the sea floor

choppedspleenMécanique

21 févr. 2014 (il y a 3 années et 6 mois)

66 vue(s)

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)
--

transform fault
--
fracture zone animation

green

are ridge segments;

red

is transform fault

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 microsopic 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 nodules