Wegener’s arguments and their reception

Wegener and Inuit guide duringthe fatal 1930 Greenlandexpedition

Chronology of Continental Drift & Plate Tectonics

• 1912 -- First publication of Wegener's hypothesis• 1920 -- 2nd edition of Wegener's book – widespread dissemination• 1926 -- New York symposium on continental drift• 1930 -- Wegener's death• 1931 -- Holmes publishes ideas about mantle convection• 1937 -- Our Wandering Continents by Du Toit• 1956 -- University of Tasmania symposium (Carey)• 1956 -- paleomagnetic evidence for North America - Europe motion (Runcorn)• 1962 -- sea floor spreading proposed (Hess, Dietz)• 1963 -- sea floor magnetic anomalies explained (Vine, Matthews, Morley)• 1965 -- transform faults explained (Wilson)• 1967 -- confirmation of transform fault motion (Sykes)• 1967 -- subduction zones proposed (Oliver and others)• 1967 -- plate tectonics described (Morgan, McKenzie)

Wegener’s evidence for continental drift

• Postglacial rebound (isostacy) shows that continents can move over a viscous substrate

• Oceanic crust is fundamentally different from continental crust, as demonstrated by the bimodal distribution of elevation

• Geodesy shows that Greenland is separating from Europe (at 36 m/yr!)• Continental coastlines fit together• Older geological units on opposite continents match• Paleontology shows that the geographical range of some species overlap

several continents, indicating the continents were joined• Paleoclimate studies show a distribution of climates that are not

compatible with present-day geography, but are compatible with pre-drift positions

Isostacy shows that continents can move over a viscous substrate

Oceanic crust is fundamentally different from continental crust

Older geological units on opposite continents

match

The geographical range of fossil species cross the present-day oceans

Paleoclimate data are more compatible with pre-drift geography

Wegener’s incorrect ideas

1) The motive forces proposed by Wegener are very small and unrelated to continental motion

2) Continents do not "plow" through oceanic crust, but instead continental and oceanic crust moves together over the weaker asthenosphere.

3) Had no concept of principal tectonic features (mid-ocean ridges, subduction zones)

4) Claimed that North America and Eurasia separated recently, and Pleistocene glacial features were evidence of drift. Also thought rate was much faster (5-30 m/yr, instead of 2-4 cm/yr)

Rebuttal -- Arguments against drift

• 1) There was no evidence that the seafloor material (sima) was weaker, so that the continental masses could move through it.

• 2) Geophysicists showed that the motive forces proposed by Wegener were insignificant.

• 3) Paleontological data for dispersal of species between continents could be explained by "land bridges”

• 4) Many geologists felt that the fit of the continents by Wegener was not very good and was due to coincidence.

• 5) The correlation of geologic units between different continents and the paleoclimate evidence was controversial, and there were proposed counter-examples.

• 6) The geodetic observations favoring drift were not accurate enough to resolve it.

• 7) Why had Pangea existed for over 4,000 Ma, and then suddenly broken up in the last 150 Ma ? Or even the last 2 Ma, as suggested by Wegener’s incorrect continental velocities?

Paleontology is explained by “land bridges”

From Texbook of GeologyPirsson & Schuchert, 1915

Explanations for Fossil Distribution

The best opponent

• Physicist Sir Harold Jeffries (1891-1989)

• The forces proposed by Wegener were miniscule

• The rheology of the earth’s interior was too rigid for plates to move

• The forces are too weak by a factor of 105

Wegener dies in Greenland, 1930

N Y Times, January 17, 1932

Movie SOS Iceberg, 1933Based on expedition torecover Wegener’s papers

Major Questions – 1930 to 19561) 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

Holmes and mantle convection

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

Early Mesozoic

Paleozoic