I. Continental Drift

A. Alfred Wegener

1. First scientific publication (1912) assembling diverse geologic data

a. proposed existence of a Late Paleozoic supercontinent (Pangea) surrounded by a superocean (Panthalassa) an arm of Panthalassa (Tethys)

i. Laurasia = N. America & Eurasia

ii. Gondwana = S. America, Africa, Antarctica, Australia, & India

2. Data

a. Shape of continents

i. quantified by Sir Edward Bullard in 1965 using computers

ii. use "real" edge of continents - continental slope

b. Geology (Match up continental geology on either side of an ocean)

i. mountain belts (Appalachians-Caledonides)

ii. radiometric age provinces (Brazil, West Africa)

iii. sedimentary basins (South Africa, Argentina)

c. Paleoclimatology

i. Earth has climate zones arranged by latitude, with distinct sedimentary deposits characteristic of each zone

ii. hard to explain tillites in India in the Permian & in the Sahara in the Ordovician, or Late Paleozoic shallow-marine limestone in the Arctic, among numerous examples

d. Paleontology (Degree of similarity or differences of fossils on various continents)

i. Similar Early Paleozoic trilobites, Middle Paleozoic fishes & Late Paleozoic amphibians in eastern N. America & north Europe

ii. Similar Mesozoic reptiles of N. America & Europe

iii. Similar Carboniferous & Permian flora (Glossopteris ) & fauna (including Mesosaurus ) in Gondwana continents

iv. All of the above contrast with the wide diversity of organisms in the Cenozoic

v. Present distribution of marsupials & lemurs

3. Wegener's mechanism for continental drift was not acceptible, however

a. he proposed that the continents plowed through the oceans

4. Mantle convection

a. alternate mechanism for continental drift now thought to be the primary mechanism for Earth's dynamic processes

i. developed by Arthur Holmes (University of Edinburgh) in 1931

B. Paleomagnetics in the 1950's - APPARENT POLAR WANDERING

1. Earth's magnetic field

a. similar to a bar magnet at the center of the Earth

i. inclination of force lines with Earth's surface is proportional to latitude, i. e., tangent of inclination = 2 X tangent of latitude

b. magnetic poles coincide with rotational poles over periods of a few thousand years

2. Remanent magnetism

a. rocks record the orientation of the Earth's field at the time they formed

i. igneous rocks when they cool below their Curie temperature (~600_C)

ii. sedimentary rocks during deposition mostly, although ground water weathering can reset

b. remanent magnetism of rocks is used to determine location of the magnetic pole with respect to a given sample area

3. Paleomagnetisists working the 1950's found that the position of the magnetic pole for a given area seemed to change over time

a. they also found that the position of the magnetic pole at a given time was not the same with respect to the different continents

4. APPARENT POLAR WANDERING is the term used for the fact that the magnetic pole seemed to move because, in fact, it is the continents that have wandered, while the magnetic pole has remained essentially stationary

II. Seafloor Spreading

A. Outgrowth of Holmes' ideas about mantle convection

1. Harry Hess, a marine geophysicist at Princeton University, proposed the concept of SEAFLOOR SPREADING in 1961

a. convection in mantle as proposed by Arthur Holmes in 1931

b. up welling of hot mantle under MOR & creation of new seafloor

c. conveyor belt that carries continents along as passengers

2. Robert Dietz of the U.S. Coast & Geodetic Survey actually coined the term seafloor spreading in 1961

3. Contributions from paleomagnetics & marine geology & geophysics

B. Paleomagnetics in the 1960's - MAGNETIC POLARITY REVERSALS

1. In the 1950's, paleomagnetisists noticed that the magnetic poles seemed to reverse periodically

a. that is, a compass needle currently points to the north magnetic pole, but at other times in the past, the same needle would have pointed to the south magnetic pole

2. To determine the exact times that the magnetic polarity reversed, an extensive program was initiated in the early 1960's

a. numerous young (<4 my) volcanic rock samples were collected from continental lava fields & from oceanic islands

b. each sample had its magnetic polarity & its Potassium-Argon radiometric age determined

c. when age was plotted versus magnetic polarity, a series of relatively long (~1 my) magnetic polarity EPOCHS was detected

i. in a few places, shorter (~100 ky) magnetic polarity Events occurred

d. thus, the magnetic polarity time scale back to 4.5 my was developed

C. Marine Geology

1. mapped the topography of the seafloor

a. central Mid-Ocean Ridge (MOR) with rift valley at crest

i. indicated tension in centers of oceans

2. collected sediment cores & rock dredges

a. oldest sediments & rocks - Mesozoic

b. age of Atlantic islands increases with distance from MOR crest

D. Marine Geophysics

1. detected magnetic polarity reversals in the ocean

a. in deep sea sediments (magnetic polarity vs. depth in core)

b. more importantly.in oceanic crust (magnetic anomaly lineations parallel to MOR crest)

c. Fred Vine recognized that these linear magnetic anomalies resulted from seafloor spreading combined with reversals of the Earth's magnetic field (magnetic polarity vs. distance from MOR crest)

i. allowed spreading rates to be calculated (<1 to >17 cm/yr)

ii. showed oldest seafloor only ~200 my old

iii. also allowed extension of magnetic polarity time scale - magnetic anomalies extended beyond Gilbert Epoch (see below)

iv. predicted age of seafloor

2. DSDP

a. Lamont folks (Heirtzler, Pitman, Dickson, & LePichon) had estimated the age of magnetic anomalies older than the Gilbert by extrapolating spreading rates & predicted the age of the seafloor in the various oceans

b. part of the rationale for the DSDP was to test this extrapolation

i. oldest sediment on top of basaltic oceanic crust should get older away from MOR crest in predicted manner

c. Leg 3 drilled a section across the South Atlantic

d. confirmed predicted ages & seafloor spreading

e. generated band wagon culminating in plate tectonics

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