Sea Floor Spreading & the Plate Tectonics
Revolution (1956-1968)
The research ship
Horizon
Scripps Institution of
Oceanography
1949-1968
Major Questions – 1920 to 1960
1) 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
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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 University
1953-1981
Exploration of the Oceans – Scripps Inst. of
Oceanography expeditions 1950-1953
Holmes and mantle convection
Continuity of the
global rift system
Heck [1938]
Discovery of the world mid-ocean
ridge system
Bruce Heezen & Marie Tharp
Lamont-Doherty
Map showing Mid-Atlantic Ridge
as continuous rift – Heezen, 1959
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