Deformation Deformation is a general term that refers to all changes in the original form and or size of a rock body Most crustal deformation occurs along plate margins Deformation involves Force that which tends to put stationary objects in motion or changes the motions of moving objects Stress force applied to a given area Strain changes in the shape or size of a rock body caused by stress Types of stress Compressional stress shortens a rock body Tensional stress tends to elongate or pull apart a rock unit Shear stress produces a motion similar to slippage that occurs between individual playing cards when the top of the stack is moved relative to the bottom How rocks deform Rocks subjected to stresses greater than their own strength begin to deform by folding flowing or fracturing Stress 1 Compressional stress differential stresses that shorten a rock body 2 Tensional stress stresses acting in opposite directions that tend to elongate or pull apart a rock body 3 Shear slippage of slabs of rock or minerals past each other like a deck of cards Stress Compression Tensional Shear Types of Strain Deformation elastic deformation changes in size and shape of a rock unit that are reversible like a rubber band plastic deformation changes in size and shape of a rock unit that are permanent or not reversible through folding and flowing brittle deformation changes in size and shape of a rock unit that are permanent through fracture and faulting ductile rocks that deform due to folding and flowing high temps confining pressures weaker bonding brittle rocks that exceed their elastic limit will behave like brittle solid and fracture low temps confining pressures stronger bonding Faults Types of dip slip faults Normal fault Hanging wall moves down relative to the footwall Accommodate lengthening or extension of the crust Most are small with displacements of a meter or so Larger scale normal faults are associated with structures called faultblock mountains Types of dip slip faults Reverse and thrust faults Hanging wall block moves up relative to the footwall block o Reverse faults have dips greater than 45 and thrust faults have dips less o than 45 Accommodate shortening of the crust Strong compressional forces Strike slip fault Dominant displacement is horizontal and parallel to the strike of the fault Types of strike slip faults Right lateral as you face the fault the opposite side of the fault moves to the right Left lateral as you face the fault the opposite side of the fault moves to the left A right lateral strike slip fault Strike slip fault Transform fault Large strike slip fault that cuts through the lithosphere Accommodates motion between two large crustal plates Joints Among the most common rock structures Technically a joint is a fracture with no movement Most occur in roughly parallel groups Dome Yosemite NP California Devil s Tower Wyoming Significance of joints Many important mineral deposits are emplaced along joint systems Highly jointed rocks often represent a risk to construction projects Chemical weathering tends to be concentrated along joints Folds During crustal deformation rocks are often bent into a series of wave like undulations called folds Most folds result from compressional stresses which shorten and thicken the crust Parts of a fold Limbs refers to the two sides of a fold Axis a line drawn down the points of maximum curvature of each layer Axial plane an imaginary surface that divides a fold symmetrically Common types of folds Anticline upfolded or arched rock layers Syncline downfolds or troughs of rock layers Depending on their orientation anticlines and synclines can be described as Symmetrical asymmetrical recumbent an overturned fold or plunging anticline a type of fold in which the rocks are arched upwards syncline a type of fold in which the rock layers are bowed down monocline possesses only one limb plunging anticline the nose points in the direction that it is plunging plunging syncline the nose points in the opposite direction of the plunge dome upwarping of layers to produce a circular or elongate structure basin downwarping of layers to produce a circular or elongate low Earthquakes Intensity a qualitative evaluation Degree of shaking Based on observed damage Magnitude a quantitative evaluation Estimate of energy released Data from seismographs Both are used to describe the strength of an earthquake Ritcher Scale a measure of earthquake magnitude determined by measuring the amplitude of largest wave recorded on the seismogram magnitude a measure of the amount of energy released during an earthquake Earthquake vibration of earths produced by a rapid release of energy Focus point in earth where energy if released Epicenter the location on the earth s surface directly above the earthquake focus Surface Waves earthquakes energy waves that travel along the outer part of the earth Body waves earthquakes energy waves that travel through earth s interior Predict earthquake seismic gaps areas in a fault zone with no seismic activity over a long time period such as a century Seismic waves P waves primary or compressional P waves have faster velocity P waves travel through solids and liquids elastic S waves secondary or shear S waves only solids liquids have no shear strength flow Left reflection Right refraction Mechanical Layers outer core a liquid metallic layer 2270km thick this zone is capable of convective flow Inner core a solid sphere having a radius of 1216km S WAVES Mohorovicic discontinuity Moho differences in arrival times of deep and shallow seismic waves Divergent Boundaries Passive continental margins Found along most coastal areas that surround the Atlantic ocean Features comprising a passive continental margin Continental shelf Continental slope Continental rise Active continental margins Continental slope descends abruptly into a deep ocean trench Abyssal plains Likely the most level places on Earth Seamounts Isolated volcanic peaks Four distinct layers Layer 1 sequence of unconsolidated sediments Layer 2 consisting of pillow lavas Layer 3 numerous interconnected dikes called sheet dikes Layer 4 gabbro in a sequence of rocks called an ophiolite complex Convergent Boundaries Orogenesis the processes that collectively produce a mountain belt Major features of subduction zones Forearc region is the area between the trench and the volcanic arc Backarc region is located on the side of the volcanic arc opposite the trench