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Experimental projectile impacts into solid intrusive rocks confirm that shock pressures on the order of tens of kilobars are sufficient to affect the natural remanent magnetism (NRM) of terrestrial materials. The principal effect is to demagnetize the preexisting remanence. A secondary effect is the acquisition of an additional component of remanence whose direction is rela•ed to the ambient field direction at the time of impact. Although longer and more intense shock pulses, may affect the highercoercivity fractions, the low-coercivity fraction is the most susceptible to magnetic resetting by shock. Thus igneous rocks with abundant lowcoercivity carriers (low remanent coercive force, HRC) are more disposed toward magnetic recording of low-pressure shock events. Evidence is presented that these low-HRc rocks are also apparently capable of recording the remanent field of a magnetized projectile at the time of impact. Analysis of the NRM of samples from a road cut in the Palisades Sill, New Jersey, demonstrated that the explosions associated with road cutting are sufficient to reset partially the magnetization in the direction of the present field. This result illustrates possible hazards in the interpretation of paleomagnetism for such samples. The variability of remanence in the Deccan basalts from the Lonar Crater, India, can be ex-the acquired remanence is transitional to thermoremanent magnetization. treme shock of hundreds of kilobars the transient and residual temperatures are high enough so that

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AF demagnetization of thermoremanent magnetization (TRM) and saturation isothermal remanent magnetization (IRM(s)) are used to establish diagnostic criteria for primary TRM and for secondary magnetization in igneous rocks. In applying the approach to the interpretation of natural remanent magnetization (NRM), the demagnetization of IRM(s) documents the nature of the magnetic phases in the rock, while the demagnetization of the NRM gives information about the process of magnetization. The ratio of NRM : IRM (s) measures the efficiency of the magnetization process.