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9.5 Option - Geophysics: 4. Magnetism and the Earth
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4. Studies of past and present physical phenomena
indicate that the Earth is dynamic
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Students learn to:
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Students: |
Extract from Physics Stage 6 Syllabus (Amended
October 2002). © Board of Studies, NSW.
[Edit 18 August 08]
Prior learning: Preliminary modules 8.2, 8.3, 8.4.
Background: Studies of the Earth's magnetism represent perhaps the oldest utilised geophysical phenomena. The humble compass as a tool for navigation has been one of the instruments that enabled the navigational conquest of the Earth.
perform an investigation that models, and present information to demonstrate how the inclination of the Earth’s magnetic field varies with latitude
- The Earth's magnetic field is much the same as that
of a large bar magnet.
- One type of investigation you may choose to
perform is to model the Earth's
magnetic field using a tennis ball cut to enable the
insertion of a bar magnet. The magnetic field can then be
studied using a device such as a magnaprobe® or a dip
needle compass. The magnaprobe® is a small pivoting
bar magnet free to pivot in three directions. By moving it
up and down next to the bar magnet in the tennis ball the
inclination variation of the Earth can be modelled.
- This model or one like it should show an inclination lack about the Equator and a strong vertical component of magnetic attraction at the poles. Remember that because you are asked to present your data to demonstrate your point that you must show something other than text. The minimum information should therefore be inclusive of accompanying labelled diagrams.
describe the Earth’s current magnetic field
- The Earth's magnetic field is similar to a bar
magnet with lines of magnetic force coming out of the south
magnetic pole (because it is a north pole) and entering the
north magnetic pole.
- The field can be divided into two components- one a
vertical field called inclination and the other horizontal
called declination. At the poles the declination component
is zero at the magnetic equator the inclination component
is zero.
- The strength and position of the magnetic field varies over time and from place to place.
account for the evidence that the Earth’s magnetic field varies over time
- There are two types of evidence that the Earth's
magnetic field varies over time. These are the secular
variation in the strength and direction of the Earth's
magnetic field that can be measured by instruments over
relatively short time frames of the order of years. Secular
variations in the Earth's magnetic field are attributed
to changes within the Earth's outer core that cause the
magnetic field to shift, wax and wane.
- There are also magnetic reversals that last for unpredictable periods of time and result in the switching of the polarity of the magnetic poles. Magnetic reversals are recorded in the magnetism of rocks forming at the time. The cause of magnetic reversals is uncertain.
summarise the geophysical evidence that supports the theory of plate tectonics
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Evidence for plate tectonics includes:
- the pattern magnetic polarity stripes mirror imaged on either side of the Mid Ocean Ridge (MOR) suggesting seafloor spreading
- the Benioff zone of earthquakes having progressively deeper earthquake focuses that are associated with oceanic trenches suggesting subduction and destruction of crustal material
- the presence of shallow earthquakes along transform faults suggesting movement and friction along these faults
solve problems and analyse information to calculate the spreading rate of an ocean using a magnetic polarity time scale and a magnetic anomaly profile
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In order to do this activity the following must be known:
- the pattern of reversals on either side of a MOR so that they can be matched to the magnetic polarity time scale,
- the distance from the MOR of the magnetic polarity
profile.
- Once the age or time of formation of the magnetic
polarity profile has been determined the spreading rate can
be calculated from the relationship spreading distance
pided by the time of spreading. Note that in practice this
is often a difficult calculation because the spreading rate
is not regular so you are only calculating an average rate
of spread.
- Modern studies of spreading rates involve the use of Global Positioning Satellite (GPS) technology with fixed position GPS monitors used to determine the change in distance and position of different plates on the Earth's surface with respect to one another. One recent study put the rate of spreading between Australia and Antarctica at around 7 cm per year with all of that spreading occurring on the Australian side of the MOR.
discuss the initial reluctance of the scientific community to accept the mobility of the Earth’s plates in the absence of a mechanism for plate movement
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When theories about continental drift were first proposed
they were rejected largely because it was not understood
how continents could move through an ocean. That problem
was largely overcome by the dating of the age of the
ocean floors that were found to be rarely more than 200
million years old and to have a maximum of around 250
million years of age. This suggested constant renewal of
the ocean floors. The discovery of evidence for seafloor
spreading was of two main types:
- The maximum age of sediments was found to increase away from the MORs
- The magnetic polarity reversal stripes detected on the seafloor by large scale magnetic surveys were discovered to mirror image about the MORs. The linking of these reversals to the magnetic timescale determined on land led to the acceptance of the concept of the mobility of the Earth's plates.
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