Chapter 23 Notes
Chapter 23 Notes GEO 100
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Popular in Geology
This 6 page Class Notes was uploaded by samantha Flavell on Friday April 8, 2016. The Class Notes belongs to GEO 100 at State University of New York at Oswego taught by Rachel Lee (P) in Fall 2015. Since its upload, it has received 23 views. For similar materials see Physical Geology in Geology at State University of New York at Oswego.
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Date Created: 04/08/16
Why are the mountains high? surface elevation Is a balance between forces: isostasy gravitational attraction pulls plates into mantle buoyancy floats lithosphere on top of the mantle adding or removing weight resets isostatic equilibrium change in lithosphere thickness or density alters isostasy Convergent margin horizontal compression causes: horizontal shortening and vertical thickening these processes can double crustal thickness a thick crustal root develops beneath mountain ranges Isostasy the gravitational balance of masses at the Earth’s surface the balance is based on contrast in density the density of the crust is lower than that of the mantle Roots due to the low density of the crust relative to the mantle young mountain belts have deep roots Sediment loading Sediment erosion and loading will result in epeirogenic uplift and subsidence this model is consistent with what we see in the major river deltas of the world Other forms of crustal loading ice is less dense than rock, but pile enough up on the crust, and... isostasy must act to reestablish equilibrium A) glacier forms adding weight to crust B) subsidence due to weight of ice Glacier rebound C) ice melts, removing weight from crust D) crustal rebound as crust rises toward original position most of Scandinavia is experiencing rapid uplift, as a result of the removal of glacial ice over the 10,000 yr. Why are mountains high? Adding igneous rock can thicken the crust volcanic material. Is added to the surface plutons are added at mid crustal levels removal of lithospheric mantle can cause uplift the Tibet plateau bears evidence of delamination What goes up?.. mountains reflect a balance between uplift and erosion mountains are steep and jagged due to erosion rock characteristics control erosion resistant layers form cliffs easily eroded rocks form slopes .... Must come down the Himalayas are the maximum height possible, why? there is an upper limit to mountain heights erosion accelerates with height weight of high mountains overwhelm rock strength deep, hot rocks eventually overflow out from beneath mountains the mountains then collapse downward like soft cheese uplift, erosion and collapse exhumed deep crustal rocks Negative feedback because of the combination of gravity and erosion mountains on earth cannot grow too large nor can they last long Why is the erosion rate higher as relief increases? Increased erosion rate Lee's elevation tectonically raised level of elevation increases erosion rock The constant battle between uplift&erosion uplift greater than erosion, the mountains rise If uplift is in balance with erosion mountains remain at high elevation if uplift is less than erosion mountains lower If uplift is almost stopped erosion is slowed forming low hills no uplift slow erosion leads to low lands and plains Earthquakes and seismology What causes an earthquake? movement along a fault release energy sudden release of energy causes an earthquake seismic waves carry the energy from the origin Earthquakes result from sudden movements on faults any kind of fault can produce an earthquake Elastic rebound theory rocks will deform if subjected to stress if fault blocks move smoothly little stress is built up if faults are "locked" the rock becomes deformed Elastic energy is stored in the deformation of the rock Rocks near the surface are fairly brittle and break when elastic limit is exceeded fault blocks move suddenly and elastic energy is released causing an earthquake What causes an earthquake? Rocks are strained beyond their elastic limit and fracture the rocks return to their ' unstrained ' state and release energy Fault motion faults move in ' jumps ‘: stick slip behavior strain builds up as a rock reforms rock breaks (ruptures) causing motion motion stops quickly because of friction Earthquake locations the exact point at which movement along a fault occurred in some depth below the surface (focus or hypocenter) the location on the surface directly above the focus is the epicenter Seismic waves radiate in all directions from the focus focus can be shallow, intermediate or deep What Causes an Earthquake? *Hypocenter (focus) The place where fault slip occurs *Usually occurs on a fault surface *Earthquake waves expand outward from the hypocenter Seismic Waves *different kinds of waves deform rocks in different ways *Body waves: travel through interior *PWaves *SWaves *surface waves: travel along the surface *Lwaves *Raleigh waves Pwaves: Primary or Compressional Waves *Fastest velocity, arrive first at recording stations *Travel through solids and liquids *Movement/deformation parallel to wave path (longitudinal wave) *Alternating pulse of compression and expansion **Compression and expansion can knock buildings off of foundation SWaves: Secondary or Shear Waves *Travel slower, arrive second at recording stations *Deformational involvement perpendicular to the direction of travel (transverse wave) *Higher amplitude than pwaves *Shear waves cause foundations to shift from side to side *Changes the shape of materials *Brick or stone buildings may be fractured by the stress Surface Waves *Travel along the Earth’s surface *slower than P or S waves *Largest amplitude of all seismic waves Cause most of the damage *Surface waves travel along Earth’s exterior *Surface waves are the slowest and most destructive *Lwaves (love waves) *swaves that intersect the land surface *move the ground back and forth like a writhing snake *Surface waves travel along Earth’s exterior *Rwaves (Raleigh waves) *Cause the ground to ripple up and down like water *surface waves die out with depth *Up and down movement by surface waves can cause fractures in roadways *Underground water and gas pipes can fracture (a major one can cause fires) Seismometers (seismographs) *Inertia *(Newton’s first law) governs the movement of Earthquakes and the functioning of instruments to measure them: seismometers Reading a Seismogram *Pwaves travel fastest, and hence arrive first. *The difference in the arrival time of the Pand Swaves is proportional to the distance to the focus Quantifying Earthquake Intensity *Richter magnitude Scale: Calculated bases on the amplitude (height) of the recorded waves and distance to the source *Moment Magnitude (M): Calculated based on amount of energy released (distance of slip x area of slip x stiffness of rock) *Modified Mercalli intensity Scale (Roman Numerals) Estimated based on damage relative to selfcriteria
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