‘Glacial Striations’ – ‘Plucking & Glaciar Abrasion’
Glacial Landscapes | ‘Svartisen og Engebreen’ in Helgeland, Norway
Fotos : Marc Ihle 2016
Plucking, also referred to as quarrying, is a glacial phenomenon that is responsible for the erosion and transportation of individual pieces of bedrock, especially large “joint blocks”. This occurs in a type of glacier called a “valley glacier”. As a glacier moves down a valley, friction causes the basal ice of the glacier to melt and infiltrate joints (cracks) in the bedrock. The freezing and thawing action of the ice enlarges, widens, or causes further cracks in the bedrock as it changes volume across the ice/water phase transition (a form of hydraulic wedging), gradually loosening the rock between the joints. This produces large pieces of rock called joint blocks. Eventually these joint blocks come loose and become trapped in the glacier.
In this way, plucking has been linked to regelation. Rocks of all sizes can become trapped in the bottom of the glacier. Joint blocks up to three meters have been “plucked” and transported. These entrained rock fragments can also cause abrasion along the subsequent bedrock and walls. Plucking also leads to chatter marks, wedge shaped indentations left on the bedrock or other rock surfaces. Glacial plucking both exploits pre-existing fractures in the bedrock and requires continued fracturing to maintain the cycle of erosion. Glacial plucking is most significant where the rock surface is well jointed or fractured or where it contains exposed bed planes, as this allows meltwater and clasts to penetrate more easily.
Plucking of bedrock also occurs in steep upland rivers, and shares a number of similarities with glacial examples. In such cases, the loosening and detachment of blocks appears to result from a combination of (1) chemical and physical weathering along joints, (2) hydraulic wedging driven by smaller rock fragments getting into existing cracks, (3) crack propagation from stresses caused by impacts of large clasts already in transport by the river, and possibly (4) crack propagation driven by flexing resulting from pressure variation in the overlying water during floods. Loosened blocks are then carried away by fast flowing water during large floods, though the entrainment is believed to be significantly less efficient than the equivalent ability of ice to carry away blocks under glaciers.