the sea floor!seafloor!• water covers 70% of Earth’s surface!seafloor!• deep seafloor largely !unknown prior to 1950’s!seafloor map from Dana (1894)!seafloor!• oceans originated mostly from volcanic de-gassing! of water vapor from Earth’s interior!additional small amount may have come from late comet impacts!after the Earth reached close to its current mass!studying the seafloor!direct methods!• rock dredges!• sea floor drilling!• submersibles!indirect methods!• sonar!• seismic reflection profiling!rock !dredges!direct !methods!sediment!corer!direct !methods!sea floor!sediment!core!direct !methods!JOIDES Resolution (1990’s-being overhauled)!DSDP (Deep Sea Drilling Project); ODP (Ocean Drilling Project)!…IODP (Integrated Ocean Drilling Project)…!Chikyu!Japanese drill! ship!submersibles!• manned or unmanned !direct methods!sonar !(sound navigation and ranging)!• sound sent from ship,!bounced off sea floor,!and recorded at ship !indirect methods!distance to seafloor!is calculated from!speed of sound in water!multiplied by!time to get return signal!divided by two!(wave goes down and up)!known for a long time that sound travels through water!1822 attempt to determine speed of sound in water !indirect methods!seismic reflection!indirect methods!• penetration of sediments by sound waves!• hydrophones record signals!echo sounding, swath bathymetry, sidescan!sea floor profile!indirect methods!South Pacific sea floor!indirect methods!sea floor was critical in development of plate tectonics!yellow lines are plate boundaries!seafloor, continents, and plate boundaries!general profile through ocean!features of the seafloor!from left to right!shelf, slope, abyssal plain, mid-oceanic ridge!seamounts, trench, slope shelf!passive continental margin!(no plate boundary)!active continental margin!(plate boundary)!mid-oceanic ridge!(plate boundary)!slope!angle is!only 4-5°!continental shelf and slope!topographic profile has 25x vertical exaggeration!(vertical and horizontal scales are not the same)!• broad, shallow shelf (100-200 m water depth)!• steeper slope dives to abyssal plain!passive margin!NO plate boundary at edge of continent!• shelf and slope!• continental rise ! (less steep than slope)!• abyssal plain ! (smooth, deep seafloor)!submarine canyons and abyssal fans!• start on shelf and end at base of slope!• allow for transport of sediment from shelf to sea floor!sand falls offshore Baja, California!submarine canyons and abyssal fans (California)!“turbidity currents” flow down canyons and deposit on fans!offshore southern California!landslide triggered ! by earthquake!submarine canyons!cable breaks in different! locations at different! times as landslide arrives!continental rises and abyssal plains !continental rise: gently sloping wedge of sediment of!" " sediment at base of slope!abyssal plain: flattest region on Earth; form where! turbidity currents bury features!sediments deposited!by turbidity currents!and contour currents!move along!elevation contours!active margin!plate boundary at edge of continent!• shelf and slope!• oceanic trench ! (deepest features in ocean)!• volcanoes (on-land) !Wadati-Benioff zone!--dipping zone of!earthquakes that!begin at trench!and extend landward!(red stars)!active margins (trenches-plates converge)!mid-ocean ridge (plate boundary-plates diverge)!NORTH AMERICA! AFRICA!sea floor spreading (divergence)!axial valley!from: http://www.geo.duke.edu/geo41/sfs.htm!• 80,000 km long; 1,500-2,500 km wide!mid-ocean ridge!• elevations of 2,000-3,000 m above sea floor!• rift valley ~1,000 m deep at crest of ridge!• basalt flows and volcanism!mid-ocean ridge!• high heat flow and small, shallow earthquakes!• hot springs supporting biological communities!black smoker!(first ever seen)!1979!life at oceanic ridge!tube worms !giant clams !spider crab !explore using submersibles!ALVIN was first one; 3 passenger!both from: http://pubs.usgs.gov/publications/text!• exposed on-land in Iceland!mid-ocean ridge!transform faults!mid-ocean ridge!fracture zones!• continuation of!transform fault!beyond ridge!--no eq’s--!• offset of mid-ocean ridge between adjacent ridges! --earthquakes occur along them (red stars)--!from: http://www.whoi.edu/page.do?pid=7545&tid=441&cid=49514&ct=61&article=29566!other sea floor features!seamount!conical mountain that!rises > 1,000 m!above sea floor;!basaltic volcanoes;!chains of seamounts!occur!(aseismic ridges)!(Emperor seamounts)!guyot!flat-topped seamount;!erosion from waves;!reefs common around!them!seamount chains and ages of seamounts in one!(hot spot track -- more later)!Emperor seamounts!pelagic: accumulate by settling through water column! …clays from wind; skeletons of microscopic organisms…!sea floor spreading leads to greater thickness of!pelagic sediments away from ridge crest!(no sediment at mid-ocean ridge)!sea floor sediments!terrigenous: derived from land and brought to sea floor! …sands/silts that make up continental rise…!composition of the oceanic crust!seismic surveys !suggest!~ 7 km thick !with 3 layers!1) marine sediments (sampled)!2) pillow basalts (sampled)!3) gabbros (not sampled)!(intrusive equivalent to basalt)!pillow basalts!resources of the ocean!offshore drilling!mining the ocean floor? manganese