Earthquakes and Mid-Ocean Ridge Spreading

1. Earthquakes and Mid-Ocean Ridge Spreading Guest Scientist: Dr. Maya Tolstoy 12 March 2005

2. What Is the Mid-Ocean Ridge? • World’s longest chain of mountains– nearly 40,000 km of basaltic ridges winding from the Artic through the Atlantic, Indian, and Pacific Oceans • Almost entirely submerged– notable exceptions include Iceland and such archipelagos as the Galapagos (Pacific) and Azores (Atlantic)

3. How Was the MOR Discovered? • Early sailors were not very interested in water deep enough to float a boat • First necessity to know details of sea floors came with efforts to lay down trans-Atlantic telegraph cables • “HMS Challenger” (1872 – 1876) used weighted piano wire to “sound” the ocean depths and produced first general idea

4. HMS Challenger circumnavigated the world from 1872 - 76 in the first major scientific study of the oceans. • http://www.wshs.fcps.k12.va.us/academic/science/bjewell/ocean/hhocean/final/chall.htm

5. What Revealed More Details? SONAR • Beginning in the 1920s, SOund Navigation and Ranging was developed to produce rapid images of what lay beneath a ship • Also called “ECO-SOUNDING,” SONAR emits sound signals and detects the returning echo to calculate depths and shapes beneath the surface • Extensively used by WW II shipping

6. P(ic)assow

7. Heezen and Tharp's "physiographic maps" • SONAR was widely employed in WW II, and many records became available after the war. • Dr. Bruce Heezen and Marie Tharp here at Lamont developed techniques beginning in the 1950s to change these 2-D records into 3-D physiographic charts, a drawing technique developed by their Columbia professors E. Raisz and A. K. Lobeck.

8. Through publication arrangements with the National Geographic Society, these physiographic images completely changed the way in which people envision the ocean floors. By the early 1970s, almost all of the oceans were depicted in such maps.

9. 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”

10. 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

11. Early Scientific Studies and Sea-Floor Spreading • Bruce Heezen and Maurice Ewing here at Lamont first connected earthquake patterns with the MOR in the mid-1950s • Harry Hess introduced the concept of sea-floor spreading (1960) • By the late 1960s, the theory of PLATE TECTONICS was beginning to be widely accepted

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. 3 Types of Plate Boundaries • Divergent • Convergent • Transform http://www.cotf.edu/ete/modules/msese/earthsysflr/plates4.html

14. Today we often represent the plate boundaries with maps like this one http://www.platetectonics.com/book/page_2.asp

15. New shipboard and airborne techniques for mapping the ocean floors include “side scan sonar” and high-resolution seismic profiling. The next slide provides examples of such images.

16. http://marine.usgs.gov/fact-sheets/fs172-97/mapping.html

17. Interesting Features of the MOR • Central Rift Valley • Basaltic composition (quite different from lava flows on continents) • Discontinuous nature of the system (Menard and Heezen found right-angle off-sets, 1960) • Transform faults (Wilson, 1965) • Propagating rifts (segments defined by two transform faults)

18. Additional Discoveries • Variations in spreading rate: East Pacific Rise (60 – 170 mm/year) Mid-Atlantic Ridge (30 mm/year) • Fast-spreading ridges generally had crests several hundred m high and 5 – 20 km wide • Slow-spreading ridges have rift valleys several km deep and 20 – 30 km wide

19. Unexpected Discoveries • Connections between ridge and the source region for their lava may remain unchanged for tens of millions of years • Segments can migrate along the ridge, lengthen or shorten • Variations in activities among types of discontinuities • Less than 5% of sea floor has been mapped

20. Small manoeuvrable research submersibles, such as the “ALVIN” operated by Woods Hole Oceanographic Institution, provide access to deep-sea features not otherwise accessible. http://www.comptons.com/encyclopedia/CAPTIONS/18005895_P.html

21. The Most Amazing Discoveries! “Black Smokers” or hydrothermal vents Spewing forth very hot water, sulphur, other minerals Basis for chemo-synthetic ecosystems unknown before the late-1970s. http://www.amnh.org/nationalcenter/expeditions/blacksmokers/smoker2.html

22. Amazing deep-sea organisms! http://www.amnh.org/nationalcenter/expeditions/blacksmokers/life_forms.html

23. Next: Dr. Maya Tolstoy Following a break, we will hear from Maya Tolstoy about her research and career as marine scientist. Although participation in “Aliens of the Deep” has brought “new fame,” Dr. Tolstoy has participated in many discoveries about the ocean floors.

24. Examples of Questions Dr. Tolstoy and Colleagues Have Investigated • Is there any correlation between tides and microearthquakes? http://www.ldeo.columbia.edu/~tolstoy/pdf/axial_tides.pdf • Can seismometers be used to identify lava eruptions? http://www.ldeo.columbia.edu/~tolstoy/pdf/Tolstoy_Gakkel.pdf • What can be learned by studying sound in the oceans?http://www.ldeo.columbia.edu/res/pi/tphase/

25. http://www.womenoceanographers.org/