Sea Floor Spreading & the Plate Tectonics Revolution (1956-1968)

1. Sea Floor Spreading & the Plate Tectonics Revolution (1956-1968) The research ship Horizon Scripps Institution of Oceanography 1949-1968

2. Major Questions – 1920 to 1960 1) What is the underlying cause of geosynclines and vertical tectonics? • The ocean floor began to be revealed – how to incorporate the new knowledge? - oceanic rocks predominantly basalt –different from continents? - can oceanic regions be transformed into continents and vice-versa? - seafloor morphology – trenches and ridges? • Deep earthquakes – Wadati Benioff zones - inclined zones of earthquakes down to 700 km - linked to oceanic trenches and volcanos? • What is the mechanism of heat loss of the earth? • Conduction or convection? - If convection how is it related to geology?

3. Developments 1930-1956 • Holmes realized with radioactivity some process must be removing heat – proposed mantle convection • Jeffries continued to insist that mantle material cannot support movement of continents • Southern Hemisphere geologists supported continental drift – Du Toit – Our Wandering Continents • Continental drift was regarded as a crazy idea by North American geologists • World War II and the cold war brought about a great increase in data about the ocean floor – how to interpet?

4. Holmes and mantle convection

5. Southern hemisphere geologists: Du Toit Our Wandering Continents (1937) Early Mesozoic Paleozoic

6. Revival of Continental Drift – Major Factors • Mapping of the ocean floor with sonar • Sampling the ocean floor • Shipboard magnetic surveys • Development of Paleomagnetism • Great improvement in seismic data (marine and global) • Recognition that the interior of the earth must convect to release heat • Most of the new data were incompatible with the existing paradigms (geosyncline, land bridge, contractionist)

7. Oceanographic ship - Vema Lamont-Doherty Geological Observatory, Columbia University 1953-1981

8. Exploration of the Oceans – Scripps Inst. of Oceanography expeditions 1950-1953

9. Holmes and mantle convection

10. Continuity of theglobal rift system Heck [1938]

11. Discovery of the world mid-ocean ridge system Map showing Mid-Atlantic Ridge as continuous rift – Heezen, 1959 Bruce Heezen & Marie Tharp Lamont-Doherty

12. Mapping the ocean basins Heezen and Tharp, 1968

13. Who discovered mid-ocean ridges?

14. Europe and North America have different polar wander paths Runcorn, 1956 (not actual image)

15. Magnetic Anomalies Raff & Mason, 1961

16. Sea floor spreading – Hess 1962

17. Comparison to modern ideas Hess [1962] Vera et al. [1990]

18. Mechanism of spreading – mantle convection

19. Evidence – crustal thickness and isostacy

20. Evidence – subsidence of seamounts and guyots

21. Vine & Matthews (1963) – The ocean crust as tape recorder

22. S. W. Carey (U Tasmania) popularized the expanding earth hypothesis S.W. Carey (1911-2002) Book by Carey geology student Lester King, published in 1983

23. Acceptance of Plate Tectonics • Major Advances • Transform faults explained (Wilson, 1965) • Magnetic reversal time scale, ocean spreading rates (1966) • Subduction zones proposed as sites of seafloor consumption (Oliver et al., 1967) • Confirmation of transform fault motion from seismology (Sykes, 1967) • Development of quantitative framework for plate tectonics (Morgan, McKenzie 1967) • Confirmation of age progression of seafloor by drilling (late 1960s) • Plate tectonics generally accepted by about 1970 • Resistance • Prominent older geoscientists continued to resist (Jeffreys, Beloussov, Meyerhoff • Soviet block scientists generally did not accept until 1980s

24. Transform Faults

25. Subduction Zones and the “New Global Tectonics” Isacks, Oliver & Sykes, 1968

26. The geometry of plate tectonics Discovered by: Morgan (1968), McKenzie et al (1967)

27. Not everybody was convinced: Beloussov (1970) 1) Magma at mid–ocean ridges must spread irregularly, so how do you get linear, parallel magnetic anomalies? • Metamorphic rocks formerly buried to 3 km exposed at ridge axes • Old rocks have been found near mid-ocean ridges (as old as 29 Ma) • Marine magnetic anomalies are not uniform and thickness not proportional to anomaly age (but he doesn’t know about magnetic “skewness” due to interaction with earth’s field) • Sediment thickness in the oceans does not correlate with the distance from ridge • Magnetic anomalies are not always symmetric about the ridge axis • Spreading ridges do not remain stationary • If spreading rate changes, why aren’t there earthquakes and deformation throughout the ocean basins? • hotspots should move with the plates • There should be accretionary prisms, instead sediments are undeformed at trenches • If lithosphere has greater density than the asthenosphere, why doesn’t it subduct immediately? And how can low density surface rocks go down to depths > 700 km where density is much greater? • Geology of Iceland does not look like a spreading center • Plate tectonics does not match the global gravity field or heat flow data • Plate tectonics ignores all the basic data of continental geology – tectonic cycles of vertical motion