Rock Behavior Under Stress PDF

How rocks behave under different types of stress  Let's define.... Strain & stress What is Stress The forces acting on rock are called stress. Stress is the force applied on a rock per unit stress area and Strain...

How rocks behave under different types of stress  Let's define.... Strain & stress What is Stress The forces acting on rock are called stress. Stress is the force applied on a rock per unit stress area and Strain When rocks deform they are said to strain. strain? A strain is a change in size, shape, or volume of a material.  tensional Three types compressional of stress shear stress  Tension stresses act in Tensional opposite directions, pulling rock apart or stress stretching it This happens as two plates shift farther away from each other Compessional This is when rock is pressed, squeezed stress or push together. When crustal platess collide, rocks are compressed or pushed. Shear Shearing is stress that pushes rocks in opposite stress directions. Shearing makes rocks break, slip apart, or change shape. Shearing happens when two plates slip past each other in opposite directions. What is deformation?  Deformation is the result of stresses that change the shape of rocks. Deformation includes faulting of rigid rocks and folding of rocks that can be bent. Rocks respond to stress differently depending on the pressure and temperature and mineralogical composition of the rock. The ability of the rock to handle stress depends on the elasticity of the rock  Elastic deformation: The rock returns to nearly its original Stages ofsize and shape when the stress is removed. deformation - Once the elastic limit (strength) of a rock is surpassed, it either bends (ductile deformation) or breaks (brittle deformation  Ductile deformation Stages of occurs when enough stress is applied to a deformation material that the changes in its shape are permanent, and the material is no longer able to revert to its original shape.  Brittle deformation or Fracture: Near the Earth’s Stages of surface rock behaves in its familiar brittle fashion. If a deformation differential stress is applied that is greater than the rock’s yield strength, the rock fractures. Fracture us an irreversible strain wherein the rock breaks.  Temperature Factors Confining pressure affecting deformation Strain Rate Composition  Temperature At high temperature molecules and their Factors bonds can stretch and affecting move, thus materials will deformation behave in more ductile manner. At low Temperature, materials are brittle.  Confining pressure At high confining pressure materials are less likely to Factors fracture because the affecting pressure of the surroundings tends to hinder the formation deformation of fractures. At low confining stress, material will be brittle and tend to fracture sooner.  Strain Rate At high strain rates material Factors tends to fracture. At low affecting strain rates more time is deformation available for individual atoms to move and therefore ductile behavior is favored.  Composition Some minerals are very brittle. Factors This is due to the chemical bond types that hold them affecting together. Thus, the deformation mineralogical composition of the rock wil be a factor in determining the deformational befiavior of the rock  Joints The Joints are fractures in rocks that show little or no movement at all. The orientation of the joints can be described as strike and dip and are from as a result of tensional stress acting perpendicular to the orientation of the produced joint on a brittle rock. Joints They provide pathways for water and thus promote chemical weathering.  Faults On the other hand, faults are extremely long and deep break or large crack in a rock as a result of continuous pulling and pushing. There are different types of faults, Dip-slip fault, strike-slip fault and ductile formation.  Dip-slip fault Different types of Strike-slip fault faults Ductile deformation  Dip-slip fault Dip-slip fault or normal fault occurs when brittle rocks are stretched-tectonic tensional forces are involved and the movement of blocks of rock is mainly in the vertical direction (sinking and rising). For dip-slip faults, the block lying on top of the fault surface is referred to as the hanging wall while the one below is referred to as the footwall.  Dip-slip fault Normal faults tend to dip about 600. The hanging wall has moved downward relative to footwall. Normal faults are the chief structural components of many sedimentary rift basins like the North Sea where they have major significance for hydrocarbon exploration. Strike-slip fault occurs when brittle rocks are sheared (the opposing tectonic forces are at right angles to compression and tension directions) and the movement of blocks of rocks is chiefly horizontal direction Strike-slip fault If the far side of the fault moves to the left relative to an observer it is called “sinistral strikeslip fault” (left-lateral). If the far side of fault moves to the right relative to an observer is called “dextral strike-slip fault (right-lateral).  Ductile Rocks buried deep within the Earth’s crust behave deformation differently when subjected to differential stress. It is impossible to produce fracture in rocks the way it is at the Earth’s surface. Rocks become thicker under compressional stress and thinner under tensional stress. Rock layers tend to bend an go out of shape.  Ductile deformation The high temperature condition makes a rock softer, less brittle and more ductile. Ductile deformation is an irreversible strain which means that the rock cannot go back to its original condition; instead it is changed into a new shape.  Ductile When rocks deform in a ductile manner, instead of fracturing to deformation form faults or joints, they may bend or fold and the resulting structure are called folds. Folds are promoted by high temperature and pressure at great depth. There are several kinds of folds: monoclines; synclines and anticlines. Kinds of folds a. Monoclines are the simplest types of folds. It occurs when the horizontal layers are bent upward so that the two limbs of the fold are still horizontal. b. Synclines are fold structures when the original rock layer have been folded downward and the two limbs of the fold dip inward toward the hinge of the fold. Kinds of folds Monocline Syncline Antincline THANK YOU!!!!