{"id":524,"date":"2019-06-11T14:51:27","date_gmt":"2019-06-11T14:51:27","guid":{"rendered":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/12-1-stress-and-strain\/"},"modified":"2021-12-08T18:07:53","modified_gmt":"2021-12-08T18:07:53","slug":"12-1-stress-and-strain","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/physicalgeology2ed\/chapter\/12-1-stress-and-strain\/","title":{"raw":"12.1 Stress and Strain","rendered":"12.1 Stress and Strain"},"content":{"raw":"Rocks are subject to [pb_glossary id=\"1689\"]stress[\/pb_glossary]<\/strong>\u2014mostly related to plate tectonics but also to the weight of overlying rocks\u2014and their response to that stress is [pb_glossary id=\"1690\"]strain[\/pb_glossary]<\/strong> (deformation).\u00a0 In regions close to where plates are converging stress is typically compressive\u2014the rocks are being squeezed.\u00a0 Where plates are diverging the stress is extensive\u2014rocks are being pulled apart.\u00a0 At transform plate boundaries, where plates are moving side by side there is sideways or [pb_glossary id=\"1691\"]shear stress[\/pb_glossary]<\/strong>\u2014meaning that there are forces in opposite directions parallel to a plane. Rocks have highly varying strain responses to stress because of their different compositions and physical properties, and because temperature is a big factor and rock temperatures within the crust can vary greatly.\r\n\r\nWe can describe the stress applied to a rock by breaking it down into three dimensions\u2014all at right angles to one-another (Figure 12.1.1).\u00a0If the rock is subject only to the pressure of burial, the stresses in all three directions will likely be the same.\u00a0 If it is subject to both burial and tectonic forces, the pressures will be different in different directions.\r\n\r\n[caption id=\"attachment_521\" align=\"aligncenter\" width=\"700\"]\"\"<\/a> Figure 12.1.1 Depiction of the stress applied to rocks within the crust.\u00a0The stress can be broken down into three components.\u00a0Assuming that we\u2019re looking down in this case, the green arrows represent north-south stress, the red arrows represent east-west stress, and the blue arrows (the one underneath is not visible) represent up-down stress. On the left, all of the stress components are the same.\u00a0On the right, the north-south stress is least and the up-down stress is greatest.[\/caption]\r\n\r\n[caption id=\"attachment_522\" align=\"alignright\" width=\"600\"]\"\"<\/a> Figure 12.1.2 The varying types of response of geological materials to stress.\u00a0The straight dashed parts are elastic strain and the curved parts are plastic strain.\u00a0In each case the X marks where the material fractures. A, the strongest material, deforms relatively little and breaks at a high stress level.\u00a0B, strong but brittle, shows no plastic deformation and breaks after relatively little elastic deformation.\u00a0C, the most deformable, breaks only after significant elastic and plastic strain. \u00a0The three deformation diagrams on the right show A and C before breaking and B after breaking.[\/caption]\r\n\r\nRock can respond to stress in three ways: it can deform elastically, it can deform plastically, and it can break or fracture.\u00a0 Elastic strain is reversible; if the stress is removed, the rock will return to its original shape just like a rubber band that is stretched and released.\u00a0Plastic strain is not reversible.\u00a0As already noted, different rocks at different temperatures will behave in different ways to stress. Higher temperatures lead to more plastic behaviour.\u00a0Some rocks or sediments\u00a0are also more plastic when they are wet. \u00a0Another factor is the rate at which the stress is applied.\u00a0 If the stress is applied quickly (for example, because of an extraterrestrial impact or an earthquake), there will be an increased tendency for the rock to fracture.\u00a0Some different types of strain response are illustrated in Figure 12.1.2.\r\n\r\nThe outcomes of placing rock under stress are highly variable, but they include fracturing, tilting and folding, stretching and squeezing, and faulting.\u00a0A fracture is a simple break that does not involve significant movement of the rock on either side.\u00a0Fracturing is particularly common in volcanic rock, which shrinks as it cools.\u00a0The basalt columns in Figure 12.1.3a are a good example of fracture.\u00a0Beds are sometimes tilted by tectonic forces, as shown in Figure 12.1.3b, or folded as shown in Figure 12.0.1.\r\n\r\n[caption id=\"attachment_523\" align=\"aligncenter\" width=\"800\"]\"\"<\/a> Figure 12.1.3 Rock structures caused by various types of strain within rocks that have been stressed. (A) Fracturing in basalt near to Whistler, BC; (B) Tilting of sedimentary rock near to Exshaw, Alberta; (C) Stretching of limestone at Quadra Island, BC. The light grey rock is limestone and the dark rock is chert. The body of rock has been stretched parallel to bedding. The chert, which is not elastic, has broken into fragments which are called boudins; (D) Faulting within shale beds at McAbee, near to Cache Creek, BC. The fault runs from the lower right to the upper left, and the upper rock body has been pushed up and to the left.[\/caption]\r\n\r\nWhen a body of rock is compressed in one direction it is typically extended (or stretched) in another.\u00a0 This is an important concept because some geological structures only form under compression, while others only form under tension. Most of the rock in Figure 12.1.3c is limestone, which is relatively easily deformed when heated.\u00a0The dark rock is chert, which remains brittle. As the limestone stretched (parallel to the hammer handle) the brittle chert was forced to break into fragments to accommodate the change in shape of the body of rock.\u00a0A fault is a rock boundary along which the rocks on either side have been displaced relative to each other (Figure 12.1.3d).\r\n

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