![]() ![]() In a syncline the youngest beds, the ones that were originally on top of the rest of the beds, are at the center, along the axis of the fold.Īnticlines and synclines form in sections of the crust that are undergoing compression, places where the crust is being pushed together. In map view, a syncline appears as a set of parallel beds that dip toward the center. The axis is an imaginary line that marks the center of the fold on the map. In an anticline, the oldest beds, the ones that were originally underneath the other beds, are at the center, along the axis of the fold. Fault, Fold, Dip, Strike, Joint (Geology) Introduction Dip and Strike Folds Faults Joints Structural geology is the study of the factors such as origin, occurrence, classification, type and effects of various secondary structures like folds, faults, joints, rock cleavage etc. In map view, an anticline appears as parallel beds of the same rock type that dip away from the center of the fold. Keep in mind that erosion has stripped away the upper parts of these structures so that map view reveals the interior of these structures. Use the block diagrams to visualize the three-dimensional shapes of the geologic structures. The colored layers represent stratified geologic formations that were originally horizontal, such as sedimentary beds or lava flows. The other two visible sides of the box are cross-sections, vertical slices through the crust. In block diagrams like those shown below, the top of the block is the horizontal surface of the earth, the map view. In terms of geologic structures, the up folds are called anticlines and the down folds are called synclines. Imagine a rug, the sides of which have been pushed toward each other forming ridges and valleys – the ridges are “up” folds and the valleys are “down” folds. ![]() The most basic types of folds are anticlines and synclines. Now let us look at the specific types of geologic structures, the breaks and bends that deform rock in response to stress. ![]() They “flow,” or bend in a plastic manner, at a geological pace. In fact, rocks deep in the continental crust and upper mantle can be so hot and soft that they behave almost like a slow-moving liquid, even though they are actually solid. The heat and pressure cause deep crustal and mantle rocks to be ductile. In the deep crust and in the earth’s mantle, rocks are very hot and subject to high pressure caused by the weight of the overlying rock. The break along which the rocks slide back to their original shape is a fault.Įarthquakes and faults occur in the shallow crust, where rocks are relatively cold and therefore brittle. The rocks on either side of a break act like rubber bands and snap back into their original shape. Rocks get bent in an elastic fashion until they reach their limit, then they break in brittle fashion. If a rock bends and stays bent after stress is released, it is said to undergo plastic behavior.Ī combination of elastic and brittle behavior causes earthquakes. If a rock bends but is able to return to its original position when the stress is released, it is said to undergo elastic behavior. If rocks tend to bend without breaking, they are said to be ductile. If a rock breaks, it is said to undergo brittle behavior. If rocks tend to break, they are said to be brittle. The bending or breaking of rock is called deformation or strain. In response to stress, rocks will undergo some form of bending or breaking, or both. Stratigraphic sections should be oriented perpendicular to depositional strike (dip or transverse section). There are three basic types of stress that deform rocks: In development geology, this information comes largely from well data (geophysical logs, mudlogs, and cores), but in some places, outcrops and seismic reflection data can be used to constrain interpretations. Stress refers to the physical forces that cause rocks to deform. PHYSICAL BEHAVIOR OF ROCKS: STRESS AND GEOLOGIC STRUCTURESīefore exploring geologic structures, we need to look at how rocks respond to the forces that create the structures. Table showing types of stress and resulting strain: Type of StressĪssociated Plate Boundary type (see Ch.\) Shear stress involves transverse forces the strain shows up as opposing blocks or regions of the material moving past each other. Compressional stress involves forces pushing together, and the compressional strain shows up as rock folding and thickening. Tensional stress involves forces pulling in opposite directions, which results in strain that stretches and thins rock. There are three types of stress: tensional, compressional, and shear. Strain in rocks can be represented as a change in rock volume and/or rock shape, as well as fracturing the rock. When the applied stress is greater than the internal strength of rock, strain results in the form of deformation of the rock caused by the stress. Stress is the force exerted per unit area and strain is the physical change that results in response to that force. Clockwise from top left: tensional stress, compressional stress, and shear stress, and some examples of resulting strain. ![]()
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