1. The Ocean Floor
Ch. 23

2. Studying the ocean floor
Submersibles, satellites, and other technology allow scientists to study the structure and composition of the ocean floor.

3. 1. Echo sounding
a system that uses transmitted and reflected sound waves to measure distances to the ocean floor; SONAR.
measure how long it takes for signal to be emitted and come back
multi-beam echo sounding measures area twice as wide as ship
use info to make seafloor maps
intensity of sound beams determine seafloor composition
rock & gravel reflect more strongly than mud

4. Echo sounding

5. 2. Sediment Sampling
Core sampling: hollow cylinder removes long cores of material from seafloor
layers are preserved
1-1500m of sediment are gathered
can analyze past climate, life, and ocean events

6. 3. Satellite observations
greater range & speed for mapping
doesn’t reach floor, bounces off surface
Ocean surface varies based on what’s below
higher over mountains, lower over trenches
determines differences down to cm’s
makes high-resolution seafloor image

7. HIGH RESOLU-TION SEAFLOOR IMAGE
Pacific and Western Atlantic Oceans

8. The CONTINENTAL MARGIN
In order to understand the continental margin, we must first understand the crust of the Earth.
The continental margin is part of the crust.

9. LAYERS OF THE earth the Earth has 4 main physical layers:
Depth from Surface (km)
Temperature (K)
State of Matter
Composition
*Crust
0-65
<1000, increases w/ depth
Solid
Rock we live on
Si, O, Al
*Mantle
increases
Solid w/ liquid properties
Fe, Si, Mg
Outer Core
increases
Liquid
Fe & Ni
Inner Core
approx. 6000
solid

10. LAYERS OF THE earth

11. two types of crust Continental Crust Oceanic Crust
lighter, less dense rock
Makes up all continents, but not necessarily all islands
Oceanic Crust
darker, dense rock
any ocean floor or ocean basin

12. THE MOVING CRUST all of the crust is divided into ‘pieces’ or plates
(Called lithospheric plates or tectonic plates)
the plates move around on the semi-solid mantle
where the plates meet or connect are called plate boundaries
at plate boundaries, the crust can be moving side by side, apart, or together
different topographic features are created, depending on the type of boundary
topography: the shape of the land

13. World plate boundaries

14. The CONTINENTAL MARGIN
The underwater portion of the continental crust.
There are two types of continental margins.

15. 1. Active CONTINENTAL margins
continental margins that occur along plate boundaries
plates are moving side by side or together
when one plate sinks under another, a trench is formed
when plates move past each other, a fault is formed
continental rise is small or nonexistent
rocky, short beach w/ cliffs
rugged, coastal mountains on land
EX: West coasts of North America and South America

16. 2. Passive continental margins
continental margins that don’t occur along plate boundaries
broad continental shelf
long, sandy beach
no trenches, mountains, or faults
EX: East coast of North America

17. Parts of the continental margin
continental shelf: part of the continent that extends from the shoreline to the continental slope
flat, lengths vary depending on location
continental slope: begins at the shelf edge where depth decreases rapidly to the rise.
20km long; descends 3.6km
sediment builds up temporarily, then falls
continental rise: descends gradually from the slope to the ocean floor
considered part of ocean basin
very long & gradual

18. Submarine canyon
an undersea gully that cuts across the continental shelf and slope
rivers erode the continental shelf and deposit sediment on continental slope
gravity and powerful turbidity currents carry sediment down to the continental rise
Very powerful agents of erosion
coarse particles settle first, then fine such as clay

19. Ocean Basin
Abyssal Plain – flattest of all Earth’s surfaces, composed of sediment from continents
Occur in all oceans
More in Atlantic Ocean where there are fewer trenches
Abyssal hills – small hills, occur in groups next to oceanic ridge systems

20. Deep-sea trenches
Deep-sea trenches – long, narrow, steep-sided troughs that run parallel to continental margins or to volcanic island chains called island arcs
Exist at subduction zones
Common sites of earthquakes and volcanic activity

21. Deep-sea trenches
A marginal trench forms if one plate is oceanic and the other is continental.
If the oceanic plate descends below the continental, a line of volcanoes stands on the overriding continental plate, forming mountain chains
If both plates are oceanic, an arc of volcanic islands forms on the overriding plate
Usually in western Pacific

22. Deep-sea trenches

23. Vents and ridges
Deep ocean vent – geyser that erupts underwater, mixing hot and cold water and bringing up minerals from beneath the surface
Mid-ocean ridges- are
undersea mountain ranges,
form at divergent plate
boundaries where 2 plates
are moving apart and
magma is rising

24. Sea mounts & guyots
Seamounts are cone-shaped mountain peaks that rise high above the ocean floor
Occur in all oceans, but more abundant in Pacific Ocean
Volcanic in origin (ex. Hawaiian islands)
Guyots are flat-topped seamounts
Waves removed their tops when they rose above sea level

25. Coral &coral Atolls
Corals are tiny sea animals that live in shallow, warm waters. Reefs form when new corals grow on top of the dead ones.
A coral atoll is a ring-shaped coral island. It forms when a coral reef develops around a volcanic island. The mountain sinks below the water, leaving a circular reef with a lagoon where the mountain was.

26. Ocean Floor Sediments
Sediment reaches the ocean floor in several ways: turbidity currents, fall from above, settle from glaciers, remains of microscopic shells
Terrigenous sediments – come from continental rocks an minerals broken down from weathering an erosion
wash into rivers and out to sea
may come from glaciers
breaking and dropping
into sea

27. Ocean Floor Sediments
Biogenous sediments come from living sources; they are oozes made mostly of shells and skeletons from tiny marine animals
Calcareous ooze=calcium carbonate
Most common from shells
and skeletons
Dissolve as they sink below
4500 meters
Siliceous ooze=silicon dioxide
More common around
Equator and Antartica

28. Ocean Floor Sediments
Hydrogenous sediments form when chemical reactions cause minerals to crystallized from seawater.
Manganese nodules are most common – has manganese, iron oxide, nickel, cobalt, copper…
They form on the sea floor from sediment that falls on them and mixes with the sea water, a few mm every million years
Important to humans but hard to get

29. Importance of sediments
By studying the layers of sediment in the ocean, scientists can determine:
the extent of former polar ice sheets
the history of water temperatures on sea floor
the pasts behaviors of prevailing winds
pattern of changes in Earth’s climate
The sediments, unique organisms, magnetic records, industrial resources on the ocean floor represent a past look at Earth’s hydrosphere, biosphere, and geosphere.

30. Ocean floor