HSHM 211: GLOBAL CATASTROPHE SINCE 1750
1/30/17: Continental drift and its discontents
1. Convection currents are the motor
2. Measure motion using GPS
1. Events causing major environmental change
2. Punctuated equilibrium: neo-Darwinian
How do uniformitarianism and catastrophism coexist?
1. Changing standards of evidence, what it takes for theory to be accepted
a. Change from the generally reasonable explanation to the smoking gun
o Smoking gun: single piece of evidence from which other things follow
Does not establish fact, we just know that this happened
o Does not establish entire explanation but establishes a fact from which
other things come
2. How theories change over time
a. Paradigm shifts? Scientific revolutions?
Late 19 through mid-20 centuries
1. Do continents move?
a. Same facts explained by different theories
b. No theories were perfect
Fixism vs. mobilism
1. Fixism (uniformitarianism): earth’s continents have stayed in the same place forever
a. Cooling earth as theory of continents
o James Dwight Dana (1813-1895): smooth earth with thin ocean that
contracted and caused earth to cool, as contraction continued oceans
o Edward Suess (1831-1914): as earth cools, periodically continents will slump
and become oceans, and ocean floors will stay in place and become
2. Mobilism (catastrophism): earth’s continents have moved over time
a. Alfred Wegener (1880-19): continental drift, 1912
b. Arthur Holmes: continent can be split apart by convection currents, 1929
Do theories need to be supported, or do theories do the supporting?
a. Wegener: mountains are zones of collision
o Explained why mountains mostly at edges but also at interior (when
continents collide and generally older)
b. Dana: oceanic crust becomes deeper/thicker by cooling temperatures
c. Dana/Suess synthesis:
2. Fossils: alignment of fossils across different continents, strata, and foldings, suggesting that
continents were once joined together a. Others: ancient continental links between present continents
o Ancient land bridges
3. Southern glaciation
4. Which is more speculative?
a. Land masses moving vertically a few thousand feet?
b. Land masses moving horizontally many thousands of miles?
Older rock sitting on top of younger rock
Only explanation: rock layers changed very radically by pressure
causing older rock to end up on top of younger rock
o Isostasy: earth’s crust floats on semi-liquid mantle below
Evidence: most of Scandinavia slowly rising
Himalayas did not affect earth’s gravity as much as
expected if earth is solid
Continental drift makes sense
US: “Pratt interpretation” crust composed of blocks of different
densities with no “roots”
Continental drift does not make sense; how can continents
move if no roots?
1. When faced with 2 theories, both were watertight that had their own strengths and
a. Why were people committed to one worldview over another? Rather than
considering merits and pitfalls of both
2. Similar kinds of science used to come to different conclusions
a. Collecting disparate evidence into a generally reasonable explanation that takes
account of everything
2/1/17: Plate tectonics and mass extinctions
Fixism vs. mobilism (uniformitarianism vs. catastrophism)
1. Both explained a lot, but both had holes
2. Both were generally reasonable explanations of the earth
Plate tectonics and mass extinction