专栏61冰川纹的形态和冰川磨损的推论
Iverson(1991)利用最近暴露在加拿大萨斯喀彻温冰川附近的碳酸盐岩基岩上的冰川纹，对冰川的力学进行推断冰川磨蚀．艾弗森测量了单个纹纹的宽度、深度和长度，并对一系列形态学标准进行了观察。条纹的形状表明，磨损碎片通常是旋转的。艾弗森定义了三种类型的冲突。第1型条纹在冰川下游逐渐变宽、变深，直到突然结束，通常是深而陡的壁沟。它们被推断为在纹状碎屑向前和向下犁犁时形成的，在纹状点断开碎屑或碎屑上的扭矩足够大以至于它旋转出沟槽之前。2型纹开始和结束时都是微弱而稀薄的痕迹。它们稳定地扩大和加深，直到在中心点附近达到最大宽度和深度。它们可能是由滑动时旋转的尖锐纹状点形成的。该点最初有一个大的犁角，导致渐进式切口的条纹。 Deeper ploughing causes more rapid clast rotation as the torque on the clast increases. Rotation, together with comminution of the clast point, reduces the ploughing angle so that there is a steady reduction in striation depth. Consequently Type 2 striae indicate clasts that slow down until the maximum striation depth is reached and then steadily accelerate, until at the striation terminus, the clast has the same velocity as the ice. Type 3 striae begin abruptly as deep gouges and then become progressively narrower and shallower down-glacier. They are inferred to form where a striator point contacts and indents the bed. Clast rotation with little displacement along the bed produces a low ploughing angle, so that a gradual reduction in indentation depth occurs as sliding proceeds. Overall, the conclusion is that clasts with steep leading edges will abrade progressively deeper into the bed with sliding, whereas those with more gently inclined leading edges will climb out of their grooves. The paper shows how, therefore, glacial striae can be used to make inferences about former subglacial processes.
资料来源:Iverson, N.R.(1991)冰川条纹的形态——对冰川床层和断层表面磨损的影响。美国地质学会通报，103,1308-16。
条纹的连续性是有效法向压力(使冰川底部与河床保持接触)和基础水压变化(使小空腔形成)之间的平衡(见第4.6节)。由不同的冰流方向形成的条纹可能以横切图案叠加(横切条纹;图6.2)。这种情况发生在冰流方向改变时，要么是由于冰在冰川消退地区的推进，要么是由于冰川内冰流方向的变化。横切的条痕记录了这样一个事实，即第二次冰流无法侵蚀第一次冰流的所有证据，要么是因为冰川磨损的效率较低，要么是因为时间不够。因此，横切条纹的出现可以用来推断前冰川动力学(框6.2)。
继续阅读:专栏62利用冰川侵蚀地貌重建冰川动态
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