1. Oceanography An Invitation to Marine Science, 8th Edition Tom Garrison
Chapter 4:
Ocean Basins
Insert: Textbook cover photo

2. Topics to be Discussed The Ocean Floor Is Mapped by Bathymetry
Ocean-Floor Topography Varies with Location
Continental Margins May Be Active or Passive
The Topology of Deep-Ocean Basins Differs from That of Continental Margins
The Grand Tour

3. Chapter 4 Main Concepts Tectonics forces shape the seabed.
The ocean floor is divided into continental margins and deep- ocean basins. The continental margins are seaward extensions of the adjacent continents and are usually underlain by granite; the deep seabeds have different features and are usually underlain by basalt.
Continental margins may be active (earthquakes, volcanoes) or passive, depending on the local sense of plate movement.
The mid-ocean ridge system is perhaps Earth’s most prominent feature. Most of the water of the world ocean circulates through hot oceanic crust in the ridges about every 10 million years.
Using remote sensing methods, oceanographers have mapped the world ocean floor in surprising detail.

4. The Ocean Floor Is Mapped by Bathymetry
The discovery and study of ocean floor contours is called Bathymetry.
An illustration from the Challenger Report (1880): Seamen handling the steam winch aboard HMS Challenger used to lower a weight on the end of a line to the seabed to find ocean depth.

5. The Ocean Floor Is Mapped by Bathymetry
How did early scientists study the ocean floor?
Early bathymetric studies were often performed using a weighted line to measure the depth of the ocean floor.
Advances in Bathymetry
Echo sounding
Multi-beam Systems
Satellite Altimetry

6. Echo Sounders Bounce Sound off the Seabed
Echo sounding is a method of measuring seafloor depth using powerful sound pulses. The pulses of sound energy, or “pings,” from the sounder spread out in a narrow cone as they travel from the ship. When depth is great, the sounds reflect from a large area of seabed. Because the first sound of the returning echo is used to sense depth, measurements over deep depressions are often inaccurate.

7. ( ) Beam of sound waves travels to bottom and
is reflected back to ship
Depth = V T — 2
( )
Figure 4.2 Echo sounding.
Stepped Art
Figure 4-2a p115

8. Multi-beam Systems Combine Many Echo Sounders
Multi-beam systems provide more accurate measurements than echo sounders. Multi-beam systems collect data from up to 121 beams to measure the contours of the ocean floor.

9. Satellites Can Be Used to Map Seabed Contours
Satellite altimetry measures the sea surface height from orbit. Satellites can bounce 1,000 pulses of radar energy off the ocean surface every second.
With satellite altimetry, sea surface levels can be measured more accurately because the precise position of the satellite can be calculated. Distortion of the sea surface above a seabed feature occurs when the extra gravitational attraction of the feature “pulls” water toward it, forming a mound of water over itself.
Geosat, a U.S. Navy satellite operated from 1985 through 1990, provided measurements of sea surface height from orbit. Height accuracy was within 0.03 meters (1 inch)!

10. Ocean-Floor Topography Varies with Location
Cross section of the Atlantic ocean basin and the continental United States, showing the range of elevations. The vertical exaggeration is 100:1. Although ocean depth is clearly greater than the average height of the continent, the general range of contours is similar.

11. Ocean-Floor Topography Varies with Location
This graph shows the distribution of elevations and depths on Earth. It is not a land-to-sea profile of Earth, but rather a plot of the area of Earth’s surface above any given elevation or depth below sea level. Note that more than half of Earth’s solid surface is at least 3,000 meters (10,000 feet) below sea level. The average depth of the ocean (12,430 feet) is much greater than the average elevation of the continents (2,760 feet).

12. Ocean-Floor Topography Varies with Location
There are two primary classifications of ocean floor
Continental Margins = the submerged outer edge of a continent
Ocean Basin = the deep seafloor beyond the continental margin

13. Ocean-Floor Topography Varies with Location
Features of Earth’s solid surface shown as percentages of the earth’s total surface.

14. Continental Margins May Be Active or Passive
There are two types of continental margins
Passive margins face the edges of diverging tectonic plates. Very little volcanic or earthquake activity is associated with passive margins.
Active margins are located near the edges of converging plates. Active margins are the site of volcanic and earthquake activity.

15. Continental Margins May Be Active or Passive
Components of Continental margins:
Continental shelf – the shallow, submerged edge of the continent.
Continental slope – the transition between the continental shelf and the deep-ocean floor.
Shelf break – the abrupt transition from continental shelf to the continental slope.
Continental rise – accumulated sediment found at the base of the continental slope.

16. Continental Margins May Be Active or Passive
(a) Vertical exaggeration 50:1 (b) No vertical exaggeration

17. Submarine Canyons
Submarine canyons are a feature of some continental margins. They cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment.

18. Submarine Canyons
Turbidity currents, occur when turbulence mixes sediments into water above a sloping bottom. Most geologists believe that the canyons have been formed by abrasive turbidity currents plunging down the canyons.
(ABOVE) A turbidity current flowing down a submerged slope off the island of Jamaica. The propeller of a submarine caused the turbidity current by disturbing sediment along the slope.

19. Features of the deep-ocean floor
The Topology of Deep-Ocean Basins Differs from That of the Continental Margin
Features of the deep-ocean floor
Oceanic Ridges
Hydrothermal Vents
Abyssal Plains and Abyssal Hills
Seamounts and Guyots
Trenches and Island Arcs

20. Oceanic Ridges Circle the World
An oceanic ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean. If the ocean evaporated, the ridge system would be Earth’s most remarkable and obvious feature. The thickness of the red lines indicate the rate of spreading for some of the most rapidly spreading sections, and the numbers give spreading rates in centimeters per year. The East Pacific Rise typically spreads about six times faster than the Mid-Atlantic Ridge.

21. Oceanic Ridges Circle the World
Hand-drawn map of a portion of the Atlantic Ocean floor showing some major oceanic features: mid-ocean ridge, transform faults, fracture zones, submarine canyons, seamounts, continental rises, trenches, and abyssal plains. The map is vertically exaggerated.

22. Oceanic Ridges Circle the World
Transform faults and fracture zones along an oceanic ridge Because the ocean floor cannot expand evenly on the surface of a sphere, plate divergence on the spherical Earth can only be irregular and asymmetrical, and transform faults and fracture zones result.

23. Hydrothermal Vents Are Hot Springs on Active Oceanic Ridges
Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents. Cool water (blue arrows) is heated as it descends toward the hot magma chamber, leaching sulfur, iron, copper, zinc, and other materials from the surrounding rocks. The heated water (red arrows) returning to the surface carries these elements upward, discharging them at hydrothermal springs on the seafloor.

24. Abyssal Plains and Abyssal Hills Cover Most of Earth’s Surface
Abyssal plains are flat areas of sediment-covered ocean floor found between the continental margins and oceanic ridges. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges.
(ABOVE) The deep, smooth sediments of the Atlantic’s Northern Madeira Abyssal Plain bury 100- million-year-old mountains. Note the one lonesome seamount emerging from the muck. This image was generated by a powerful echo sounder.

25. Volcanic Seamounts and Guyots Project above the Seabed
Seamounts are volcanic projections from the ocean floor that do not rise above sea level. Flat-topped seamounts eroded by wave action are called guyots.

26. Trenches and Island Arcs Form in Subduction Zones
Trenches are arc-shaped depressions in the ocean floor caused by the subduction of a converging ocean plate. Most trenches are around the edges of the active Pacific. Trenches are the deepest places in Earth’s crust.

27. Trenches and Island Arcs Form in Subduction Zones
The Mariana Trench
(a) Comparing the Challenger Deep and Mount Everest at the same scale shows that the deepest part of the Mariana Trench is about 20% deeper than the mountain is high. (b) The Mariana Trench shown without vertical exaggeration.

28. Chapter 4 in Perspective
In this chapter you learned how difficult it has been to discover the shape of the seabed. Even today, the surface contours of Mars are better known than those of our ocean floor. We now know that seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. The ocean floor can be divided into two regions: continental margins and deep- ocean basins. The continental margin, the relatively shallow ocean floor nearest the shore, consists of the continental shelf and the continental slope. The continental margin shares the structure of the adjacent continents, but the deep ocean floor away from land has a much different origin and history. Prominent features of the deep ocean basins include rugged oceanic ridges, flat abyssal plains, occasional deep trenches, and curving chains of volcanic islands. The processes of plate tectonics, erosion, and sediment deposition have shaped the continental margins and ocean basins.