的温度以及高达2.2的应变。在钻孔壁的应变曲线（孔直径与其初始直径之比的对数）随时间的变化显示了一个阶段的恒定闭合速率对应于二次（稳态）蠕变的冰，然后由加速闭合速率归因于重结晶的冰（三级蠕变）。低应力曲线也显示了闭合速率下降的初始瞬态阶段。第三蠕变的开始主要取决于应变;临界值范围从0.03到0.10，温度越低，临界值越高。二次蠕变速率在不同的钻孔是一致的，彼此的数据产生的蠕变激活能为54千焦/摩尔和流动定律指数接近3。将钻孔数据简化到-22 °C的普通温度，并与该温度下的两个实验室实验结果进行比较。对于给定的应力，Steinemann（1958 a，B）测得的应变率是钻孔中应变率的2-3倍，而对于巴恩斯等人（1971）的实验，应变率系数约为8。实验室和冰川冰之间的差异，可能在粒度上，可以解释钻孔数据和Steinemann的结果之间的差异。有证据表明，巴恩斯等人在此温度下测得的蠕变速率可能包含瞬态蠕变的重要组成部分;这可能是他们的结果与Steinemann的结果之间存在较大差异的原因。第三蠕变速率与第二蠕变速率之比随应变近似线性增加。即使在1.5的应变下也没有观察到稳态第三蠕变速率，在该点处第三蠕变速率与第二蠕变速率的比率为约10。然而，在钻孔闭合期间，冰没有均匀地应变。即使钻孔壁附近的冰已经完成了再结晶，更远的冰由于应变较小，仍然可能再结晶。这可能是未能观察到稳态第三蠕变的原因。在一个钻孔的底部附近，蠕变速率（第三级）大约是其上方冰中蠕变速率的4倍。两个变形区之间的边界密切对应于威斯康星州冰川作用期间沉积的冰和自那时以来沉积的冰之间的边界。威斯康星州冰中的晶体比其他地方的更小，尺寸变化更小，更接近等维。此外，威斯康星州冰具有高得多的微粒含量和低得多的海洋来源的盐含量。这表明，一个或多个这些差异使威斯康星州冰"软"比其余的冰。晶粒尺寸的减小被认为是最可能的因素。
Published and previously unpublished measurements of closure rates of five boreholes in polar ice caps are reviewed. The data cover effective shear stresses between 0.15 and 1.0 MN m−2, temperatures between −16° and −28°C, and strains up to 2.2. Curves of strain at the borehole wall (logarithm of the ratio of hole diameter to its initial diameter) against time show a stage of constant closure rate corresponding to secondary (steady state) creep of the ice followed by accelerating closure rate attributed to recrystallization of the ice (tertiary creep). Curves for low stresses also show an initial transient stage of decreasing closure rate. The onset of tertiary creep is largely determined by the strain; critical values range from 0.03 to 0.10, and the lower the temperature, the higher the critical value. Secondary creep rates in the different boreholes are consistent with each other; the data yield a creep activation energy of 54 kJ/mol and a flow law index close to 3. The borehole data reduced to a common temperature of −22°C are compared with the results of two laboratory experiments at this temperature. For a given stress the strain rates measured by Steinemann (1958a, b) are 2–3 times those in the boreholes, and for the experiments of Barnes et al. (1971) the factor is about 8. Differences between laboratory and glacier ice, probably in grain size, may explain the differences between the borehole data and the results of Steinemann. Some evidence is presented that the creep rates measured by Barnes et al. at this temperature may contain a significant component of transient creep; this might account for the large difference between their results and those of Steinemann. The ratio of tertiary to secondary creep rate increases approximately linearly with the strain. No steady state tertiary creep rate is observed even at a strain of 1.5, at which point the ratio of tertiary to secondary creep rate is about 10. However, the ice is not strained uniformly during borehole closure. Even if recrystallization has been completed in the ice near the borehole wall, the ice further away, having been strained less, may still be recrystallizing. This may account for the failure to observe steady state tertiary creep. Near the bottom of one borehole, creep rates (tertiary) are about 4 times those in the ice immediately above. The boundary between the two deformation regions corresponds closely to the boundary between ice deposited during the Wisconsin glaciation and ice deposited since that time. The crystals in the Wisconsin ice are smaller, much less variable in size, and more nearly equidimensional than those elsewhere. Moreover, the Wisconsin ice has a much higher microparticle content and a much lower content of salts of marine origin. It is suggested that one or more of these differences make the Wisconsin ice ‘softer’ than the remainder of the ice. The decrease in grain size is considered to be the most likely factor.