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Transcript
The Earth’s Magnetic Field!
Geomagnetic reversal
Geomagnetic Reversal
• A geomagnetic reversal is a change in the orientation of
Earth's magnetic field such that the positions of magnetic north
and magnetic south become interchanged.
• Over very long periods, geomagnetic reversal seems to have
occurred with a frequency of 1 to 5 times per million years
• The last reversal happened approximately 780,000 years ago.
• Earth's magnetic field is fading. Today it is about 10 percent
weaker than it was when German mathematician Carl Friedrich
Gauss started keeping tabs on it in 1845, scientists say
• The earth's magnetic field has reversed approximately 170
times over the last 100 million years.
Causes of Reversals
• Scientific opinion is divided on what causes geomagnetic
reversals.
• Many scientists believe that reversals are an inherent aspect of
the dynamo theory of how the geomagnetic field is generated.
• Dynamo theory describes the process through which motion of
a conductive body in the presence of a magnetic field acts to
regenerate that magnetic field.
• In computer simulations, it is observed that magnetic field lines
can sometimes become tangled and disorganized through the
chaotic motions of liquid metal in the Earth's core.
• In some simulations, this leads to an instability in which the
magnetic field spontaneously flips over into the opposite
orientation.
Observing Past Fields
• past record of geomagnetic reversals was first
noticed by observing the magnetic stripe "anomalies"
on the ocean floor
• Since the sea floor spreads at an essentially constant
rate, this results in broadly evident "stripes" from
which the past magnetic field polarity can be inferred
by looking at the data gathered from simply towing a
magnetometer along the sea floor.
This diagram shows just how the earths ocean floor
conveniently records the direction of the earths polarity.
Old Age Dating
• The previous method mentioned works great for
detecting the polarity of earth up to about 180 million
years ago
• Most sedimentary rocks incorporate tiny amounts of
iron rich minerals, whose orientation is influenced by
the ambient magnetic field at the time at which they
formed.
• Under favorable conditions, it is thus possible to
extract information of the variations in magnetic field
from many kinds of sedimentary rocks.
• However it is very difficult to find rocks that are in
good enough condition to perform this procedure
History of Reversals
• A long period of time during which there were no magnetic pole
reversals, the Cretaceous Long Normal lasted from about 120 to
83 million years ago.
• An interesting trend can be seen when looking at the frequency
of magnetic reversals approaching and following the Cretaceous
Long Normal.
• The frequency steadily decreased prior to the period, reaching
its low point (no reversals) during the period.
• Following the Cretaceous Superchron the frequency of reversals
slowly increased over the next 80 million years ago, to the
present.
• There has been two other long periods of time that a reversal
did not take place
What's Happening Now?
• At present, the overall geomagnetic field is
becoming weaker at a rate which would, if it
continues, cause the current field to
temporarily collapse by 3000-4000 AD.
• The Earth's magnetic north pole is drifting
from northern Canada towards Siberia with a
presently accelerating rate -- 10km per year
at the beginning of the 20th century, up to
40km per year in 2003.
• It is unknown if this drift will continue to
accelerate.
Consequences
• Some speculate that a greatly diminished magnetic field during
a reversal period will expose the surface of the earth to a
substantial and potentially damaging increase in cosmic
radiation.
• However, Homo erectus and their ancestors certainly survived
many previous reversals. There is no uncontested evidence that
a magnetic field reversal has ever caused any biological
extinctions.
• Although the inspection of past reversals does not indicate
biological extinctions, present society with its reliance on
electricity and electromagnetic effects (e.g. radio, satellite
communications) may be vulnerable to technological disruptions
in the event of a full field reversal.
Aurora
• Typically the aurora appears either as a
diffuse glow or as "curtains" that
approximately extend in the east-west
direction.
• Each curtain consists of many parallel rays,
each lined up with the local direction of the
magnetic field lines, suggesting that aurora is
shaped by the earth's magnetic field.
• Indeed, satellites show aurora electrons to be
guided by magnetic field lines, spiraling
around them while moving earthwards.
Animals and Earth’s Magnetic Field
• Many animals depend on the earths magnetic
field for a sense of direction while migrating
• Turtles, frogs, fruit fly’s, birds and everything
in-between use the polarity for direction
• Blind mole rats use the field to determine
how deep they are under the ground, turtles
do the same under water.
• If the poles of the earth were to switch, what
problems would these animals face?
• One would think they would be able to adapt
as they must have already survived 170
reversals just to be alive today.
The End