Download Continental Drift and Sea-Floor Spreading

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Department of Marine Science
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 continental
geology. 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 to
complement the military effort. So, what could geologists see from the land before all of this
war-driven exploration?
1. Hypotheses
a. 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. The
trough accumulates sediment in deep water at the axis of the trough and in shallow
water at its margins. The trough subsides under the weight of added sediment. At
some magical moment, the trough of sediments is compressed, deformed, rebounds
upward into new mountains and also metamorphoses at the same time from all the
pressure and heat. A contracting (cooling ) Earth was cited as a possible mechanism,
but the mechanism failed the tests of observation. No good mechanism was ever
brought forward, but geosynclinal theory describes the patterns seen in many
mountains. They look like this happened.
b. Contracting Earth hypothesis
i. The contracting Earth hypothesis was developed to explain the apparent crustal
shortening evident in folded mountain ranges. The interior of the planet (core and
mantle) 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 he
attempted to estimate the age of the earth based on its heat flux. Common wisdom
says that the amount of cooling estimated by Lord Kelvin (known also as William
Thomson)--and, hence, the age of Earth--was initially too large because Kelvin did
not know about radioactivity and the heat it adds to the mantle. The story common
told is that after radioactivity was discovered, geophysicists found that the Earth
cooled much less than Kelvin estimated because radioactive decay provided heat that
Kelvin did not account for in his equation. However, this idea was not verified at the
time by any sort of attempt to estimate the effect of radioactive decay on Kelvin’s
calculation of heat flux. Phillip England (2007) re-evaluated the history and the
calculations of Lord Kelvin and found several interesting facts. First, the same year
that 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 Kelvin
assumed 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, when
convection throughout the mantle and core is considered (Kelvin thought the
mechanism was conduction), then the convective loss of heat from the lower mantle
and 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 of
kilometers, so wrinkling of the crust into mountains is untenable.
iv. Furthermore, the hypothesis could not explain the episodic nature of mountain
building events, nor the fact that mountain ranges vary in age from very old to
juveniles less than 40 millions years old rather than clustering in the early stage of
cooling when heat loss (which is an inverse exponential) was fastest and the
shrinkage would have been largest.
c. Continental Drift
i. Alfred Wegener developed this concept. See Kennett’s text for a more complete
explanation of the history of the concept
ii. Lines of evidence
(1) Distribution of identical species 200 Ma on landmasses that are presently
separate.
(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 the
Sahara Desert are at the poles at that time according to Wegener’s
reconstructions)
(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, he
was not the first to propose this (Perry was the first), but this time people paid
attention. He suggested that the ocean floor was generated by a diffuse intrusion of
magma into ocean crust in a sporadic pattern.
ii. Vening Meinesz added that ocean trenches were formed by drag of downwardmoving limbs of convection cells and the drag also produced mountain-building
compression.
iii. The mechanism for compression was used to explain folded and thrust-faulted
mountains. This is tantalizingly close to modern plate tectonic theory.
e. Expanding Earth hypothesis
i. 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 tectonic
theory.
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 to
660 km below the surface of the earth. The deepest earthquakes occur in the
mantle!
(2) Benioff seismic zone: the plane of earthquakes beneath island arcs that extends to
as 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 wondering
i. 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 geographic
north pole. One way to explain this is assume that the magnetic pole hasn’t changed
its 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 one
continent has moved relative to the other. In fact, the pathway the continents must
have traveled can be backtracked.
iii. The method was used to backtrack the continents to their positions about 200million
years ago, when they appear to be all joined into a supercontinent, Pangaea. The
results supported the proposals of Wegener.
iv. As a side note, there is a very interesting article in the January 2001 issue of Science
about true polar wandering, intervals where continental drift and sea floor spreading
can’t explain the movement of the magnetic pole, suggesting internal redistribution of
mass 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 ocean
trenches, a concept dubbed seafloor spreading by Bob Dietz. Hess published a formal
paper explaining his ideas in 1962.
d. Magnetic anomaly patterns around oceanic ridges (OR) could be explained if new
seafloor 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 fresh
basalt at the ridge crest. This provided a specific geographic site for creation of new
sea floor, an idea suggest by Holmes (1931) who imagined that emplacement was
widespread and diffuse rather than concentrated at a single feature like the OR. And
of course it directly supported the suggestions of Harry Hess (1962).
iv. Vine and Mathhews invited others to explain their data in another way, but they
couldn’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 in
your 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 between
segments of the oceanic ridge (see diagram in the presentation).
iv. Seismographs (Lynn Sykes, 1966) placed on the seafloor showed that movement
along the faults is away from the OR and must be caused by movement of new crust
away from the spreading centers. The zone where the relative movement occurs is
called a transform fault.
v. This last experiment was the final straw that convinced many geologists that
spreading 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 and
drilled to solid ocean crust in more than 1329 sites as of December 2005 in all major
oceans except the Arctic Ocean using drilling ships, first the Glomar Challenger and
then the JOIDES Resolution. The Chikyo is now operational.
ii. The age of the sediment overlying the ocean crust at many of the sites has been
determined using biostratigraphy, and all are consistent with sea floor spreading. The
age of the ocean crust increases virtually everywhere in the ocean with distance from
the OR (oceanic ridge). This is a very convincing piece of evidence in support of sea
floor spreading.
iii. Evidence that poured forth in support of the hypothesis after the mid 1960s
revelations. For example, the thickness of ocean sediment increases from millimeters
at the spreading centers to kilometers near the continental margins. Kennett mentions
some, 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 is
destroyed, hence mass balance between formation and destruction of the seafloor. The
earth 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 of
the uppermost mantle. They called this the lithosphere (crust + uppermost mantle, more
information later). The plates can have one or more of three types of margins: a margin
where they form, a margin where they are destroyed, and a margin where one plate simply
moves past another plate without growth or destruction of ocean crust.
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