Geology Study Guide Part 3 The Ocean Floor Mid Ocean Ridge where new crust is created diverging plates plates move away from each other creating gap between the two plates magma rises to fill gap and is cooled forming new crust sea floor spreading starts here most important feature of earths surface 70 of volcanic activity happens here 1 3 of heat lost from earths interior is lost here Abyssal Hills mainly faulted oceanic crust with thin sediment if any crust moving away from ridge is cold and sinks ocean depth increasing faulted crust is slowly being covered by oceanic sediments crust created at ridge moves away and gets older as you move further away as it gets older it gets colder and heavier denser and literally sinks as it moves away depth increases as you move away from ridge faulted oceanic crust that is exposed in this area is known as abyssal hill Abyssal Plains oceanic material in which sediment covers oceanic crust made at the ridge smooth flattest portions of the ocean floor very thick pockets of sediment sediment from above falls onto AH and faulted crust and covers over faulted materials Deep Ocean Trenches occur at convergent plate boundaries where oceanic lithosphere is being subducted or diving down beneath other material long linear deepest depth ing the oceans occur here no trenches occur off the east cost of the US don t happen in middle of ocean occur on boundaries of continents where plates are converging Seamounts and Guyotes periodically magma will rise and explode onto areas of MOR Seamount underwater volcanoes occur individually or as chains Guyotes flat topped seamounts erosion by waves when seamount was near at surface indicated that at one point was at surface then rode plate and sunk western part of the pacific ocean Continental Margins transition between deep ocean floor and land divided into 3 zones continental shelf slope and rise a Continental Shelf shallow portion of the ocean extends from 100 300 m gentle slopes can be exposed as land during glacial time global sea level is lower b Continental Slope extends from shelf to water depth approaching 3000m steep slopes submarine canyons can form here b c of turbidity currents dissect continental shelf and slopes rivers cut into the continental shelf at times when sea level was low and shelf was exposed what begins canyon but canyons at such great depth depth occurs bc of turbidity currents avalanches of sediment 30x size of grand canyon turbidity currents sediment laden current that flow down slope instable steep slope things fall down and current picks it up c continental rise from slope abyssal plain 3000 4000m gentle slopes slowing turbidity drops sediment submarine fan like alluvial fan Oceanic Sediments a terrigenous sediments weathered rock on land that is transported to the oceans and deposited turbidite deposits b pelagic sediments from chemical rxn in ocean that form sediments calcite from shells Earthquakes and Earth s Interior Earthquake sudden release of energy from the earth stresses build up in rock due to plate tectonic forces acting on the rock when built up stresses overcome frictional forces rock moves and releases energy Focus actual location where earthquake occurred defined by long x lat y and depth z Epicenter surface location directly above focus position long and lat or geographic location epicenter is NOT where the most destruction occurs energy travels outward as waves seismic waves waves displace rock material as it passes by Body Waves travel within the earth two major types p waves travel by displacing rocks back and forth in the direction that wave travels compressional waves or primary waves can travel through liquid b c it can be compressd fastest traveling seismic waves highest velocity s waves travel at same time as P but move particles in different direction as it encounters rock it moves material perpendicular to the traveling wave cannot travel through liquid slower than p waves shear wave or secondary wave L waves love ground ripples back and forth snake like R waves Rayleigh cause ground to ripple up and down Surface waves travel with combo of P and S wave motion complex motion that travels even slower than P and S have higher amplitude and greater displacement creates the most destruction p wave occurs first then s back and forth then side to side can use difference in P and S waves to locate where quake occurs Seismograph can systematically record ground displacement two basic configurations one for up and down one for back and forth motion How to calculate where earthquake occurred 1 measure change in time between first p and s wave 2 plot difference on curve and determine how far away the station is from quake 3 plot distance determined in step 2 as radius away from location of seismograoh quake has occurred somewhere in the circle 4 repeat at least 2 more times where circles meet denotes epicenter most earthquakes occur at boundaries between lithospheric plates where plates used to be or where boundary may be in the future Earthquake Intensity physical destruction caused by the earthquake Earthquake magnitude measure of energy released by earthquake by looking at amp of waves Modified Mercalli Scale describes intensity not good measure of size b c intensity can cause more damage in different places Richter Scale measures magnitude logarithmic not linear measures energy released by earthquake by relating amp of waves to amount of energy as magnitude increases by a factor of 1 amplitude increases by factor of 10 and energy released is 30 times greater ex what is difference between M5 and M2 105 2 103 amplitude is 1000 times greater 305 2 303 energy is 27 000 times greater Tsunami series of water eaves caused by displacement of large volumes of water ocean can be created by earthquakes volcanic erup or underwater slides turbidity currents as waves approach shallow waters velocity slows down and height increases energy concentrated and wave height dramatically increases propagate as a set of waves whose energy is concentrated at wavelengths corresponding to the earth movements 100km at wave heights determined by vertical displacement 1m and at wave directions determined by the adjacent coastline 4 major layers of the earth a crust outermost layer continental crust 20 40km thick felsic in composition granite rhyolite less dense waves travel faster through CC than OC oceanic crust 0 10km thick mafic in composition gabbro basalt more Mg and Fe more dense b