The Floors of the Oceans: Concealed Revealers of Change

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. 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 World Ocean Facts

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 http://www.ngdc.noaa.gov/mgg/image/2minrelief.html

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 -- 170 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. http://www.pmel.noaa.gov/vents/acoustics/seismicity/seismicity.html

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 http://www.cotf.edu/ete/modules/msese/earthsysflr/plates4.html

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 http://csmres.jmu.edu/geollab/Fichter/Wilson/Wilson.html 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: www.ametsoc.org/amsedu/DS-Ocean/home.html

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 http://www.ldeo.columbia.edu/CORE_REPOSITORY/RHP1.html

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 http://walrus.wr.usgs.gov/infobank/

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