1. The Floors of the Oceans: Concealed Revealers of Change
Earth2Class Workshops for Teachers
Originally presented 20 Sep 2003

2. with Guest Scientists Dr. Gerard Bond
Doherty Senior Scholar, Biology and
Paleoenvironment Division
Ramona (Rusty) Lotti
Curator, L-DEO Deep Sea Sample
Repository

3. World Ocean Facts Oceans cover 71% of Earth’s surface
Northern Hemisphere is 39% land, 61% sea
Southern Hemisphere is 19% land, 81% sea
Pacific is largest basin, followed by Atlantic, Indian, Southern, and Arctic
Southern is defined as waters between 60o S and Antarctica, where the Antarctic Circumpolar Current flows
Numerous seas and gulfs lie adjacent to the major ocean basins

4. Earth’s Crust Continental crust granitic thicker less dense
Average elevation 838 m (2,750 ft.)
Oceanic Crust
basaltic
thinner
Denser
Average depth 3800 m (12,500 ft)

5. Continental Margins Continental shelf
gentle slope out to average depth of 130 m (430 ft.)
variations in width, materials, origins, etc.
coral reefs
Continental slope
more steeply sloping edge of continent
Continental rise
apron of sediments

6. Submarine Canyons Cut through continental slope and shelf
Probably created through erosion by turbidity currents
Also connected to Pleistocene river drainage; e.g., Hudson Canyon
Differences between East and West Coast canyons
Submarine fans at base of many canyons

7. Ocean Basins Abyssal Plains vast, flat basins
abyssal hills generally less than 100 m
sediment blankets from terrestrial and pelagic sources
Ocean Ridges
volcanic mountain range with rift valley
about 23% of sea floor

8. Other Ocean Basin Features
Trench and island arc systems
Volcano and seamount chains
Atolls
Guyots
Hydrothermal vents

9. “Heezen-Tharp Maps”
Perhaps the most famous version of what the floors of the oceans look like were the series of maps created beginning in the 1950s by Bruce Heezen and Marie Tharp of Lamont-Doherty. Using thousands of sonar tracks, they produced physiographic charts of the ocean basins. These were later published in enhanced versions by the National Geographic Society, and are still widely used.

10. Online Bathymetry Maps

11. Ocean Basins and Plate Tectonics
Francis Bacon (1561 – 1626) noted “fit” of South America and Africa
Alfred Wegener (1880 – 1930) and Frank Taylor (1860 – 1939) independently proposed continental drift
Challenged existing theories, could not provide satisfactory mechanism

12. Supporting Evidence Earthquake and volcano patterns
SONAR and other observation techniques
Geomagnetic patterns
-- “normal” and “reverse” polarity
-- “mirror” images on opposite side of MOR
reversals over past 76 million years
Increasing age away from MOR rift valley
Greatest heat flow along MOR
Increasing thickness of sediments away from MOR

13. Modern Exploration Technologies
Acoustic Monitoring of Ocean Seismicity
The hydroacoustic method allows detection of low-magnitude seismicity and volcanic activity and provides more accurate source locations than land seismic networks.

14. Evolution of a theory
During the past century, understanding of the ocean floors changed dramatically.
“Continental Drift”
“Convection” in the upper mantle
“Sea-floor Spreading”
Plate Tectonics

15. 3 Types of Plate Boundaries
Divergent
Convergent
Transform

16. Spreading and Closing Cycles
J. Tuzo Wilson (U. Toronto) proposed that oceans open and close over cycles lasting 500 – 600 million years.
An excellent explanation has been created by Prof. Lynn Fichter of James Madison University
The cycle starts with a stable craton (core of a continent.)

17. “Embryonic” stage
Supercontinents remain in one spot for hundred of millions of years, acting like a blanket
Mantle convedction warms the overlying supercontinent, causing expansion and stretching
Fracturing forms a rift valley
Examples: East African Rift Valley

18. “Juvenile” stage Rift valleys gradually widen and connect to ocean
Examples: Red Sea, Gulf of California

19. “Mature” stage
Lateral spreading of rift valley leads to development of a divergent plate boundary and new oceanic crust
Atlantic is a mature ocean with passive edges

20. “Declining” stage Subduction becomes more widespread
Convection in mantle drags lithosphere plates into subduction zones
Examples: Scotia Arc in South Atlantic, West Indies near Barbados
Explains why there is little oceanic crust more than 200 my

21. Ocean basin begins to close as subduction rates exceed spreading rates
Sediment deposits are deformed and uplifted, creating mountain ranges in new supercontinent on site of former ocean
Example: Appalachians (formed at end of previous Wilson cycle)

22. Wilson cycles influence sea level relative to continents of 105 years
When supercontinents dominate, land stands higher as mantle warms them
As spreading begins, land is lowered and seas encroach, flooding low-lying continental areas
As basin widens and crust ages, it deepens and sea level falls
During periods of higher sea level, oceans covered about 80% of surface
During periods of lower sea level, oceans covered only about 65% of surface

23. Sea Floor Sediments Terrestrial and pelagic origins
(important types in next slide)
Provide evidence of processes and environmental conditions at time of origin
Relatively protected from erosion, so more complete “record” than continental deposits
Connection with rocks now forming continents (ex., limestones, shales)

24. Types of Deep Sea Sediments
Terrigenous muds
Turbidites
Glacial (“ice-rafted”)
Brown/red clays
Authigenic (hydrogenous) deposits
Manganese nodules
Pelagic oozes—diatoms, “forams,” “rads”
Volcanic

25. Ocean Sediment Distribution
Sediment thickness is influenced by:
Age of the underlying crust
Tectonic history of the underlying crust
Structural trends in the basement rocks
Location and nature of sediment sources
Nature of depositional processes delivering the sediments
You can see a digital image of worldwide sediment distribution
through the DataStreme Ocean web page:

26. LDEO Deep Sea Sample Repository
Largest collection in the world
E2C has visited many times – see archived series for related PowerPoints
Web site provides many useful starting points for education

27. Sediments and Climate Change
Dr. Gerard Bond will discuss some of his research about climate change based on interpretation of deep-sea sediments. Of particular interest for much of his study are ice-rafted materials collected in the North Atlantic. You can see versions of some of his work in the Core Repository pages.

28. Resources for Educational Projects
In addition to what is available through the L-DEO and other university-related web sites, one of the best sources for information about this topic may be found in the USGS Coast & Marine Geology Program web pages

29. Special Reference “DataStreme Ocean” Preview Package
Ch. 2: Ocean Basins and Plate Tectonics
Authors: M. Grant Gross and Elizabeth Gross
Sponsor: AMS/NOAA Cooperative Program for Earth System Education