How do we develop scientific ideas?

• Undergraduates are often presented with “Encyclopedia science”• Science as a collection of well-solved problems and facts• History of science is only touched on to tell us how we arrived at the

“truth”• This does not present a realistic picture of science as a dynamic discipline

• In this class we will get inside the minds of some of the great earth scientists as they were developing new theories of the earth• We will try to overcome the “triumphalist” view, where everything is seen

from the modern perspective, and understand what science is like as new discoveries are made.

Philosophical Issues• Does science develop smoothly, or does it go through

long periods of stasis followed by sudden “revolutions”? How do the revolutions happen?

• Is there progress in science? Does progress happen by adding to previous knowledge (accumulation) or by replacing previous knowledge?

• Is science more than a social construct?

• To start thinking about these issues, we will look at the development of plate tectonics, one of the grand unifying themes in earth sciences that has developed over the last 100 years.

Tectonic ideas before Continental Drift

• Not one dominant theory, but general consensus on the fixity of the major continents. Most geologists held to some combination of these major ideas:

• Contractionist theory (Beaumont, Suess, Stille) -- Earth is slowly contracting as it cools -- contraction produces periods of compression and mountain building

• Geosynclinal theory (Hall, Dana) -- Paired regions of uplift and depression formed large sedimentary basins which were later uplifted to produce mountain ranges.

• Both ideas suggested that geology was dominated by vertical motions of uplift and subsidence, with only limited horizontal tectonic motion.

Mountain building through contraction“Collapsing Earth” theories

• Basic process of mountain building was thermal contraction• Mountains resulted from wrinkling of the Earth’s crust• Shrinkage of the Earth’s interior caused some regions to collapse and become ocean basins; uncollapsed regions are continents• Further shrinkage may cause uncollapsed regions to collapse continents and oceans trade places!

Successes of Contraction Theory• Explained the presence of marine deposits on

continents• Explained the similarity of fossils on different

continents• Volcanism was attributed to the process of

continental collapse (e.g. Mediterranean Sea)• Simple thermal models predicted reduction of the

earth’s radius from contraction (e.g. Lord Kelvin)

Geosyncline Theory

From textbookThe Outlines of Physical GeologyBy Longwell, 1941

Example: Appalachian Geosyncline

Reinterpretation of Geosynclines

Classic AppalachianGeosyncline(after Kay, 1948)

ModernReinterpretation

From S. Dutch website

Early ideas of continental drift

Antonio Snider-Pellegrini (1858)"The Creation and its Mysteries Unveiled”

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)

Alfred Wegener (1880-1930)

• Alfred Wegener was a German natural scientist in meteorology, astronomy, and geology

• PhD in astronomy from U. of Berlin in 1905 but he became interested in climatology and meteorology

• His first academic position (tutor) was at the University of Marburg in meteorology

• He was fascinated by Greenland and participated in 3 expeditions (1906, 1912, and 1930). He died during the last one.

How did Wegener get the idea of continental drift?

• While at University of Marburg in 1911, Wegener was browsing in the library when he came across a paper that listed fossils of identical plants and animals found on opposite sides of the Atlantic (Brazil and Africa)

• Intrigued, Wegener began to look for, and find, more cases of similar fossils separated by oceans.

• Wegener was fascinated by the close fit between the coastlines of South America and Africa.

• Wegener decided that the similar fossils and coastlines indicated that the continents were once joined and subsequently moved apart to their current positions.

• He later wrote: "A conviction of the fundamental soundness of the idea took root in my mind."

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