Continental Drift
Tectonics around 1910
- Continental Gravity Surveys 1900-1910
- Continents are in equilibrium everywhere!
- No real drive, excepting what may happen with erosion and deposition
- Joly 1909: Radioactivity
- Radioactivity provides heat, indicating the earth is not cooling and may be heating up
- Disproved basic tenet of contraction model, and Kelvin’s argument
- Tectonics based on idea that
- Radioactive materials are concentrated in crust
- These get preferentially concentrated in geosynclines
- Heat makes material “more plastic” and deformable
- This can deform “Dana-style” geosynclines
- Chamberlin 1910: Mega-Earth prisms
- See notes from prior class
- Permanent ocean and continents
- Prisms undergo episodic adjustment due to contraction
- Taylor 1908: Continental displacements
- Continental movement to explain compressive mountain ranges – Suess model used to determine continental motions
- Continents start near the poles
- Earth’s rotation moves them toward the equator
- Baker 1912: Continental Motions
- Envisions one large supercontinent with huge Pacific Ocean
- Venus – Earth interactions cause tidal distortions that rip away material to form the Moon
- Result is the Pacific Ocean
- Continents motions as they slipped toward the resulting void
Wegener in two versions
- 1912: initial presentation
- More of a theoretic idea intended to provoke future work
- Relation to former tectonic models
- Contraction: Argues against it
- Radioactivity cuts at basic premise
- Oceans and continental different materials
- Does adopt the idea of compressive mountain features (Alps, Appalachians, etc.)
- Isostasy
- Favors Airy isostasy
- Uses the idea that light continents “float” on denser substratum (= ocean crust)
- Recognizes that this doesn’t really build mountains due to isostatic balance
- Evidence
- Structural Parallelism of mountains
- Fit of continental shelves
- Distribution of ancient terrestrial faunas and floras indicate link of Africa and South America
- Difference between oceanic and continental crust indicate not change one to other
- Mechanism
- Vague (at best)
- Viscosity at depth to allow motion but what drives?
- Postulates continents “flight from the poles” ormove westward
- Unable to explain how continents really move or what force could possibly move them
- 1924: English edition of fourth edition
- This is the version with which we are familiar
- Incorporates much more geological information
- More information on terrestrial faunas and floras
- Ancient climate indicators (reefs, glacial deposits)
- Detailed geological trends/formations
- Geophysics of Drift still problematic
- No clear mechanism; isostasy removes much of the “driver”
- Argued for very recent opening of northern Atlantic; sees North America, Greenland and Europe as one landmass until very recently
- Requires very rapid movement of Greenland, that was proved untrue by 1930s
Reactions
- Land Bridges (1920s-40s)
- Wegener’s best argument was probably fauna/flora connections
- Counter-argument was “land bridges”
- Oceanic islands are remnants of former land connections
- These emerged to allow terrestrial faunal exchange and then (conveniently) subsided away
- Isostasy argues against this kind of “see-saw” crustal behavior
- Popular with American paleontologists
- State of debate by 1930
- Very divided community – no consensus
- Southern Hemisphere and some European geologists (ex: Dutch) were “Mobilists”
- Reflect influence of Suess and knowledge of Gondwana fossils
- North American and some European geologists were “Fixists”
- Reflect history of Dana, Chamberlin, vertical isostasy