![]() ![]() Strike-slip faults tend to occur along the boundaries of plates that are sliding past each other. The San Andreas Fault-made infamous by theġ906 San Francisco earthquake-is a strike-slip fault. There are three main types of faults, based on how adjacent blocks of rock move Shift depends on the type of fault the movement occurs in. They'll suddenly slip, making a big move all at once. Too slowly for us to notice, but sometimes stress (pressure) builds up and Usually, the rocks on either side of the fracture are moving past each other These cracks may be small and localized or can stretch thousands of miles where tectonic plates meet. Most earthquakes occur along cracks in the planet's surface called faults. Photo: Macelwane Archives, Saint Louis Universityįrom an earthquake's point of view, there's more than one way to devastate a city. 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.The force of the two sides of the San AndreasĪult sliding past each other bent railroad tracks between Los Gatos and Santa Cruz in 1906. 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. 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. There are three basic types of stress that deform rocks: 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. ![]()
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