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Magnetic Stripes

Magnetic stripes: look again

John Michael Fischer, 2004
www.newgeology.us

In the mid 1960s, evidence of magnetic striping on the seafloor
convinced the geologic community of the validity of plate tectonics
theory.  It is often pointed to today as clinching evidence for the
theory.  Since then, the public has not heard much about this pivotal
find.  It is time to look again.

The Geodynamo

Without looking into the center of the Earth, we cannot know with
certainty the source of the geomagnetic field.  The current
thinking is explained by Dr. Gary A. Glatzmaier, Professor of
EarthSciences at UCSC, Santa Cruz, on the geodynamo page of his
website.  His work on computer modelling in the mid-1990s demonstrated
how polarity reversals might occur in the geodynamo.  Quoting him (my
emphasis in blue):
"Paleomagnetic records indicate that the geomagnetic field has existed
for at least three billion years.  However, based on the size and
electrical conductivity of the Earth's core, the field, if it were not
continually being generated, would decay away in only about 20,000
years since the temperature of the core is too high to sustain
permanent magnetism.  In addition, paleomagnetic records show that the
dipole polarity of the geomagnetic field has reversed many times in
the past, the mean time between reversals being roughly 200,000 years
with individual reversal events taking only a couple thousand years.
These observations argue for a mechanism within the Earth's interior
that continually generates the geomagnetic field.  It has long been
speculated that this mechanism is a convective dynamo operating in the
Earth's fluid outer core, which surrounds its solid inner core, both
being mainly composed of iron.  The solid inner core is roughly the
size of the moon but at the temperature of the surface of the sun.
The convection in the fluid outer core is thought to be driven by both
thermal and compositional buoyancy sources at the inner core boundary
that are produced as the Earth slowly cools and iron in the iron-rich
fluid alloy solidifies onto the inner core giving off latent heat and
the light constituent of the alloy.  These buoyancy forces cause fluid
to rise and the Coriolis forces, due to the Earth's rotation, cause
the fluid flows to be helical.
Presumably this fluid motion twists and shears magnetic field,
generating new magnetic field to replace that which diffuses away.
However, until now, no detailed dynamically self-consistent model
existed that demonstrated this could actually work or explained why
the geomagnetic field has the intensity it does, has a strongly
dipole-dominated structure with a dipole axis nearly aligned with the
Earth's rotation axis, has non-dipolar field structure that varies on
the time scale of ten to one hundred years and why the field
occasionally undergoes dipole reversals.  In order to test the
convective dynamo hypothesis and attempt to answer these longstanding
questions, the first self-consistent numerical model, the
Glatzmaier-Roberts model, was developed.  Our solution illustrates how
the influence of the Earth's rotation on convection in the fluid outer
core is responsible for this magnetic field structure.  About 36,000
years into the simulation the magnetic field underwent a reversal of
its dipole moment, over a period of a little more than a thousand
years.  The intensity of the magnetic dipole moment decreased by about
a factor of ten during the reversal and recovered immediately after,
similar to what is seen in the Earth's paleomagnetic reversal record.
Our solution shows how convection in the fluid outer core is
continually trying to reverse the field but that the solid inner core
inhibits magnetic reversals because the field in the inner core can
only change on the much longer time scale of diffusion.  Only once in
many attempts is a reversal successful, which is probably the reason
why the times between reversals of the Earth's field are long and
randomly distributed.  After the first magnetic reversal, we continued
our simulation."  Using "a heterogeneous heat flux similar to the
Earth's present pattern", the model "underwent two more reversals,
roughly 100,000 years apart.  This demonstrates the influence the
thermal structure in the lower mantle has on the style of convection
and magnetic field generation in the fluid core below.

The popular story

The general impression is that nice, even bands of alternating
polarized rock form as new seafloor is made by the slow, steady
movement of plates over millions of years and regular reversals of the
geomagnetic field about every 200,000 years.  It is often illustrated
with this kind of diagram:

Reality

The stripes themselves are better described with words like irregular,
blobbed, marbled, blended, meandering, as if shaken while in a fluid
state and divided by segments diverging along transform faults.

On the left below is the familiar global map of the age of the sea
floor using presumed magnetic isochrons.  See how even the bands are.
On the right is a global map of sea floor spreading rates, based on
the measurable width of rock segments.  What plate tectonic forces do
you suppose caused such jerky movement in many individual segments?
The chaotic jumble on the right in many areas should be matched on the
left, but it has been nicely smoothed out.

From McElhinny, Michael W., Phillip L. McFadden, Paleomagnetism:
Continents and Oceans. 2000. Academic Press, San Diego, CA.
Maps opposite pages 206 and 207

Now look at the frequency of reversals (data from Cande and Kent,
1995).

As Dr. Glatzmaier wrote, the mean frequency of reversals is about
200,000 years.  How many would you expect to be within, say, 50,000
years of 200,000?  It is 19% (35 of 185), only one-third of what one
would expect in the standard normal distribution.  The intervals fall
primarily between 30,000 and 500,000 years, with another spike at
1,000,000 to 2,000,000 years and a significant number of intervals
across the rest of the spectrum.  Rather than a regular tempo of
reversals, it instead seems fairly chaotic.  And what does the
supposed 35 million year "Cretaceous Normal Superchron" represent?
One would expect over 150 reversals in that period of time, but there
are none.  Earth's magnetic field may in fact occasionally reverse as
Dr. Glatzmaier has described, but perhaps that is not what the
magnetic stripes have recorded.

The Shock Dynamics correlation

Notice that the start and end positions, and the "Cretaceous Normal
Superchron" have normal polarity.

If the continents were rapidly divided and seafloor spreading occurred
quickly, as described in the Shock Dynamics theory
(www.newgeology.us), then the geomagnetic field would have been in a
state of normal polarity at the time.  Reversals would have been
momentary oscillations caused by some aspect of the movement of the
continents.
That aspect appears to be related to the formation of mountain ranges.

I propose that the normally-polarized field oscillated at the surface
during the two periods of major mountain building in the Shock
Dynamics theory.  An analogy for illustration might be a
spring-mounted guage that oscillates when bumped but eventually
returns to its starting position.

The bump in this case may have been something well known on a much
smaller scale. Fracturing of quartz and igneous rock produces
electromagnetic emissions in the lab and has been associated with
earthquakes.  The range of emitted frequencies is broad, and includes
ultra-low.  Rock fracturing during the lateral crush of rapid mountain
building throughout the Shock Dynamics event would have been on a
colossal scale.  The consequent electromagnetic activity may have been
sufficient to perturb the global magnetic field (Fraser-Smith, 1990;
Freund, 2002; Takeuchi, 2002; Yoshida, 2004).

Whether the cause was compression of the crust while raising thousands
of miles of mountain chains in a matter of hours, or jolts transferred
from the lithosphere to the core, or some other effect as yet unknown
(and too large for man to experiment with) that shook the geomagnetic
field, the correlation of the Geomagnetic Polarity Time Scale with
Shock Dynamics mountain building is intriguing.

Other observations

Here are excerpts from geologist David Pratt on marine magnetic
anomalies from his webpage Problems with plate tectonics: reply.

"Seafloor spreading, in combination with global magnetic reversals, is
supposed to produce the alternating bands of slightly higher and lower
magnetic intensity on either side of ocean ridges.  However, linear
magnetic anomalies are known from only 70% of the seismically active
midocean ridges, and the diagrams of symmetrical, parallel, linear
bands of anomalies displayed in many plate-tectonics publications bear
little resemblance to reality.  The anomalies are symmetrical to the
ridge axis in less than 50% of the ridge system where they are
present, and in about 21% of it they are oblique to the trend of the
ridge.  Linear anomalies are sometimes present where a ridge system is
completely absent, and not all the charted anomalies are formed of
oceanic crustal materials.  A complicating feature is that each
magnetic lineation consists in detail of numerous, narrow,
high-amplitude anomalies."

"Correlations between magnetic stripes on either side of a ridge or in
different parts of the ocean have been largely qualitative and
subjective, and are therefore highly suspect.
The data are subject to significant manipulation and smoothing, and
virtually no effort has been made to test correlations quantitatively
by transforming them to the pole (i.e. recalculating each magnetic
profile to a common latitude).  The magnetic anomalies of the
Reykjanes Ridge are supposed to be a classic example of ridge-parallel
symmetry, but Agocs et al. (1992) concluded from a detailed,
quantitative study that the correlations were very poor.  The distance
between bands of magnetic anomalies is not proportional in most cases
to the length of the geomagnetic epochs.  The lengths of the Brunhes,
Matuyama, and Gauss epochs have the ratio 1.0 : 2.4 : 1.6.  However,
on the Reykjanes Ridge, the distances between the anomalies closest to
its axis have the ratio 1.0 : 0.5 : 0.4.  An equally great breach of
proportionality is observed on the East Pacific Rise, which can be
explained from the angle of plate tectonics only by invoking
considerable changes in the spreading rate.  Asymmetric seafloor
spreading frequently has to be invoked as well."

"The initial, highly simplistic seafloor-spreading model for the
origin of ocean magnetic anomalies has been disproven by ocean
drilling (Hall and Robinson, 1979; Pratsch, 1986).  First, the
hypothesis that the anomalies are produced in the upper 500 metres of
oceanic crust has had to be abandoned.  The existence of different
polarity zones at different depths suggest that the source of magnetic
anomalies lies in deeper levels of ocean crust not yet drilled or
dated.  Second," there are "vertically alternating layers of opposing
magnetic polarization directions.  Given that the magnetic-stripe
timescale is probably fictional, any correspondence between
plate-motion estimates derived from it and space-geodetic data is
likely to be either coincidental or the result of biased
interpretation."

***********
References

Agocs, W.B., Meyerhoff, A.A., and Kis, K., 1992. Reykjanes Ridge:
quantitative determinations from magnetic anomalies. In Chatterjee, S.
and Hotton, N., III, eds., New Concepts in Global Tectonics, Lubbock,
TX: Texas Tech University Press, pp. 221-238.

Cande, S.C. and D.V. Kent, 1995. Revised calibration of the
geomagnetic polarity time scale for the Late  Cretaceous and Cenozoic.
Journal of Geophysical Research, Vol. 100, No. B4, pp. 6093-6095.

Fraser-Smith, A.C., A. Bernardi, P.R. McGill, M.E. Ladd, R.A.
Helliwell, O.G. Villard, Jr., August 1990. Low-frequency magnetic
field measurements near the epicenter of the Ms 7.1 Loma Prieta
earthquake. Geophysical Research Letters, Vol. 17, No. 9, pp.
1465-1468.

Freund, Friedemann, 2002. Charge generation and propagation in igneous
rocks. Journal of Geodynamics, Vol. 33, pp. 543-570.

Hall, J.M. and Robinson, P.T., 1979. Deep crustal drilling in the
North Atlantic Ocean. Science, Vol. 204, pp. 573-586.

Pratsch, J.C., July 14, 1986. Petroleum geologist's view of oceanic
crust age. Oil and Gas Journal, Vol. 84, pp. 112-116.

Takeuchi, Akihiro, Hiroyuki Nagahama, 2002. Interpretation of charging
on fracture or frictional slip surface of rocks. Physics of the Earth
and Planetary Interiors, Vol. 130, pp. 285-291.

Yoshida, Shingo, Tsutomu Ogawa, 2004. Electromagnetic emissions from
dry and wet granite associated with acoustic emissions. Journal of
Geophysical Research, Vol. 109, B09204, 11 pages.

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