The sea floor. seafloor water covers 70% of Earth’s surface

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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

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additional small amount may have come from late comet impactsafter 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

sedimentcorer

direct methods

sea floorsediment

core

direct methods

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

submersibles

• manned or unmanned

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direct methods

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)

known for a long time that sound travels through water

1822 attempt to determine speed of sound in water

indirect methods

seismic reflectionindirect methods

• penetration of sediments by sound waves• hydrophones record signals

echo sounding, swath bathymetry, sidescan

sea floor profile

indirect methods

South Pacific sea floorindirect methods

sea floor was critical in development of plate tectonics

yellow lines are plate boundariesseafloor, continents, and plate boundaries

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)

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

passive marginNO 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 depositedby turbidity currentsand contour currents

move alongelevation contours

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)

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 widemid-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 volcanismmid-ocean ridge

• high heat flow and small, shallow earthquakes• hot springs supporting biological communities

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

both from: http://pubs.usgs.gov/publications/text

• exposed on-land in Iceland

mid-ocean ridge

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)--

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 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

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 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…

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)

pillow basalts

resources of the ocean

offshore drilling

mining the ocean floor? manganese nodules