Jökull


Jökull - 01.12.1984, Side 70

Jökull - 01.12.1984, Side 70
V.R.M. is mostly in the direction of the present geomagnetic field, but it has not yet been at- tempted to separate these two by quantitative analyses of intensity and direction changes during demagnetization. The induced magnetization of a sample in the earth’s magnetic field F is the vector k-F, where k is the susceptibility and F is ~ 0.5 Oe. The Königsberger ratio Q is the intensity of reman- ence (N.R.M. or T.R.M.) over the magnitude of k-F. If Qt < 1, it may be difficult to measure the original polarity using a fluxgate meter in the earth’s field, but the fluxgate probe should be directed as nearly as possible at right angles to the actual field to minimize this interference. Within a lava flow having a very variable oxida- tion state, a negative correlation between k-F and T.R.M. has been observed (Wilson et al. 1968). However, if the oxidation state within the lava is high or constant, or between flows, the magni- tude of both magnetization vectors may be expected to be proportional to the amount of magnetite in a sample (Deutsch et al. 1971). Hence, a positive correlation between k-F and T.R.M. may be expected from that cause. In actual samples collected from a large number of lava flows, the positive and negative correlations seem to cancel, yielding the graph of Fig. 1. Note that the proportion of samples having Q < Vi, where confusing results are liable to be obtained during field measurements, is at least 10—15%. To circumvent this problem it may actually be helpful to use an insensitive magneto- meter or compass. That way, more effort and patience has to be spent on locating sufficiently strongly magnetized samples, but these may be expected to give much more reliable directional results than the weakly magnetized ones. To give an idea of the strength of remanence in a vertically magnetized hand sample, it may be noted that a sample of 10 cm height and 30 cm2 horizontal cross-section having J = 0.5 A/m will give a field of 50 nT at 8 cm distance from its center. Smaller deflections should be regarded as possibly suspect and influenced by V.R.M. or induced magnetization. DISTRIBUTION OF DIRECTIONS AND INTENSITIES It is often advantageous to envisage the geo- magnetic field as being a purely central dipole field whose dipole moment direction „wobbles“ with respect to the geographic pole (i.e. spin axis) of the earth. In this way, one may plot positions of virtual poles (V.G.P.’s) for comparison of directional data between different areas. A norm- al-polarity field is generally taken to be a field direction to which corresponds a south magnetic pole north of the geographic equator. The general relation between pole latitude and field inclination in Iceland is illustrated in Fig. 2. 3.0 *10'3 2.0-■ 1.0 vol. susc. c.g.s. Profiles HA-HH and GD—GH (1980-82) ■ Qt=1/io 45 ‘Qt=1/3 ,. k 'b •Q,=1 'Qt=3 80 95 88 75 Remanence (J100), A/m (=10 G) 0.1 —h 0.3 4- 1.0 -)--------h- 3.0 -l-/ •Qt=io 54 10.0 Fig. 1. Relation between mean magnetic susceptibility and T.R.M. intensity (Jtoo) in lava samples from Snaefellsnes and Myrar, W-Iceland. Bars just above abscissae show range of J-values in samples (one per flow) whose susceptibility values were averaged for each data point. Standard-error bars inclu- ded- Dotted curves indicate Q-ratios for the primary T.R.M. in 0.5 Oe field. 1. mynd. Vensl segulhrifstuðuls og styrks upprunalegrar hitaseg- ulmögnunar í 437 bergsýnum úr jafnmörgum hraunum af Snae- fellsnesi og Mýrum (óbirt gögn). 68 JÖKULL 34. ÁR
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