Download Physiography of the Ocean Floor Distribution of topography and

Physiography of the Ocean Floor
Take away the water and what do you see?
Distribution of topography and
bathymetry
Bimodal
distribution
Average land
elevation 0.84 km
Average ocean
depth 3.8 km
Deepest depth
(Mariana Trench)
more extreme than
highest height
(Mt. Everest)
Earth’s dominant topographic features lie beneath the ocean’s surface.
Ocean provinces
The submarine “landscape,” or sea floor, can be subdivided into
three distinct provinces:
continental margins
~22% of total area
deep ocean basins
~42% of total area
mid-ocean ridges
~31% of total area
(remaining ~5% of total
area occupied by deep-sea
trenches)
Continental margins
submerged edges of
the continents
massive wedges of
sediment eroded from
the land and deposited
along continental edge
wide, flat in Atlantic
thin, steep in Pacific
can be divided into
three parts:
continental shelf – continuation of land below sea surface (slope < 1o)
continental slope – where margin breaks (slope ~ 4o); often contain
huge submarine canyons where sediment cascades down
continental rise – sedimentary wedge at base of slope (slope 1o or less)
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Continental margins
Deep ocean basins
submarine canyons
Monterey
Canyon
Grand
Canyon
- 6,672 ft.
Submarine canyon
off Barcelona, Spain
between the
continental
margins and the
mid-oceanic ridge
includes a variety
of features from
mountainous to
flat plains:
abyssal plains – flattest parts of the world
abyssal hills – elongated dome-shaped hills of oceanic crust
seamounts – abyssal mountains, largely volcanic (active and extinct);
includes flat-topped guyots formed by wave erosion
deep sea trenches – deepest regions on Earth, found close to land
Mid-ocean ridges
continuous
submarine
mountain range
extends for about
60,000 km around
the Earth
contains rift valley
in center of ridge
site of new oceanic
crust formation
flanked by
transform faults and
fracture zones
Mapping ocean depths
Prior to the early 20th
century, “soundings”
were the only means to
determine water depth
weighted lines lowered
from ships
time-consuming, relatively
few, doubtful accuracy
HMS Challenger one of
the first to do this
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Mapping ocean depths
Mapping ocean depths
Matthew Maury (the “father of oceanography”) constructed
the first ocean-wide bathymetric map (N. Atlantic, 1855).
Echo Sounder
Seismic Reflection
Profiler (sound source
and hydrophone)
Bathymetric maps
depict the topography
of the seafloor;
isobaths connect points
of equal elevation.
Mapping ocean depths
echo sounders
sound source and receiver
(hydrophone) on hull of ship
high frequency sound waves
travel through the water, reflect
off the seafloor (and off subbottom sediment layers), and are
recorded by the hydrophone
based on travel time (t) of sound
waves with known velocity (v),
get depth [d = v x (t/2)]
provide continuous depth profiles
along a ship’s cruise track
but only 2-dimensional profile
(horizontal, vertical)
vs.
Mapping ocean depths
seismic reflection
profilers
sound source and
hydrophone towed by ship
continuous but also only
2-dimensional profile
lower frequency energy
source (stronger sound
source with fewer sound
waves per second)
deeper penetration into
sedimentary layers and ocean
crust
seismic
reflection survey
guyot
seamount
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Mapping ocean depths
Mapping ocean depths
Multi-beam sonar
and
Side scan sonar
Seabeam Multibeam bathymetry
like echo sounder, but
images a 60 km swath
of seafloor
overlapping swaths =
complete coverage
(3-dimensional)
produces 3-dimensional
bathymetric map of
seafloor
Sea ice
keel scour
Mapping ocean depths
Mapping ocean depths
Satellites
bathymetric map based on a
seabeam survey for Deep
Sea Drilling Project sites
off northwest Africa
(contour interval = 50 m);
note the very steep
Mazagan Escarpment
dropping off to the deep-sea
Precise altimeters (using
microwaves) can map the relief
of the ocean surface with
centimeter-scale resolution.
isobaths
Altimetry uses satellites to
determine bathymetry based
upon slight changes in the
elevation of the sea surface
Gravitational attraction of large
rock masses on the seafloor
distorts the ocean surface, very
slightly but enough to be
measured from space!
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Mapping ocean depths
North
Atlantic
Ocean
relief
from
satellite
altimetry
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