The effect of shock on the magnetism of terrestrial rocks

Abstract

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 related to the ambient field direction at the time of impact. Although longer and more intense shock pulses may affect the higher‐coercivity fractions, the low‐coercivity fraction is the most susceptible to magnetic resetting by shock. Thus igneous rocks with abundant low‐coercivity 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 explained in terms of their observed differing response to shock, as a function of HRC. However, the lack of oriented samples precludes a conclusive test that the secondary component observed in nature is a shock remanent magnetization. The magnetism of Lonar microbreccias seems to be related to the temperatures they reached during the impact event. Despite widespread remagnetization of the Kaibab formation since the impact event, certain oriented dolomite samples from Meteor Crater, Arizona, retain evidence of partial magnetic resetting at the time of the crater‐forming event, which may well be a shock remanence. The Coconino sandstone from Meteor Crater shows changes in remanence and hysteresis properties with increasing degrees of shock metamorphism. An important application of the effects of shock upon magnetization lies in the interpretation of lunar magnetism, since all lunar samples were collected from an impact‐generated regolith. The ‘hard’ NRM curves of many mare basalts may in part be due to preferential shock demagnetization of ‘soft’ NRM. In the soil breccias the observed NRM is likely to be directly associated with the lithification process. The mechanism of shock‐associated magnetization is not well understood. In the low‐shock range of a few kilobars it appears to be similar to static‐stress‐related magnetization. In extreme shock of hundreds of kilobars the transient and residual temperatures are high enough so that the acquired remanence is transitional to thermoremanent magnetization.