GEOPHYSICS (08/430/0012) PRACTICAL 10: LOCATION OF THE GEOMAGNETIC POLE The aim of this exercise is to estimate the location of the Earth's north magnetic pole using data at 11 observatories from 1975 (listed in Parasnis, 1997, Principles of Applied Geophysics, Chapman and Hall, page 13). The methods applied here are the same as those used in estimating VGPs (virtual geomagnetic poles) from palaeomagnetic inclinations and declinations at a set of sites. The following table lists the magnetic inclinations and declinations at the observatories derived from the data in Parasnis. Longitudes and declinations are in degrees East and latitudes in degrees North. Latitudes of observatories in the southern hemisphere are negative. Inclinations are positive when the magnetic field dips into the earth and negative when it points upwards out of the earth. Observatory Latitude Longitude Inclination Declination Mag.Lat. VGP lat. VGP long. Chelskyin 77.7 104.3 86.6 16.9 83.2 83.9 123.0 Sodankyla 67.4 26.6 76.7 6.2 Hartland 51.0 355.5 66.3 351.5 San Miguel 37.8 334.4 56.1 346.7 36.7 79.4 -117.6 Hyderabad 17.4 78.6 20.5 358.4 Tangerang -6.2 106.6 -32.6 0.8 -17.8 78.4 -77.2 Apia -13.8 188.2 -30.3 12.3 -16.3 77.9 -68.5 Mauritius -20.4 57.7 -53.2 343.1 Gnangara -31.8 116.0 -66.2 356.8 -48.6 73.1 -56.7 Macquarie Is -54.5 159.0 -78.6 27.7 Scott Base -77.8 166.8 -81.2 156.4 -72.8 61.2 -27.4 At some observatories the magnetic latitude and location of the `virtual' geomagnetic pole have been computed from the inclination and declination. You can check your method of calculation using these values. GEOPHYSICS (08/430/0012) PRACTICAL 10: LOCATION OF THE GEOMAGNETIC POLE (CTD) 1. Compute the missing values of the magnetic latitudes at the remaining observatories either from the graph of magnetic latitude λ versus inclination i in the notes or from the formula 1 tan λ = tan ã 2 2. Compute the missing values of the latitude «V GP and longitude ¬V GP of the `virtual' geomagnetic pole from the magnetic latitude λ, declination d, latitude «S and longitude ¬S of each observatory. You can either use the formulae sin «V GP = sin «S sin λ + cos «S cos λ cos d cos λ sin d sin(¬V GP ¬S) = − cos «V GP or you can use a stereographic net (Box 3–8 in Cox and Hart, 1986, "Plate Tectonics – How It Works" explains how to do this). Note that Lowrie has an error in the formulae on page 291. 3. Plot the latitudes «V GP and longitudes ¬V GP of the `virtual' geomagnetic poles on the equal area polar net provided on the next page and estimate by eye the average location of the geomagnetic pole. 4. What, roughly, do you reckon the angle of the cone of confidence is for this average location? GEOPHYSICS (08/430/0012) EQUAL AREA STEROGRAPHIC POLAR PROJECTION 180 90 W 90 E 0.