the sea floor

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the sea floor
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seafloor
water covers 70 of Earths surface
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seafloor
deep seafloor largely unknown prior to 1950s
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seafloor
oceans originated mostly from volcanic
de-gassing of water vapor from Earths
interior
additional small amount may have come from late
comet impacts after 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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direct methods
rock dredges
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direct methods
sediment corer
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direct methods
sea floor sediment core
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JOIDES Resolution (1990s-being overhauled)
DSDP (Deep Sea Drilling Project) ODP (Ocean
Drilling Project) IODP (Integrated Ocean
Drilling Project)
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Chikyu Japanese drill ship
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direct methods
submersibles
manned or unmanned
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indirect methods
sonar (sound navigation and ranging)
sound sent from ship, bounced off sea
floor, and recorded at ship
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)
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indirect methods
known for a long time that sound travels through
water
1822 attempt to determine speed of sound in water
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indirect methods
seismic reflection
penetration of sediments by sound waves
hydrophones record signals
echo sounding, swath bathymetry, sidescan
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indirect methods
sea floor profile
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indirect methods
South Pacific sea floor
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seafloor, continents, and plate boundaries
yellow lines are plate boundaries
sea floor was critical in development of plate
tectonics
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features of the seafloor
general profile through ocean
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)
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continental shelf and slope
broad, shallow shelf (100-200 m water depth)
steeper slope dives to abyssal plain
topographic profile has 25x vertical
exaggeration (vertical and horizontal scales are
not the same)
slope angle is only 4-5
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passive margin
NO 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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submarine canyons
landslide triggered by earthquake
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
sediments deposited by turbidity currents and
contour currents move along elevation contours
abyssal plain flattest region on Earth form
where turbidity currents bury
features
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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)
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active margins (trenches-plates converge)
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mid-ocean ridge (plate boundary-plates diverge)
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mid-ocean ridge
80,000 km long 1,500-2,500 km wide
elevations of 2,000-3,000 m above sea floor
rift valley 1,000 m deep at crest of ridge
axial valley
NORTH AMERICA
AFRICA
sea floor spreading (divergence)
from http//www.geo.duke.edu/geo41/sfs.htm
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mid-ocean ridge
basalt flows and volcanism
high heat flow and small, shallow earthquakes
hot springs supporting biological communities
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black smoker (first ever seen) 1979
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life at oceanic ridge
tube worms
spider crab
giant clams
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explore using submersibles
ALVIN was first one 3 passenger
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mid-ocean ridge
exposed on-land in Iceland
both from http//pubs.usgs.gov/publications/text
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mid-ocean ridge
transform faults
offset of mid-ocean ridge between adjacent
ridges --earthquakes occur along them (red
stars)--
fracture zones
continuation of transform fault beyond
ridge --no eqs--
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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?pid7545tid441
cid49514ct61article29566
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other sea floor features
seamount
conical mountain that rises gt 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
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seamount chains and ages of seamounts in one (hot
spot track -- more later)
Emperor seamounts
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sea floor sediments
terrigenous derived from land and brought to sea
floor sands/silts that make up continental
rise
pelagic accumulate by settling through water
column clays from wind skeletons of
microsopic organisms
sea floor spreading leads to greater thickness
of pelagic sediments away from ridge crest (no
sediment at mid-ocean ridge)
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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)
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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