![]() ![]() Specifically, because, a particular event will create deformation structures with similar orientations, the orientation of the deformation structures give clues to the sequence of deformation events. Geologic map of northern Ontario, with a cross-section Geological Survey of Canada, “A” Series Map 1263A, 1969, 1 sheet, (Open Access)ĭeformation information is also collected from outcrops and displayed on geologic maps because this information is import to understanding the forces that acted on the rock as well as the sequence of events leading up to its present morphology. Furthermore, the geologic map can be used to create a geologic cross – section (Figure 5.4), which shows what the rocks would look like if you were able to cut a slice through Earth’s crust like a red velvet layer cake. The geologic information and the inferences are recorded on a geologic map. Rocks exposed in outcrops at Earth’s surface are used to infer the lateral and vertical extent of rock formations. (b) Rock layer folding indicating ductile deformation 5.3 Mapping Geologic Deformation Structures (a) Fault lines indicating brittle deformation of sandstone layers. Ultimately, it will be the net effect of all three controlling factors that will determine whether a particular rock will deform as a brittle or ductile solid (Figure 5.3). However, wood can be bent into curved shapes for construction purposes through a process that requires slow application of force to encourage bending and prevent breaking. ![]() For example, wood is brittle at room temperature and tends to break if a large force is applied quickly. The faster force is applied, the greater the likelihood of the rock breaking in a brittle fashion, while a slow application of force can allow the rock to deform in a ductile manner. Deformation Rate The rate at which force is applied will also influence whether the rock will undergo brittle or ductile deformation.That means that a particular rock could deform in a brittle manner closer to the surface but the same rock could undergo ductile deformation deeper in the crust, where it is hotter. As a result, the deeper rock is in the crust, the more it will behave in a ductile manner in response to an incident force. Temperature Rock deeper in Earth’s crust is subjected to higher temperatures than rock that is shallower.If the minerals are primarily soft or platy, then deformation will tend to be ductile. Generally speaking, if the rock is made up primarily of minerals that are hard (6 or above on Moh’s scale) and non-platy, then it will tend to deform in a brittle manner. ![]() Rock Type The type of rock will influence the way it deforms in response to forces acting on it.Furthermore, the rate at which the force is applied will influence how the rock will deform, too. However, temperature also influences the nature of the rock and therefore also influences the way rocks will deform. Ductile deformation means the rock accommodates a force by “flowing” instead of breaking. Brittle deformation means the rock cannot accommodate a force by flowing but will instead “snap,” or break. Rocks will deform in brittle or ductile manner in response to the tectonic forces acting on them. Outcrop showing deformation of sedimentary rocks on the island of Crete, Greece 5.2 Rock Deformation It is from outcrops that we gather the geologic information required to identify the rock and the processes that form and deform it (Figure 5.2). In order to observe the effect of these forces on rocks, the rocks must be exposed at the surface in an outcrop. Time-lapse of the deformation caused by compressive forces from continent–continent convergenceĬlick on the image to load the animation. Location of compressional, tensional, and shear tectonic forces Animation 5.1. Crust is conserved that is, it is neither extended or shortened (Figure 5.1 and Animation 5.1). Shearing forces are typically associated with transform fault plate boundaries but can be associated with spreading centres with a steplike divergent boundary. Compressional forces occur around convergent plate boundaries and lead to the shortening of the crust, while tensional forces occur around divergent boundaries and lead to an extension of the crust. ![]()
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