Download The Sea Floor

the sea floor
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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
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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
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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)
indirect methods
known for a long time that sound travels through water
1822 attempt to determine speed of sound in water
indirect methods
seismic reflection
• penetration of sediments by sound waves
• hydrophones record signals
echo sounding, swath bathymetry, sidescan
indirect methods
sea floor profile
indirect methods
South Pacific sea floor
seafloor, continents, and plate boundaries
yellow lines are plate boundaries
sea floor was critical in development of plate tectonics
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)
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°
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
submarine canyons
landslide triggered
by earthquake
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
sediments deposited
by turbidity currents
and contour currents
move along
elevation contours
abyssal plain: flattest region on Earth; form where
turbidity currents bury features
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)
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
mid-ocean ridge
• basalt flows and volcanism
• high heat flow and small, shallow earthquakes
• hot springs supporting biological communities
black smoker
(first ever seen)
1979
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tube worms
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life at oceanic ridge
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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
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 eq’s--
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
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
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)
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 nodules