Chris Goldfinger

1
Chris Goldfinger Burt 282 7-5214 gold_at_coas.ore
gonstate.edu
OCE 661 Plate Tectonics
Course notes at www.activetectonics.coas.oregonst
ate.edu
Course notes modified in part after notes
developed by D. Muller, University of Sydney
School of Geosciences, Division of Geology and
Geophysics, Sydney Australia
2
What is this?
Where is it?
3
How does the magnetic striping pattern form?
Why are the stripes symmetrical around mid-ocean
ridges? 1961 scientists began to
theorize that mid-ocean ridges mark structurally
weak zones where the ocean floor was being ripped
in two lengthwise along the ridge crest.
4
What causes the Earths magnetic field? 1. The
permanent magnet hypothesis-inner core (iron,
nickel)-problem-ferromagnetic substances lose
their ferromagnetism above the Curie point
(temp less than 1000deg)-in the core the metals
are liquid (therefore temp gt1000deg) 2. Dynamo
hypothesis-the magnetic field caused by electric
currents within the Earth
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Dynamo- A system that uses electromagnetic inducti
on to convert mechanical energy (motion) into a
magnetic field
The Earth's magnetic field is produced by dynamo
action the movement of fluid in the core
through a magnetic field produces electrical
currents, that, in return maintain the magnetic
field gtwhat moves the fluid?
7
Energy sources for driving convection in the
outer core Radioactivity from K, Th, U
(reside in Earth's crust, in small amount in the
core) Compositional convection, in which
heavier iron-nickel crystallizes in the outer
core and sink onto the inner core boundary.
Lighter residual elements rise buoyantly.
Latent heat is released as the inner core
material crystallizes.
8
A recent study showed that a conducting fluid
needs to have a small seed magnetic field
before it can generate a self-sustaining
field. The creation of this field relies on
positive feedback mechanism. This process works
if the fluid moves very rapidly and is in large
quantity. (from Gailitis et al.,1984 Phys. Rev
Let)
9
Origin The origin of the Earth's magnetic field
is due to convection of a electrically conducting
fluid outer core. This is an example of a
"homogeneous dynamo", or a dynamo without
wires, consisting of currents in a continuum
Most likely pattern of convection consists
of cylindrical rolls aligned with the Earth's
axis of rotation.
A snapshot of the lines of force of the magnetic
field generated in the simulated fluid core of
the Earth.
10
Character of the magnetic field The Earth's
Magnetic Field is primarily a dipole
field, exhibiting some small non-dipole
components. (Note field lines are the directions
of magnetic force, a compass needle orients
itself parallel to the local field line. Field
lines are perpendicular surfaces of constant
potential.)
11
The parameters conventionally used are
intensity B, declination D, and inclination I. I
and D are measured in degrees. inclination I
angle between the field vector B and the
horizontal, measured positive down, i.e. it
goes from 90 to -90. declination D angle in
the horizontal (N-E plane) between the field
vector and the direction to the geographic north
pole. In the horizontal (N-E plane) the
declination D is the angle between the field
vector and the direction to the geographic north
pole. D is measured positive in a clockwise
direction from 0-360.
Bh is the projection of the field vector onto
the horizontal plane and Bz is the projection
onto the vertical axis Ng denotes geographic
north, and Nm magnetic north, E geographic east,
and Z down.
12
Declination across Australia varies from 5W in
SW Australia to 15E in SE Australia
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The Earths field is not a perfect dipole.
Variations on the order of 10 contribute to what
is called the non-dipole field. The
geomagnetic poles are presently at 79N, 71W and
79S, 109E. The intensity of the magnetic field
B is measured in nannoTeslas(nT) (SI-unit). Some
useful conversions are 1 nT 10-5 G (Gauss), 1
nT1g (gamma).
14
Main field declination, 1995
180 0 -180
15
Colatitude and inclination are related by the
dipole formula tan I 2 cot (q) q 90
l. tan (l ) (tan I)/2.
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Geomagnetic field reversals the Earth's dipole
field flips polarity at irregular intervals
the polarity is said to be "normal" when it is
oriented the same as today on average, the
field spends about half its time in each state
reversals are observed from Precambrian times to
the present although the frequency of reversals
has changed considerably through time during a
reversal, the intensity usually decreases by
about an order of magnitude for several thousand
years, while the field maintains its direction.
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During a reversal, the intensity usually
decreases by about an order of magnitude for
several thousand years, while the field maintains
its direction. The field then undergoes
complicated directional changes over a period of
1000- 4000 years and finally intensity grows with
the field having reversed polarity
18
Robert and Glatzmaier model (1995) first
3D computer simulation of an Earth-like magnetic
field that reverses itself -the objective was to
get a geomagnetic field that would maintain
itself longer than the decay time -after running
for 1 year on a supercomputer the model showed a
reversal -the model shows that in the inner core
the magnetic field has an opposite polarity from
the outer core, and this stabilizes the field
against a tendency to reverse more
frequently. -the model simulated the behavior of
the geomagnetic field for 40000 yr. After 36,000
years, the magnetic field reversed its dipole
polarity over a period of only 1200 years
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Magnetic field lines Normal
transition reversed
- The total time span of a reversal is
up to 10.000 years - The reversal sequence has
been calibrated for the last 5 million years
by dating basalts of known polarity.
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What causes the reversal? If outer core acts as
a dynamogtthe equations to describe it have 2
solutions-normal and reverse, more
complicated dynamos have spontaneous
reversals -postulated that the reversal begins
in a localized region of the core, usually S
hemisphere -possible coupling between core and
mantle that determine the frequency of reversals
22
Is the Earths magnetic field is about to
change its polarity ? French and Danish
scientists have plotted the currents of molten
iron that create the magnetic dipole over the
last 20 years, -gtthe vortices in the molten iron
rotate in a direction that reinforces a reversed
magnetic field (Hulot et al., 2002,
Nature) By combining data from
satellite missions with measurements of
the paleomagnetic field recorded in lava and
sediments, there is potential for extending
our picture of the geodynamo over thousands,
even millions, of years into the past.
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Paleomagnetism Study of fossil magnetism
retained in rocks -rocks that form oceanic floor
acquired magnetic imprints of the magnetic field
of the time on it cooled -on land volcanic lava
preserve ancient magnetic field signatures minera
ls responsible for ferromagnetic
properties -magnetite, hematite (iron
oxides)-only 1 of rock ferromagnetic
magnetization below the Curie temperature (580C
for magnetite, 680 for Hematite) gt permanent
or remanent magnetism DRM (detrital remanent
magnetization)-process by which a sediment
acquires magnetic memory, the ferr. Grains behave
like compass needle TRM (thermoremanent
magnetization)-lava flows, igneous rocks CRM
(chemical remanent magnetization) VRM (viscous
remanent magnetization)
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Portions of the time scale which are of
one dominant polarity are called chrons, and
the most recent four chrons are named after
scientists who contributed significantly to our
understanding of the geomagnetic field (Brunhes,
Matuyama, Gauss, Gilbert).
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Several lines of evidence (1) at or near the
crest of the ridge, the rocks are very young, and
they become progressively older away from the
ridge crest (2) the youngest rocks at the ridge
crest always have present-day (normal) polarity
and (3) stripes of rock parallel to the ridge
crest alternated in magnetic polarity
(normal-reversed-normal, etc.), suggesting that
the Earth's magnetic field has flip-flopped many
times.
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Magnetic striping and polar reversals Early
1950s, scientists, using magnetometers adapted
from airborne devices developed during World War
II to detect submarines, began recognizing odd
magnetic variations across the ocean floor.
basalt contains a strongly magnetic mineral
(magnetite) and can locally distort compass
readings.
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We map the oceanic magnetic field by using a
proton precession magnetometer. It is based on
the fact that nuclear magnetic moments posses a
spin, which will precess about the earth's
magnetic field. In the magnetometer the
free-precession of hydrogen nuclei ( protons) is
measured. In the absence of a magnetic field the
dipole moments of protons in water are randomly
oriented. In the presence of a strong magnetic
field the dipoles become polarized in the
direction of the field.
When the field is removed the protons spin is
oriented around the direction of the Earths
magnetic field for a short time, until they
return to their random state. After the
polarizing field has been switched off, the
frequency of the spinning protons is counted.
The precession frequency is proportional to the
field strength. As a consequence the proton
precession magnetometer produces a number of
discrete measurements of the absolute field
strength by means of the proton precession
frequency. The advantage of this type of
magnetometer is that the orientation of the
instrument is not critical.
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Early in the 20th century, paleomagnetists --
such as Bernard Brunhes in France (in 1906) and
Motonari Matuyama in Japan (in the 1920s) --
recognized that rocks generally belong to two
groups according to their magnetic
properties gt normal polarity and reversed
polarity ocean floor shows a zebra-like pattern
of alternating stripes of magnetically different
rock gt magnetic striping.
29
The total magnetic field B that we measure with
a magnetometer, either over continental or ocean
crust, is always the sum of the ambient field Ba
and the field originating from magnetized rocks
Br. B Ba Br
30
In general, the ambient field, Ba, is much
stronger than the field generated by magnetized
rocks, Br. Ba gtgt Br
31
A magnetic anomaly is caused by an edge effect
between two bodies with different magnetization.
A series of magnetic lineations on the ocean
floor will cause the magnetic anomalies caused by
the individual edge effects to be superimposed on
each other to give an observed anomaly.
32
When both the crustal magnetization and
geomagnetic field vectors are steep (i.e. in the
vicinity of the magnetic pole), the normal blocks
cause positive anomalies. However, near the
equator, east-west striking blocks magnetized in
the same direction as the geomagnetic field
produce negative anomalies.
The top profile is produced by a model of an
east-west oriented spreading ridge, equivalent to
north south spreading, at the magnetic south
pole, and the bottom profile shows the same ridge
at the equator.
33
The shape and intensity of magnetic anomalies
depends on (1) the segmentation of the mid-ocean
ridge by fracture zones (i.e. length of
magnetized blocks along-axis), (2) spreading
velocity (length of blocks across-axis). Fast
spreading causes relatively longer blocks to form
than slow spreading. (3) frequency of polarity
reversals (length of blocks across-axis), (4) the
direction of magnetization in a given block.
34
Reversal time scale from marine magnetic
anomalies a) Determine the time scale (i.e.
transform the peaks and lows into a set of time
intervals-length-mag anom are produced by highly
magnetized basalt flow-layer2 -requires
comparison of many mag profiles from different
oceans b) Calibrate the age of time scale -by
known ages of oceanic crust (drilling or age of
sediments) Superchrons-large period of time with
constant reversal freq. -K-Tertiary-Quaternary
mixed polarity 0-83 Ma -Cretaceous Normal
Polarity superchron 83-119 Ma (K quiet
zone) -Jr-K mixed polariy 119-165 Ma
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Paleomagnetic investigation of deep sea cores
(using detrital remanent magnetism of sediments)
was used to extend the timescale back to 20 Ma by
Opdyke et al (1974). meanwhile even the oldest
preserved ocean crust (180Ma, west Pacific) has
been surveyed and drilled, extending the
magnetic timescale to about the Middle
Jurassic. paleomagnetic investigations on land
have shown that geomagnetic reversals have
occurred at least back to 2.1 Ga ("Gigannum",
billion years before present). The first
magnetic polarity timescale for the Late
Cretaceous to Quaternary was constructed by
Heirtzler et al. (1968). They used magnetic
anomalies along a single ship track in the South
Atlantic to calibrate their timescale by assuming
that seafloor spreading rates had been
approximately constant.
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Magnetic reversals correlated with other
geological observations
38
Past and present geomagnetic field secular
variation the geomagnetic field
undergoes progressive changes from variations in
the convection in the fluid outer
core. Assumption the Earth's magnetic field
averages a dipole field in geological time
spans - paleomagnetic measurements provide
intensity, azimuth, and inclination of the
primary remanent magnetization - inclination is
related to paleolatitude - azimuth is related to
rotation - paleolongitudes of a continent can
never be resolved due to radial symmetry of
magnetic field
39
Paleomagnetic measurements can be presented in
two ways 1) A succession of paleomagnetic poles
with respect to one plate can be used to
reconstruct the plates' positions through time in
terms of lat. and a rotation about an axis
centered on the plate. gt It is not possible to
reconstruct its longitudinal position due to the
radial symmetry of the magnetic field.
2) The plate can be held fixed and the positions
of the pole through time with respect to the
plate can be plotted on one map, with the plate
in its present day coordinates. This
representation is called an apparent polar
wander (APW) curve. It is an apparent wandering
curve, since in reality the plate moves, and not
the pole.
40
The curves on this map trace the apparent path
followed by the north magnetic pole through the
past 600 million years. The apparent polar
wandering path for Europe is different from the
path determined from measurements made in North
America. If the continents are reassembled into a
single supercontinent, the two paths coincide,
indicating that Europe and North America moved as
one continent during this period.
41
First it was shrinking, now its expanding?
A profound consequence of seafloor
spreading is that new crust was, and is now,
being continually created along the oceanic
ridges. This idea found great favor with some
scientists who claimed that the shifting of the
continents can be simply explained by a large
increase in size of the Earth since its
formation. However, this so-called "expanding
Earth" hypothesis was unsatisfactory because its
supporters could offer no convincing geologic
mechanism to produce such a huge, sudden
expansion. Most geologists believe that the Earth
has changed little, if at all, in size since its
formation 4.6 billion years ago, raising a key
question how can new crust be continuously added
along the oceanic ridges without increasing the
size of the Earth? This question
particularly intrigued Harry H. Hess, a Princeton
University geologist and a Naval Reserve Rear
Admiral, and Robert S. Dietz, a scientist with
the U.S. Coast and Geodetic Survey who first
coined the term seafloor spreading. Dietz and
Hess were among the small handful who really
understood the broad implications of sea floor
spreading. If the Earth's crust was expanding
along the oceanic ridges, Hess reasoned, it must
be shrinking elsewhere. He suggested that new
oceanic crust continuously spread away from the
ridges in a conveyor belt-like motion. Many
millions of years later, the oceanic crust
eventually descends into the oceanic trenches --
very deep, narrow canyons along the rim of the
Pacific Ocean basin. According to Hess, the
Atlantic Ocean was expanding while the Pacific
Ocean was shrinking. As old oceanic crust was
consumed in the trenches, new magma rose and
erupted along the spreading ridges to form new
crust.
42
In effect, the ocean basins were perpetually
being "recycled," with the creation of new crust
and the destruction of old oceanic lithosphere
occurring simultaneously. Thus, Hess' ideas
neatly explained why the Earth does not get
bigger with sea floor spreading, why there is so
little sediment accumulation on the ocean floor,
and why oceanic rocks are much younger than
continental rocks. Deep Sea Drilling Additional
evidence of seafloor spreading came from an
unexpected source petroleum exploration. In the
years following World War II, continental oil
reserves were being depleted rapidly and the
search for offshore oil was on. To conduct
offshore exploration, oil companies built ships
equipped with a special drilling rig and the
capacity to carry many kilometers of drill pipe.
This basic idea later was adapted in constructing
a research vessel, named the Glomar Challenger,
designed specifically for marine geology studies,
including the collection of drill-core samples
from the deep ocean floor. In 1968, the vessel
embarked on a year-long scientific expedition,
criss-crossing the Mid-Atlantic Ridge between
South America and Africa and drilling core
samples at specific locations. the third leg
of the Deep Sea Drilling Program (DSDP) drilled a
number of holes in the South Atlantic at right
angles to the mid-Atlantic Ridge to test the SFS
hypothesis. gt the oldest sediments overlying the
ocean crust were drilled and dated
paleontologically gt the agreement with ages
predicted from magnetostratigraphy was excellent
43
Concentration of earthquakes During the 20th
century, improvements in seismic instrumentation
and greater use of earthquake-recording
instruments (seismographs) worldwide enabled
scientists to learn that earthquakes tend to be
concentrated in certain areas, most notably along
the oceanic trenches and spreading ridges. By the
late 1920s, seismologists were beginning to
identify several prominent earthquake zones
parallel to the trenches that typically were
inclined 40-60 from the horizontal and extended
several hundred kilometers into the Earth. These
zones later became known as Wadati-Benioff zones,
or simply Benioff zones, in honor of
the seismologists who first recognized them,
Kiyoo Wadati of Japan and Hugo Benioff of the
United States. The study of global seismicity
greatly advanced in the 1960s with the
establishment of the Worldwide Standardized
Seismograph Network (WWSSN) to monitor the
compliance of the 1963 treaty banning
above-ground testing of nuclear weapons. The
much-improved data from the WWSSN instruments
allowed seismologists to map precisely the zones
of earthquake concentration worldwide, as shown
below.
44
Earthquakes in subduction zones. In subduction
zones, earthquake foci vary from shallow, near
the trench, to deep, farther away from the trench
in the direction of plate subduction. This
drawing shows earthquakes that occurred beneath
the Tonga Trench in the Pacific Ocean, over a
period of several months. Earthquakes in this
region are generated by the downward movement of
the Pacific Plate. Zones of shallow-to-deep
earthquakes like this one are also called
Wadati-Benioff zones.
45
The theory of plate tectonics The concept of sea
floor spreading was originally proposed by Hess
(1962) and Dietz (1961), who suggested that new
sea floor is created at mid-ocean ridges and
spreads away form them as it ages. It must be
stressed that this idea is significantly
different from the proposal by Wegener (1924)
that continents drift on a passive ocean floor.
However it is not greatly different from Holmes
1945, who proposed the correct mechanism A
major contribution came from Wilson (1965), who
developed the concept of plates and transform
faults. He suggested that (1) the active mobile
belts on the surface of the Earth are not
isolated but continuous (2) these mobile belts,
marked by active epicenters, separate the Earth
into a rigid set of plates (3) these active
mobile belts consist of (a) ridges where plate is
created, (b) trenches where plate is destroyed.
and (c) transform faults, which connect the other
two belts to each other. Plate tectonic
concepts (1) Continuity of plate
boundaries Plate boundaries are outlined by
active Earthquake epicenters. Morgan (1968)
separated the world into 10 plates. Today, we
know that the actual number of plates is much
larger. All major plates are surrounded by
spreading centers, subduction zones, and
transform faults.
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(2) Rigidity The concept of internal rigidity of
tectonic plates together with Euler's Theorem
allows us to model the relative motion of plates
quantitatively. (3) Relative motion All plates
can be viewed as rigid caps on the surface of a
sphere. The motion of a plate can be described
by a rotation about a virtual axis which passes
through the center of the sphere (Euler's
Theorem). In terms of the Earth this implies
that motions of plates on a sphere can be
described by an angular velocity vector
originating at the center of the globe. The
most widespread parametrization of such a vector
is using latitude, longitude, describing the
location where the rotation axis cuts the surface
of the Earth, and a rotation rate that
corresponds to the magnitude of the angular
velocity (degrees per m.y. or microradians
per year). The latitude and longitude of the
angular velocity vector are called the Euler
pole. Because angular velocities behave as
vectors, the motion of a plate can be expressed
as a rotation w w k, where w is the angular
velocity, k is a unit vector along the rotation
axis, w the rotation rate.
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The motion of individual plates can be described
by an absolute motion angular velocity. The
motion between two plates, which have different
absolute motion poles, can be expressed by an
angular velocity of relative motion. Plate
tectonic theory was developed by determining
relative motion between plates, which - in
general - is easier to measure than their
absolute motions.
49
w angular velocity, also called Euler vector w
rotation rate at point on sphere, measured in
radians/year (rad/yr) r vector pointing to a
position on sphere. The magnitude of this vector
corresponds to the radius of the sphere, measured
in meters (m). v linear velocity vector at r v
speed at r, measured in millimeters/year
(mm/yr). The rotation of a plate can be
represented as angular velocity w about a fixed
axis originating at the center of the sphere. The
Euler pole is the intersection of the Euler
vector w and the surface of the sphere.
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The following figure illustrates how the rotation
speed increases from the pole of rotation, and
that transform faults offsetting both ridges and
trenches are small circles about the rotation
pole. The first figure shows a counterclockwise
rotation of plate B relative to plate A,
separated by a ridge, whereas the figure on the
right shows a counterclockwise rotation,
separated by a subduction zone. Notice the
difference in the sense of the rotation.
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h
Flat Earth Plate Geometry Simple vector addition
can be used to approximate motion of plates on a
sphere for a local area.
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• How can you determine the quantities
• needed to solve these types of vector problems
  (in general)?
• Geological and geophysical methods
• Spreading rates
• Angles at triple junctions
• Azimuths of transform faults
• Slip rates on faults
• Slip vectors of earthquakes
• Geodetic Methods
• GPS (almost cheating!)
• VLBI, SLR
• Seafloor geodesy
• Strain meters

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Approximating a pole of rotation using ridge and
transform orientation
The relative motion of two plates sharing a
mid-ocean ridge is assumed to be parallel to the
transform faults, because the arcs of the faults
are expected to be small circles. (What if this
is not the case, is this possible?) This would
imply that the rotation pole must lie somewhere
on the great circle perpendicular to the small
circles defined by the transform faults. Hence,
if two or more transform faults between a plate
pair are used, the intersection of the great
circles approximates the position of the rotation
pole.