GLY 326 Exam 1

-Explain how our understanding of deformation is related to stratigraphy Deformation alters the arrangement, orientation, etc. which is recorded in the rock record. Compression, tension, and shear stress are the forces responsible for the folding, faulting, and titling of rock layers. -Why does the surface of the Earth experience stress? Tectonic plates, convergent boundaries cause compressional stress on each other, divergent boundaries cause tensional stress as the lithosphere is stretched, transform boundaries cause shear stress along faults. -How is Earth’s structure able to drive motion of the lithosphere? The Earth is layered, and the lithosphere is the rigid outer layer, beneath is the asthenosphere which is semi-fluid. The heat flow from the interior of the Earth drives mantle convection. -How do we quantify deformation? Strain analysis -What are the different rigid and nonrigid transformations? Rigid deformation consists of translation and rotation. Nonrigid deformation consists of dilation and distortion. -Where do we see outcrop/regional examples of transformations on Earth? Near plate boundaries. Transform boundaries we can observe lateral displacement from the horizontal motion. Convergent boundaries we can observe from mountain ranges, due to the uplift. -How do we quantify displacement? Displacement = x f − x i -How can an ellipse be used to show strain? Because when an object undergoes deformation, its shape can change, and the change in shape can be described using the strain ellipses. An object is deformed by applying stress in the Y direction, causing shortening along Y, and extension along X. -What causes the differences between coaxial and noncoaxial deformation? Coaxial deformation occurs when all particles move parallel to one another during deformation, so the material maintains its original shape while changing in size. This can occur in areas of high pressure and temperature. Noncoaxial deformation occurs when particles within a material do not move parallel, instead they move relative along parallel planes, causing a change in shape as well as in size. This can occur in areas with lower pressures. -What are the different forces acting on the lithosphere in subduction zones and rift zones? See notes below. -How does resistance in the mantle affect the motions of tectonic plates? Viscosity of the mantle has a large impact on the motions of tectonic plates. A higher mantle viscosity means the plates will move slower, due to greater resistance. A lower mantle viscosity means the plates will move faster, due to less resistance. -How do we determine the normal and shear tractions on a body of rock? Normal traction acts perpendicular to the surface. It’s equal to on of the principal stresses, either sigma 1 or sigma 3. Shear traction acts along the surface, calculated with the difference between the two principal stresses. | sigma 1 – sigma 3 | / 2 -What are the ways traction is expressed in the Earth? The movement of plate tectonics is the primary expression of traction. -How can we use stress equations to determine the stresses on a surface? Define the problem (axial, shear, or torsional), Identify stress components. Normal stress = elastic module * strain. Shear stress = shear modulus * shear strain -How can a stress ellipse show the different stresses on a body? Deformation , change in shape or structure of a rock due to stress that is placed on it.

F = ma Stress = force/area = pressure Rigid Body Deformation, each point within the body of rock maintains the same location related to the other points in the body of rock during the deformation. Nonrigid Body Deformation , the spacing of points within the body of rock change with respect to one another during the deformation Displacement Vector, final position – initial position, then can construct a displacement field. Displacement Field, the whole array of displacement vectors for a given setting. Rigid Transformations Translation: points have not changed locations with respect to each other, each traveled along the same vector path. Change in position, no change in size, no change in shape. Ex: sliding of fault blocks Rotation: point do no change location with respect to each other but do with respect to the start; each point travels along a different vector path. A change in the orientation of a body of rock. No change in size, no change in shape. Sense , ex clockwise or counterclockwise. Magnitude of rotation , ex 45° Nonrigid Transformations Dilation: points all get closer to each other, or all move away from each other – but orientation remains the same. Distortion: points change orientation and distance with respect to each other. Strain, the change in shape and/or size of a body, this is the nonrigid component of deformation . Dilation, A change in the size of a body of rock, without a change in shape. In pure dilation, there is no change in shape, just a change in size. Ex: Boudins, start from originally continuous layer of rock that is deformed so it’s no longer horizontal. Distortion, A change in the shape of a body of rock. Takes place through systematic change in the spacing arrangement between material points in the object. In pure distortion, there is no change in size, just a change in shape. Extension: e = l 1 − l 0 l 0 = ∆l l 0 , extension will have a positive value when |l|>0, when the deformation produces an elongation, negative for |l|<0, for shortening. Stretch: S = l f l 0 , S = e + 1, stretch is the measure of the change in the length of a line. %Lengthening (or %Shortening): e *100% or (S-1)*100% Normal Stress: when a rock is being compressed, it is because forces on opposite sides of the rock are oriented directly toward each other. Strain: change in shape and/or size of a body – the nonrigid component of deformation. Expressed as dilation or distortion. Strain Ellipse: quantified by the major axis S 1 , direction of greatest extension (or maximum stretch) , the minor axis S 3 , the direction of least extension. Plane Strain, restricted to 2-dimensions, with no stretching or shortening in the direction perpendicular to the plane containing the maximum and minimum stretch directions. Motion is restricted to parallel planes. An object is deformed by applying stress in

Overriding plate resistance, R o , counters the slab pull due to friction between subducting and overlying plates. Slab-drag resistance, R SD , overlying slab motion is slowed by friction. This resistance treats the asthenosphere more as a solid. The friction is between the surfaces of the ocean lithosphere and the asthenosphere. Mantle drag, R DO ∧ R DC , counters each plate’s movement towards subduction, friction between lithosphere and asthenosphere. Bending resistance, R B , counters the slab pull and negative buoyancy of the subducting slab. The weight and density of the ocean plate force it down, but as a rigid solid it resists that force to bend. R B is not friction , is more like the force of a spring, or bending a pencil or a plastic frisbee. In some cases, R B can be greatly exceeded to cause a plate to break. Forces in a Rift Zone: the ocean ridge is pushed open by rising magma. Hot, fluid asthenosphere pushes up through plate boundary forming magma. Ridge push, F RP , a force resulting from gravity acting down the slope of the ridge, causing the material to slump downward, pushing the slab away from the ridge. This force drives the expansion of ocean plates, pushing them towards subduction zones. Slab pull and ridge push can be viewed as the main drivers of plate motion on Earth’s surface. Ridge resistance, R R , a force that acts directly opposite of the force of ridge push, slowing the spreading rate. Mantle drag under ocean, R DO , is the friction of the oceanic plate against the asthenosphere. This assumes the mantle is behaving more as a solid. Ridge push, F RP , a force resulting from gravity acting down the slope of the ridge, causing the material to slump downward, pushing the slab away from the ridge. This force drives the expansion of ocean plates, pushing them towards subduction zones. Traction = Force Area in units of pascals (N/ m 2 ) Buoyancy, F b =− ρgV