Shock magnetization and demagnetization of basalt by transient stress up to 10 kbar

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J. Pohl
U. Bleil
U. Bornemann


The effect of stress waves on the magnetization of basalt was studied. The stress waves were generated by impacting cylindrical basalt samples with aluminium projectiles. The 3 mm thick aluminium plates were accelerated in a non-magnetic compressed air gun accelerator to velocities ranging from 20 to 160 m/s, corresponding to peak stresses in the basalt between 2.5 and 10 kbar. The duration of the stress impulse was about several micro-seconds. For the experiments a basalt with well-known magnetic properties was used (Rauher Kulm, Germany).  The magnetizing effect of the stress waves was determined as a function of the number of impacts, the intensity and direction of the applied magnetic field ( < 10 Oe) and the peak stress amplitude. In the used stress range the measured shock remanent magnetization (SRM) tends to a final steady value after 5 or 6 impacts. This value is proportional to the intensity of the applied field and increases with the peak stress applied. The produced SRM can be erased with maximum ac-fields of about 150 to 200 Oe. Any dependance of SRM on the direction of the applied magnetic field could not be recognized within the accuracy limits of the experiments.  The demagnetizing effect of stress waves on the highfield (1,000 Oe) isothermal remanent magnetization, the low-field (1 Oe) thermoremanent magnetization and the natural remanent magnetization was studied as a function of the number of impacts and the peak stress. A final steady state of magnetization is generally obtained after 4 or 5 impacts. With increasing peak stresses increasingly harder remanent magnetizations can be demagnetized, with stresses of 2.5 kbar corresponding to coercive forces of about 75 Oe, 5.5 kbar to about 125 Oe and 8 kbar to about 175 Oe.

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Pohl, J., U. Bleil, and U. Bornemann. 1975. “Shock Magnetization and Demagnetization of Basalt by Transient Stress up to 10 Kbar”. Journal of Geophysics 41 (1), 23-41.
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