#App. Polar Wandering
2 messages in this thread
Questions on Apparent Polar Wandering and the determination of continental land
masses, and how this determination relates to the magnetic pole reversals.
I'm not sure I fully understand this concept of Apparent Polar Wandering and
the determination of the early earths continental areas if there is evidence
that the magnetic poles have gone through complete reversals over the ages.
And if the mag. poles have gone through these complete reversals are they by a
gradual process ( where the N. pole slides around to the south, like turning a
bar mag. clockwise ), or was it instantaneous? And how have we discovered
these reversals??
Let me see if I understand this right from my readings;
The assumption in the book I am studying is that the mag poles have coincided
with the geog poles over "x" amount of years. This book stated that for the
last few thousand years the avg positions of these 2 have coincided. First of
all what is this "few thousand" years? On a geological timescale this does not
seem like a long time. From this perspective I understand that we presently
take magnetite samples from different continents (same age of course) and just
run the clock back to determine position. So does this mean that we can only
determine the continents positions accurately only back to a few thousand
years. Yet I have seen drawings that seem to take the land masses back much
further than a few thousand years. And how does the mag. pole reversals play
into this picture?
It would seem that we would need to determine these reversal periods and find
magnetite samples within this range to complete the picture backwards ( a
tedious process?), and how accurate can we measure these million year+ aged
magnetite samples?
Not sure if this all came across as I intended, but any help would be
appreciated.
Thanks,
gary
Gary,
Let me see if I can help any re: polar wandering.
You've got two separate and (probably) unrelated phenomena here. One is _polar
wandering_, wherein the Earth's magnetic north pole moves in relation to 'true'
geographic north, ie, the northern end of Earth's axis of rotation. We can
detect and track polar wandering because many volcanic minerals orient along
magnetic lines when they solidify, so they are essentially fossil magnets.
That gives us the alignment of the magnetic field lines at that site when that
mineral solidified. Do that in rocks of the same age from half a dozen places
all over the planet's surface, and you can triangulate the position of the
north magnetic pole with very high precision. When we do this and correlate
the location of the pole against the age of the rocks, we can see that the pole
moved over time.
The other phenomenon is _magnetic field reversal_. For reasons that aren't yet
known, the Earth's magnetic field intensity varies steadily through time,
rising to a high somewhat higher than it is now and dropping eventually to
zero. At zero the field actually reverses – magnetic north becomes magnetic
south, and vice versa, so a compass needle points in the opposite direction.
Then it builds to a maximum oriented that way, then drops back to zero and
reverses again. Since we have those fossil-compass minerals, and different
minerals react to the magnetic field in different ways, we can tell not only
which way the magnetic field went at a given time, but also how strong it was.
The two effects are independent. Whether the mineral crystals pointed north or
south when they formed isn't important when plotting the location of the
magnetic poles. All you need is the direction of the field lines.
Does that help make it any clearer?
Jon W.