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the sea floor
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seafloor• water covers 70% of Earth’s surface
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seafloor
• deep seafloor largely unknown prior to 1950’s
seafloor map from Dana (1894)
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seafloor• oceans originated mostly from volcanic de-gassing of water vapor from Earth’s interior
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additional small amount may have come from late comet impactsafter the Earth reached close to its current mass
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studying the seafloor
direct methods
• rock dredges
• sea floor drilling
• submersibles
indirect methods
• sonar
• seismic reflection profiling
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rock dredges
direct methods
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sedimentcorer
direct methods
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sea floorsediment
core
direct methods
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JOIDES Resolution (1990’s-being overhauled)
DSDP (Deep Sea Drilling Project); ODP (Ocean Drilling Project)…IODP (Integrated Ocean Drilling Project)…
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ChikyuJapanese drill ship
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submersibles
• manned or unmanned
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direct methods
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sonar (sound navigation and ranging)
• sound sent from ship,bounced off sea floor,and recorded at ship
indirect methods
distance to seaflooris calculated from
speed of sound in watermultiplied by
time to get return signaldivided by two
(wave goes down and up)
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known for a long time that sound travels through water
1822 attempt to determine speed of sound in water
indirect methods
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seismic reflectionindirect methods
• penetration of sediments by sound waves• hydrophones record signals
echo sounding, swath bathymetry, sidescan
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sea floor profile
indirect methods
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South Pacific sea floorindirect methods
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sea floor was critical in development of plate tectonics
yellow lines are plate boundariesseafloor, continents, and plate boundaries
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general profile through ocean
features of the seafloor
from left to right
shelf, slope, abyssal plain, mid-oceanic ridgeseamounts, trench, slope shelf
passive continental margin(no plate boundary)
active continental margin(plate boundary)
mid-oceanic ridge(plate boundary)
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slopeangle 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
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passive marginNO plate boundary at edge of continent
• shelf and slope• continental rise (less steep than slope)• abyssal plain (smooth, deep seafloor)
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submarine canyons and abyssal fans• start on shelf and end at base of slope• allow for transport of sediment from shelf to sea floor
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sand falls offshore Baja, California
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submarine canyons and abyssal fans (California)
“turbidity currents” flow down canyons and deposit on fans
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offshore southern California
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landslide triggered by earthquake
submarine canyons
cable breaks in different locations at different times as landslide arrives
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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 depositedby turbidity currentsand contour currents
move alongelevation contours
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active marginplate boundary at edge of continent
• shelf and slope• oceanic trench (deepest features in ocean)• volcanoes (on-land)
Wadati-Benioff zone--dipping zone ofearthquakes thatbegin at trenchand extend landward(red stars)
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active margins (trenches-plates converge)
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mid-ocean ridge (plate boundary-plates diverge)
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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 widemid-ocean ridge
• elevations of 2,000-3,000 m above sea floor• rift valley ~1,000 m deep at crest of ridge
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• basalt flows and volcanismmid-ocean ridge
• high heat flow and small, shallow earthquakes• hot springs supporting biological communities
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black smoker
(first ever seen)
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life at oceanic ridgetube worms
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giant clams
spider crab
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explore using submersibles
ALVIN was first one; 3 passenger
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both from: http://pubs.usgs.gov/publications/text
• exposed on-land in Iceland
mid-ocean ridge
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transform faultsmid-ocean ridge
fracture zones• continuation oftransform faultbeyond ridge--no eq’s--
• offset of mid-ocean ridge between adjacent ridges --earthquakes occur along them (red stars)--
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transform fault--fracture zone animation
green are ridge segments; red is transform fault
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from: http://www.whoi.edu/page.do?pid=7545&tid=441&cid=49514&ct=61&article=29566
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other sea floor featuresseamountconical mountain that
rises > 1,000 mabove sea floor;
basaltic volcanoes;chains of seamounts
occur(aseismic ridges)
(Emperor seamounts)
guyotflat-topped seamount;erosion from waves;
reefs common aroundthem
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seamount chains and ages of seamounts in one(hot spot track -- more later)
Emperor seamounts
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pelagic: accumulate by settling through water column …clays from wind; skeletons of microsopic organisms…
sea floor spreading leads to greater thickness ofpelagic sediments away from ridge crest
(no sediment at mid-ocean ridge)
sea floor sedimentsterrigenous: derived from land and brought to sea floor …sands/silts that make up continental rise…
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composition of the oceanic crustseismic surveys
suggest~ 7 km thick with 3 layers
1) marine sediments (sampled)
2) pillow basalts (sampled)
3) gabbros (not sampled)(intrusive equivalent to basalt)
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pillow basalts
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resources of the ocean
offshore drilling
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mining the ocean floor? manganese nodules