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Bathymetry
of the
Ocean Floor
The ocean floor is mapped by SONAR.
(Sound navigation and ranging)
Depth = (time x 1500 m/sec)/2 (round trip)
At 25 degrees Celsius
Relationship above and below the water
Ocean Floor Topography
 Major features: continental shelf, trenches, abyssal
plains, mid-ocean ridges.
 After 1920’s, these features could be mapped in detail
by sonar (bouncing sound waves off the ocean floor).
Continental Shelf
 Area of shallow water along passive continental margin (not an
active plate boundary).
 Continental Margin – consists of continental crust (think shallow end of pool)
 Shelf drops off abruptly at outer edge.
 Much of shelf was above sea level during past glacial periods.
Continental Rise
 gentle slope at the base of the continental slope caused
by:
- turbidity currents - deposition of sediment by
underwater landslides and other processes that
carry mud, sand & silt down the slope
- occurs at the base of the slopes & gently slope
seaward to the deep sea floor
Submarine Canyons
 Submarine Canyons - ripple marks observed on the
floor of submerged canyons and sediments fanning out
at the end suggest they were formed by moving
sediments and water
- as flow reaches bottom, it slows down, fans out
and the sediment settles out
 Submarine canyons cut through the shelves and slopes,
look like river valleys on land.
Trenches
 Occur along active margins (present-day plate
boundaries) where subduction is taking place
 deepest part of the ocean floor, typically 3 - 4 km
deeper than surrounding seafloor
 most occur in the Pacific, mostly western Pacific, but most of
the Pacific is surrounded by trenches
 deepest spot in the oceans is the Challenger Deep in the
Marianas Trench, 11,035 m
 trenches are associated with active volcanoes and earthquakes
 most are near chains of volcanic islands
Deep Ocean Basin - Abyssal
Plane
Flat areas making up much of the ocean
floor apart from trenches and mid-ocean
ridges
Deep Ocean Basin - seaward of the
continental margin
 In most places, seafloor is a flat plain - Abyssal Plain
 covered by sediment deposits of turbidity currents
covering an irregular seafloor plain interrupted by:
 seamounts - individual mountains made of volcanic material.
Rise steeply, sometimes above the surface to form islands.
 guyots - (flat topped seamounts found most often in the Pacific)
 table mounts, - many have ancient coral reefs on top indicating that
they were once at the surface
 - flat tops due to wind & rain erosion
 - subsided due to their own weight & crustal movement
Mid-Ocean Ridges
Largest topographic features on
earth.
Broad ridges (where plates pull
apart from one another) with a
deep rift valley down the center.
Rift Valley-forms between the
ridges of mountains.
Location where basaltic lava
flows erupt.
Age of the ocean floor
 Detailed studies in 1950’s showed that
there are bands of similar aged rocks
parallel to mid-ocean ridges.
 Bands get older with greater distance from
the ridges.
Basins
Abyssal plains
Seamount
Guyot
Trenches
Mid Ocean ridges
Rift Valley