Chris Goldfinger

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Chris Goldfinger Burt 280 or 282
7-5214 gold_at_coas.oregonstate.edu http//activetect
onics.coas.oregonstate.edu Reading for next
class Richards et al., 2000 (paper
copy) Bercovici et al., 2000 web site Oreskes
chapter on origins of mountain building models
(paper copy)
OCE 661 Plate Tectonics
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Science doesnt always progress in a straight
line. The development of plate tectonics
revolutionized solid earth sciences, but it
wasnt easy Youve heard of Hess, Vine,
Mathews, Wilson, and probably Wegener, but did
you know
1596 Dutch map maker Abraham Ortelius produced
Thesaurus Geographicus. Ortelius suggested
that the Americas were "torn away from Europe and
Africa . . . by earthquakes and floods" and went
on to say "The vestiges of the rupture reveal
themselves, if someone brings forward a map of
the world and considers carefully the coasts of
the three continents."
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In 1858, geographer Antonio Snider-Pellegrini
made these two maps showing his version of how
the American and African continents may once
have fit together, then later separated.
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Continental Drift - introduced in two articles
published by a German meteorologist named Alfred
Wegener. The origin of continents and oceans
(1915) First person to present evidence other
than continental margin fit 200 m.y. ago,
the supercontinent Pangaea (all Earth) began to
split apart.
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• Evidence for continental drift
• Geological evidence
• fold belts
• 2) age provinces
• 3) igneous provinces
• 4) stratigraphic sections
• 5) metallogenic provinces

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Paleontological evidence 1) distribution of
tetrapods-early distrib-easy communication in
Pangaea 2) early Permian reptile
Mesosaurus-found in S Africa and Brazil 3)
marine invertebrates-distrib of cont and oceans
different from today 4) Cambrian trilobites 5)
ammonites (shallow seas between India, Madagascar
and Africa in J) 6) Glossopteris and
Gangamopteris fauna in Gondwana (cold climate),
tropical flora in Laurasia 7) diversity of
species (increases towards Eqdrifting N-S
controls the diversity)
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Fauna in Pangaea-similar climates
Geographical proximity of fossil fauna in
reconstructed Pangaea -Mesosaurus, Lystrosaurus,
Cynognathus (lizards) -Glossopteris (seed fern)
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Paleoclimatic evidence Some lithologies are
indicative of particular climates and hence
global position 1) carbonates and reef
deposits-warm water, 30deg from eq 2)
evaporates-hot, arid conditions 3) red beds-hot
climate for form of hematite 4) coal and
oil-warm, humid climate 5) phosphorites-within
45 deg of eq 6) bauxite and laterite-tropical,
subtropical weathering 7) desert deposits-both
warm and cold cond, direction of wind,
continental rotation 8) glacial deposits-30 deg
from poles
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Wegener - a continental ice sheet covered parts
of South America, southern Africa, India and
southern Australia about 300 million years ago.
Glacial striations on rocks show that glaciers
moved from Africa toward the Atlantic Ocean and
from the Atlantic Ocean onto South America. Such
glaciation is most likely if the Atlantic Ocean
were missing and the continents joined
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Mechanism for continental drifting? (in
Wegeners hypothesis) 1. continental crust plows
through oceanic crust like an icebreaker through
ice (pole-fleeing force-movement of continents
toward Equator) -continental crust is too weak to
do this (2.7g/cm3 vs 3.3g/cm3) the force is
several millions times smaller than force of
gravity) 2. Centrifugal (rotational) and tidal
forces are responsible for movement of land
masses (westward movement of continents) - tidal
forces are too weak to do this
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Alexander Du Toit, Professor of Geology at
Johannesburg University proposed that Pangaea
first broke into two large continental
landmasses, Laurasia in the northern hemisphere
and Gondwanaland in the southern hemisphere (Our
wandering continents, 1937). He proposed as a
mechanism of continental drift a combination of
geological forces-erosion, deposition, subsidence
and the ultimate ascent of molten
magmageosyncline concept A. Holmes (Edinburgh
University) Principles of Physical Geology
(1945) radioactivity concept - first to suggest
that the continental drift might be explained by
thermal convection continents do not sail
over the mantle, but they are carried by
a conveyor belt represented by mantles
convection cells formed due to gravitational and
thermal forces
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The hypothesis was widely rejected for many
decades, elaborate but incorrect schemes were
concocted to explain mountain ranges and large
synclines. In the 1950s a wealth of new
evidence emerged to revive the debate about
Wegener's ideas (1) demonstration of the
ruggedness and youth of the ocean floor (2)
confirmation of repeated reversals of the Earths
magnetic field in the geologic past (3)
emergence of the seafloor-spreading hypothesis
and associated recycling of oceanic crust
and (4) precise documentation that the world's
earthquake and volcanic activity is concentrated
along oceanic trenches and submarine mountain
ranges.
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Ocean floor mapping About two thirds of the
Earth's surface lies beneath the oceans.
Before the 19th century, the depths of the open
ocean were largely a matter of speculation, and
most people thought that the ocean floor was
relatively flat and featureless. 19th century
deep-sea line soundings (bathymetric surveys) in
the Atlantic and Caribbean. 1855, a
bathymetric chart published by U.S. Navy
Lieutenant Matthew Maury revealed the first
evidence of underwater mountains in the central
Atlantic (which he called "Middle Ground").
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later confirmed by survey ships laying the
trans-Atlantic telegraph cable. after World
War I (1914-18) echo-sounding devices began to
measure ocean depth by recording the time it took
for a sound signal from the ship to bounce off
the ocean floor and return. ocean floor is
much more rugged than previously thought.
demonstrated the continuity and roughness of
the submarine mountain chain in the central
Atlantic (later called the Mid-Atlantic Ridge)
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Chronology of recent discoveries in this area a.
1947 Maurice Ewing determines the existence of a
vast abyssal plain. b. 1947 Oceanographers
abandon the idea that the Oceans' floors are flat
and produce arguments in favor of the continental
drift hypothesis. c. 1950's Prompted by studies
of paleomagnetism in the 1940's, geologists and
oceanographers begin to accept the concept of
continents moving relative to magnetic poles and
to one another. This change represents the
beginning of sea floor spreading and plate
tectonics. d. 1953 American geologists Maurice
Ewing and Bruce Heezen discover an underwater
canyon , and in 1956 they propose the existence
of the Mid-Oceanic Ridge, accompanied by a
formation of canyons called the Great Global
Rift. The discovery that the rift separates the
Earth's crust into plates contributes to the
development of the theory of plate tectonics. e.
1960 US Geologist Harry Hess proposes the
concept of sea floor spreading, a key idea in
plate tectonics. Hess suggests that new crust
forms at rifts, especially on the sea floor,
where the lithosphere moves apart mantle
convection.
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f. 1963 Geologists (Frederick Vine) discover the
phenomenon of periodic magnetic reversals in the
earth's crust, evident from the pattern
of alternating magnetic polarity in the ocean
floor near mid-ocean rifts. This discovery
supports Hess' idea of sea-floor spreading. g.
1965 Scientist now suspect the existence of "hot
spots", junctures at tectonic plates through
which heat leaks up into the ocean hot spots are
not detected until the 70s. h. 1977 Scientists
using the submersible ALVIN discover deep ocean
vents near the Galapogos Islands, where hot,
mineral-laden water spews into the sea. i. DSDP
(Deep Sea Drilling Project) began in 1963, funded
by NSF, Joint Oceanographic Institutes for Deep
Earth Sampling (JOIDES ) now headed by TAMU.
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Meanwhile, back closer to the surface, Holmes
provided a possible mechanism for driving the
plates, convection.

• Simple thermal convection like this is based on
• Equal input from below and loss from the top.
• Flat earth
• No internal heating
• Constant viscosity and rheological properties.
• How realistic is this? Are there solutions to
  these problems?

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After almost 4 decades, many problems are
unresolved. Simple convection cannot produce
such things as strike-slip plate boundaries,
narrow mid-ocean ridges, or hotspot plumes. If
you are a little confused about how plate driving
forces are described, and how a plate can rotate
around a vertical axis due to convection, thats
ok. There is no consensus about the relative
importance of these forces, or on the existence
of some of them. How the above problems are
being addressed is complex. While convection is
almost certain to be responsible, we know little
of the deep structure. First lets look at the
current view of forces
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What to we really know about mantle convection?
In reality, not very much. We live at the
upper boundary layer, on a plate, at least I do.
So we think we know a little bit about what must
be happening nearby, at plate boundaries, and
maybe at the bottom surface of the plate we are
riding on. But even that knowledge is mostly
conjecture.
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So, rather than gloss over the issues, lets use
this as a starting point for discussion of the
outstanding issues. This discussion will be
woven into the course throughout, but well get
started with an overview of what works and what
doesnt work so well with mantle convection,
plate driving forces, and plate tectonics. To
begin, we have to let go of the idea that plate
tectonics and mantle convection form a well
defined Unified Theory. Much like the elusive
Unified Field Theory that frustrates physicists
today, the commonly taught version of plate
tectonics and mantle convection isnt what it
appears. Lets start with what we think we know
about basic convection.
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Simple convection of a fluid heated at is base is
well described by Rayleighs hydrodynamic
stability theory (1916). The unstable fluid
will tend to form convection cells about as wide
as they are deep (unit aspect ratio). Rayleighs
theory is a search for the least perturbation
that will cause overturning of the unstable
fluid. This mode is called the least stable or
most unstable mode. The Rayleigh number of a
fluid yields the relative ease with which a
temperature perturbation, DT, will start
convection. This is called the critical Rayleigh
number Racr Ra rgaDTd3 mk r density g
gravitation, d layer thickness a thermal
expansivity m dynamic viscosity k thermal
diffusivity
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Ra for fluids is on the order of 1000. The
greater the number, the greater the vigor of
convection. Ra for the mantle is on the order of
107-109. While mantle convection is very slow,
it is considered vigorous in hydrodynamic terms.
It acts over huge distances against extreme
viscosity ( 19 orders of magnitude greater than
molasses!). Ok, Lets consider simple
symmetrical convection, where heat from the
bottom is equal to loss off the top, viscosity is
constant, and both top and bottom boundaries are
free-slip. Cold downwelling will be the same
difference from average as the warm upwelling,
And both top and bottom will have symmetrical
Thermal Boundary Layers. These are the
transition zones that are unstable, and are in
contact with either the heating or cooling
source. Tectonic plates are the upper boundary
layer. Much of the rest of the fluid is
gravitationally stable.
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The upper boundary layer thickens and cools as it
moves away from the divergence zone and toward
the downwelling zone, forming a horizontal
boundary current. It flows as far as it can
before cooling enough to sink, thus defining the
size of a tectonics plate in the simplest sense.
An aspect ratio of 1 is typical. Forces
Gravity drives the vertical currents, but the
horizontal currents are driven by pressure
gradients. High pressure forms as the plumes
reach the surface, and flow toward low pressure
in the more dense downwelling zones, similar to
high and low pressure systems in the atmosphere.
This convection based view is somewhat
different that the plate forces we just
discussed. Does basal traction have any
meaning? Are plates passive or active?
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In the earth, symmetrical convection does not
exist. 85-90 of the Earths heat comes from
radiogenic heating, not from contact with the
molten Iron outer core (Turcotte and Schubert,
1982). How does this change the picture?
The dominance of internal heating means that the
convection becomes asymetrical. The bottom
boundary layer only has to transfer heat from the
core, while the upper boundary layer has to
transfer heat from radiogenic heating, plus the
heat from the core, to remain in balance. This
means that the cold downwellings dominate the
system, and the upwellings are relatively weak
and diffuse (Davies and Richards, 1992).
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What about all our other assumptions, like
constant viscosity? Also incorrect. Mantle
viscosity is highly temperature dependent.
Whether motion takes place as diffusion creep, or
dislocation power-law creep, mobility is highly
temperature dependant, and has a quantum
mechanical probability distribution. A few
hundred degrees temperature change results in
many orders of magnitude viscosity change.
How does this change the picture?
This effect means that the upper boundary layer
is also mechanically stronger (a plate), and
serves to further cap temperature loss,
increasing the tendency of cold downwellings to
dominate the system, and reducing the upwellings
(Davies and Richards, 1992).
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If the viscosity change is too great, the plates
will form a stagnant lid, and convection will
go on below, perhaps this is happening on Mars
and or Venus??
What is this?
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The Earth is also round, more or less, what
effect does this have? Convection becomes more
complex, but the main difference is that the
symmetry of upper and boundary layers is again
broken. Why?
The surface area of the CMB is much less than the
area of the Earths surface, so for thermal
symmetry, the temperature anomaly would have to
be much greater. This effect is somewhat counter
to the effects of internal heating and viscosity
variability.
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So where do we stand with convection and plate
tectonics. Many things work well

• Dominance of subduction zones and slab pull is
  consistent with theory, and rates are
  approximately correct (see Bercovici et al.
  2000). Asymmetry consistent with internal
  heating.
• Ridge push is consistent with a convective
  pressure gradient (not an edge force as
  originally conceived), and consistent with
  tensional ridge structure, and weak upwelling.
• Ocean basin structure and depth profiles fit the
  thermal boundary layer model reasonably well.

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So where do we stand with convection and plate
tectonics. Many things work well

• Downwelling zones correspond reasonably well with
  mantle heterogeneity in the form of velocity
  anomalies

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So where do we stand with convection and plate
tectonics. Some things seem to work, and some
things dont work quite so well

• Convection models do not predict narrow intense
  divergence zones (ridges)
• Collision resistance not predicted (because
  convection models do not predict plates of
  plateness. This results from chemical
  differentiation and continent formation.
• Ocean basins deviate from the age-2 relationship
  near ridges (hydrothermal circulation?) and far
  from ridges (secondary convection??)
• Dynamic topography, expected from mantle
  heterogeneity, seems inconsistent. The South
  African superswell makes sense, but some other
  areas of expected dynamic topography have not
  been found.

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So where do we stand with convection and plate
tectonics. And some things dont work quite so
well

• Plate motion changes Abrupt changes in plate
  motion, such as the Hawaiian-Emperor bend, also
  reflected in many other seamount chains on the
  Pacific Plate, seems to require a 45 degree
  change in less than 5 my. How does convection do
  this? Some have suggested that all the hotspots
  changed direction simultaneously instead.
  Attempts to quantitatively fit this change to the
  collision of India with Asia, while the timing is
  ok, have otherwise not fared well (Richards and
  Lithgow-Bertelloni, 1996).

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So where do we stand with convection and plate
tectonics. And some things dont work quite so
well

• By the way, what are hotspots anyway? Where do
  they fit in? Compatible in mid plate settings,
  but not so much in ridge centered settings, and
  where downwelling zones cross them (Yellowstone
  hot spot).
• For that matter, how can slab pull drive India
  through Asia? Will it stop?
• Models dont predict plates at all, or
  plateness. Other features are required, such
  as strong interiors and weak edges (plate
  evolution)

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So where do we stand with convection and plate
tectonics. And some things dont work quite so
well

• One of the biggest missing pieces, is toroidal
  motion, that is rotations about vertical axes and
  the attendant strike slip motion. This may
  account for as much as 50 of plate kinetic
  energy. Where does this come from?
• You can impose plate shapes or fault systems to
  generate toroidal motion (not very satisfying)
• The existence of such motion may minimize energy
  dissipation in 3D convection models, and thus be
  favored energetically.
• All of these problems require very soft plate
  boundaries, and seem to require non Newtonian
  lithospheric layers, i.e. strain-softening non
  linear behavior. This means the greater the
  force, the weaker it gets. Can you think of an
  example?