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Continental Drift and Sea-Floor Spreading

What was known about functioning of the planet that set the stage for plate tectonic theory?Most of what was known about Earth before the Second World War was based on continentalgeology. Not very much was known about the face of the seafloor until military interests(submarine warfare) drove massive surveys. Funding was awarded to academic research tocomplement the military effort. So, what could geologists see from the land before all of thiswar-driven exploration?

1. Hypothesesa. Isostasy and geosynclines

i. This idea was still being taught when plate tectonics swept the scene in the 1960's.ii. Sediments eroded from a new mountain range are deposited on the adjacent crust in

an arcuate swath that subsides due to isostasy as the mass of the pile builds. Thetrough accumulates sediment in deep water at the axis of the trough and in shallowwater at its margins. The trough subsides under the weight of added sediment. Atsome magical moment, the trough of sediments is compressed, deformed, reboundsupward into new mountains and also metamorphoses at the same time from all thepressure and heat. A contracting (cooling ) Earth was cited as a possible mechanism,but the mechanism failed the tests of observation. No good mechanism was everbrought forward, but geosynclinal theory describes the patterns seen in manymountains. They look like this happened.

b. Contracting Earth hypothesisi. The contracting Earth hypothesis was developed to explain the apparent crustal

shortening evident in folded mountain ranges. The interior of the planet (core andmantle) cooled and shrank, decreasing Earth’s radius by hundreds of kilometers,according to this hypothesis.

ii. The shrinkage was based on the amount of cooling estimated by Lord Kelvin when heattempted to estimate the age of the earth based on its heat flux. Common wisdomsays that the amount of cooling estimated by Lord Kelvin (known also as WilliamThomson)--and, hence, the age of Earth--was initially too large because Kelvin didnot know about radioactivity and the heat it adds to the mantle. The story commontold is that after radioactivity was discovered, geophysicists found that the Earthcooled much less than Kelvin estimated because radioactive decay provided heat thatKelvin did not account for in his equation. However, this idea was not verified at thetime by any sort of attempt to estimate the effect of radioactive decay on Kelvin’scalculation of heat flux. Phillip England (2007) re-evaluated the history and thecalculations of Lord Kelvin and found several interesting facts. First, the same yearthat Kelvin (1895) published his model of heat loss and estimated the age of the earth,John Perry (1895) rebutted the paper in Nature. Perry pointed out that Kelvinassumed that the heat was from a thin band at the top of the mantle and did not

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consider heat in the whole volume of the planet. Further, Perry showed that, whenconvection throughout the mantle and core is considered (Kelvin thought themechanism was conduction), then the convective loss of heat from the lower mantleand core would result in an age of Earth on the order of 2 Ga.

iii. In any case, the earth has shrunk no more than 10 km tops, not hundreds ofkilometers, so wrinkling of the crust into mountains is untenable.

iv. Furthermore, the hypothesis could not explain the episodic nature of mountainbuilding events, nor the fact that mountain ranges vary in age from very old tojuveniles less than 40 millions years old rather than clustering in the early stage ofcooling when heat loss (which is an inverse exponential) was fastest and theshrinkage would have been largest.

c. Continental Drifti. Alfred Wegener developed this concept. See Kennett’s text for a more complete

explanation of the history of the conceptii. Lines of evidence

(1) Distribution of identical species 200 Ma on landmasses that are presentlyseparate.

(2) Jigsaw puzzle fit of the continents into Pangaea.(3) The continuation of rock formations and even tectonic trends across continents on

opposite sides of the Atlantic Ocean.(4) The match of climate zones marked by coal measures (tropics) and glacial

deposits (the late Permian glacial tills rejoined; Devonian tills in the middle of theSahara Desert are at the poles at that time according to Wegener’sreconstructions)

(5) His hypothesis stalled when an adequate mechanism could not be found.d. Mantle Convection (Holmes, 1931)

i. Holmes proposed thermal convection cells in the mantle. As you noted above, hewas not the first to propose this (Perry was the first), but this time people paidattention. He suggested that the ocean floor was generated by a diffuse intrusion ofmagma into ocean crust in a sporadic pattern.

ii. Vening Meinesz added that ocean trenches were formed by drag of downward-moving limbs of convection cells and the drag also produced mountain-buildingcompression.

iii. The mechanism for compression was used to explain folded and thrust-faultedmountains. This is tantalizingly close to modern plate tectonic theory.

e. Expanding Earth hypothesisi. In the 1950's, surveys of the seafloor revealed that the ocean ridges were very

extensive and were clearly under extensional stress.ii. Carey (1958) proposed that Earth is expanding to explain the extensional ridges that

wrap around the entire planet (60,000 km of oceanic ridges).iii. But, how can the compression features of continental mountains be explained? And

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why would the Earth be expanding, excess heat from radioactive decay?

2. New evidence from work in the 1950's and 1960's that lead to formulation of plate tectonictheory.a. Distribution of earthquakes (seismic activity) relative to geologic features: Why is there

such a distinctive pattern?i. beneath trenches and island arcs

(1) there are shallow (<10 km), intermediate (10-100 km), and deep earthquakes to660 km below the surface of the earth. The deepest earthquakes occur in themantle!

(2) Benioff seismic zone: the plane of earthquakes beneath island arcs that extends toas much as 660 km.

ii. At oceanic ridge (OR)(1) only shallow earthquakes occur here along the rift valley and (2) transform faults: shallow-depth earthquakes between offset segments of OR.(3) shallow earthquakes are less than 10 km deep, and mostly in the brittle crust.

b. Polar wonderingi. Measure the paleomagnetism of successively older rocks on a continent, and the pole

appears to have wondered much more than the ~1100 km ring around the geographicnorth pole. One way to explain this is assume that the magnetic pole hasn’t changedits behavior, but that the continents have moved.

ii. In fact, if the path of the magnetic pole is measured on Europe and North America,then compared, they are distinctly different. This can only be explained if onecontinent has moved relative to the other. In fact, the pathway the continents musthave traveled can be backtracked.

iii. The method was used to backtrack the continents to their positions about 200millionyears ago, when they appear to be all joined into a supercontinent, Pangaea. Theresults supported the proposals of Wegener.

iv. As a side note, there is a very interesting article in the January 2001 issue of Scienceabout true polar wandering, intervals where continental drift and sea floor spreadingcan’t explain the movement of the magnetic pole, suggesting internal redistribution ofmass that has caused the geographic poles to shift!!!

c. Sea floor spreading model (Hess, 1962)i. In 1959 Harry Hess suggested that the ocean floor was relatively young and

constantly being created at the mid-ocean ridges and destroyed at marginal oceantrenches, a concept dubbed seafloor spreading by Bob Dietz. Hess published a formalpaper explaining his ideas in 1962.

d. Magnetic anomaly patterns around oceanic ridges (OR) could be explained if newseafloor was being created at the ridges. i. Data and concept published in 1963 by Fred Vine, a graduate student, and Drummond

Matthews, his major professor.

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ii. They worked on the Juan de Fuca and Gorda Ridges (where are they located?)iii. Found linear anomaly patterns symmetrically arranged around the rift valleys of these

ridges. The only way they could think to explain it was by emplacement of freshbasalt at the ridge crest. This provided a specific geographic site for creation of newsea floor, an idea suggest by Holmes (1931) who imagined that emplacement waswidespread and diffuse rather than concentrated at a single feature like the OR. Andof course it directly supported the suggestions of Harry Hess (1962).

iv. Vine and Mathhews invited others to explain their data in another way, but theycouldn’t think of anything better than the model of Harry Hess!

v. (What do you do with the rest of your career if you discover sea floor spreading inyour thesis research? May you all have to face this problem!)

e. Transform faults between segments of oceanic ridge (hypothesis of Tuzo Wilson, 1965)i. a fault: the rocky crust on either side of a fault moves in opposite directions by

definition.ii. Segments of the OR are offset from one another as though movement along faults has

torn apart the ridge into the segments. But, is this what is happening?iii. If spreading occurs at the ridge, then movement of the crust will be away from the

crest of the ORs, opposite to what is predicted if faulting caused the offsets betweensegments of the oceanic ridge (see diagram in the presentation).

iv. Seismographs (Lynn Sykes, 1966) placed on the seafloor showed that movementalong the faults is away from the OR and must be caused by movement of new crustaway from the spreading centers. The zone where the relative movement occurs iscalled a transform fault.

v. This last experiment was the final straw that convinced many geologists thatspreading occurs.

f. Age of oceanic crust–the final piece of evidence that convince most of the hold-outs. This evidence took many years to gather.i. Since 1968, the Deep Sea Drilling Project (DSDP), which was renamed the Ocean

Drilling Project (ODP) in 1985, and the IODP since 2001, have taken cores anddrilled to solid ocean crust in more than 1329 sites as of December 2005 in all majoroceans except the Arctic Ocean using drilling ships, first the Glomar Challenger andthen the JOIDES Resolution. The Chikyo is now operational.

ii. The age of the sediment overlying the ocean crust at many of the sites has beendetermined using biostratigraphy, and all are consistent with sea floor spreading. Theage of the ocean crust increases virtually everywhere in the ocean with distance fromthe OR (oceanic ridge). This is a very convincing piece of evidence in support of seafloor spreading.

iii. Evidence that poured forth in support of the hypothesis after the mid 1960srevelations. For example, the thickness of ocean sediment increases from millimetersat the spreading centers to kilometers near the continental margins. Kennett mentionssome, and you can probably think of some patterns that are explained by the theory.

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g. Attention was turned to subduction zones as well. This is the site where sea floor isdestroyed, hence mass balance between formation and destruction of the seafloor. Theearth doesn’t expand or contract within our ability to measure it.

3. Very soon scientists proposed that the surface of the earth is made up of about 20 rigid plates(12 major ones and a number of little ones) that included the crust and about 90-100 km ofthe uppermost mantle. They called this the lithosphere (crust + uppermost mantle, moreinformation later). The plates can have one or more of three types of margins: a marginwhere they form, a margin where they are destroyed, and a margin where one plate simplymoves past another plate without growth or destruction of ocean crust.