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

The research shipHorizonScripps Institution of Oceanography1949-1968

Major Questions – 1920 to 19601) What is the underlying cause of geosynclines and vertical tectonics?2) 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?

3) Deep earthquakes – Wadati Benioff zones- inclined zones of earthquakes down to 700 km- linked to oceanic trenches and volcanos?

4) What is the mechanism of heat loss of the earth?- Conduction or convection?- If convection how is it related to geology?

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?

Holmes and mantle convection

Southern hemisphere geologists: Du Toit Our Wandering Continents (1937)

Early Mesozoic

Paleozoic

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)

Oceanographic ship - Vema

Lamont-Doherty Geological Observatory, Columbia University1953-1981

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

Holmes and mantle convection

Continuity of theglobal rift system

Heck [1938]

Discovery of the world mid-ocean ridge system

Map showing Mid-Atlantic Ridgeas continuous rift – Heezen, 1959Bruce Heezen & Marie Tharp

Lamont-Doherty

Mapping the ocean basins

Heezen and Tharp, 1968

Who discovered mid-ocean ridges?

Europe and North America have different polar wander paths

Runcorn, 1956(not actual image)

Magnetic Anomalies

Raff & Mason, 1961

Sea floor spreading – Hess 1962

Comparison to modern ideas

Hess [1962] Vera et al. [1990]

Mechanism of spreading – mantle convection

Evidence – crustal thickness and isostacy

Evidence – subsidence of seamounts and guyots

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

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

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

Transform Faults

Subduction Zones and the “New Global Tectonics”

Isacks, Oliver & Sykes, 1968

The geometry of plate tectonics

Discovered by: Morgan (1968), McKenzie et al (1967)

Not everybody was convinced: Beloussov (1970)

1) Magma at mid–ocean ridges must spread irregularly, so how do you get linear, parallel magnetic anomalies?

2) Metamorphic rocks formerly buried to 3 km exposed at ridge axes

3) Old rocks have been found near mid-ocean ridges (as old as 29 Ma)

4) 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)

5) Sediment thickness in the oceans does not correlate with the distance from ridge

6) Magnetic anomalies are not always symmetric about the ridge axis

7) Spreading ridges do not remain stationary

8) If spreading rate changes, why aren’t there earthquakes and deformation throughout the ocean basins?

9) hotspots should move with the plates

10) There should be accretionary prisms, instead sediments are undeformed at trenches

11) 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?

12) Geology of Iceland does not look like a spreading center

13) Plate tectonics does not match the global gravity field or heat flow data

14) Plate tectonics ignores all the basic data of continental geology – tectonic cycles of vertical motion