Shock Remanent Magnetization Research Articles

Assessing lunar paleointensity variability during the 3.9 - 3.5 Ga high field epoch

Numerous paleomagnetic studies suggest that a lunar dynamo, with surface field intensities potentially as high as 40-100 µT, existed between ∼3.9 Ga and ∼3.5 Ga. This period is referred to as the High Field Epoch (HFE). However, the debate over the origin of magnetization recorded in lunar rocks still persists. In addition, whether the Moon could have sustained a continuously strong dynamo during the HFE remains unclear. To unravel the origin of magnetization preserved in lunar rocks and to better characterize the evolution of the ancient lunar dynamo, we conducted a comprehensive set of experiments including rock magnetic tests, electron microscopy, and paleomagnetic investigations on four HFE-aged Apollo 11 mare basalt samples: 10003, 10044, 10069, and 10071. Rock magnetic experiments and electron microscopy indicate that the remanence carriers are kamacite grains of varying sizes and domain states. Sample 10003 recorded a paleointensity of 54.10 ± 4.66 µT. Sample 10044, which was shocked (peak pressure >5 GPa), did not preserve a stable high coercivity remanent magnetization. Samples 10069 and 10071 recorded paleointensities of 61.46 ± 26.09 µT and 10.69 ± 2.87 µT, respectively. A series of hydrostatic pressure experiments, isothermal remanent magnetization (IRM) acquisition experiments, and viscous remanent magnetization (VRM) tests preclude the possibility of our samples containing shock remanent magnetization from transient impact-generated fields, IRM acquisition from exposure to spacecraft fields, and VRM acquisition from exposure to the Earth magnetic field. Overall, our study suggests that the source of these magnetizations was likely the lunar dynamo and may indicate nearly order-of-magnitude magnetic field fluctuations during the HFE.

Effects of Pressure and Temperature Changes on Shock Remanence Acquisition for Single‐Domain Titanomagnetite‐Bearing Basalt

AbstractKnowledge of the shock remanent magnetization (SRM) property is crucial for interpreting the spatial change in a magnetic anomaly observed over an impact crater. This study conducted two series of impact‐induced SRM acquisition experiments by varying the applied field intensity (0–400 μT) and impact conditions. Systematic remanence measurements of cube‐shaped subsamples cut from shocked basalt containing single‐domain titanomagnetite were conducted to investigate the effects of changes in pressure and temperature on the SRM acquisition. The peak pressure and temperature distributions in the shocked samples were estimated using shock‐physics modeling. SRM intensity was proportional to the applied field intensity of up to 400 μT. SRM intensity data for peak pressure and temperature of up to 8.0 GPa and 530 K, respectively, clearly show that it increases with increasing pressure and decreases with increasing temperature. The SRM has unblocking temperature components up to a Curie temperature of 510 K, and it easily demagnetizes with alternating field demagnetization. The observed SRM properties can be explained by the pressure‐induced microcoercivity reduction and temperature‐induced modification of the blocking curve. Although the remanence acquisition efficiency of the SRM is significantly lower than that of the thermoremanent magnetization (TRM), the magnetic anomaly originating from the SRM distribution in a broader region may show a contribution comparable to that of the impact‐induced TRM distribution in a narrow region.

Shock Remanent Magnetization Intensity and Stability Distributions of Single‐Domain Titanomagnetite‐Bearing Basalt Sample Under the Pressure Range of 0.1–10 GPa

AbstractKnowledge of the shock remanent magnetization (SRM) property is crucial to interpret the spatial changes in magnetic anomalies observed over the impact crater. This study reports the spatial distributions of SRM intensity and stability of single‐domain titanomagnetite‐bearing basalt based on the SRM acquisition experiments using a two‐stage light gas gun with Al projectiles, remanence measurements for divided subsamples, and impact simulations. The SRM properties systematically change with increasing pressure, and three distinctive aspects are recognized at different pressure ranges: (1) constant intensity below 0.1 GPa, (2) linear trend as intensity is proportional to pressure up to 1.1 GPa, and (3) constant intensity and increasing stability above 1.9 GPa. The SRM intensity and stability distributions suggest that the crustal rocks containing the single‐domain titanomagnetite originally had an SRM intensity distribution according to the distance from the impact point, which changed depending on the remanence stability after the impact.

Preservation and detectability of shock‐induced magnetization

AbstractAn understanding of the effects of hypervelocity impacts on the magnetization of natural samples is required for interpreting paleomagnetic records of meteorites, lunar rocks, and cratered planetary surfaces. Rocks containing ferromagnetic minerals have been shown to acquire shock remanent magnetization (SRM) due to the passage of a shock wave in the presence of an ambient magnetic field. In this study, we conducted pressure remanent magnetization (PRM) acquisition experiments on a variety of natural samples as an analog for SRM acquisition at pressures ranging up to 1.8 GPa. Comparison of the alternating field (AF) and thermal demagnetization behavior of PRM confirms that AF demagnetization is a more efficient method for removing SRM overprints than thermal demagnetization because SRM may persist to unblocking temperatures approaching the Curie temperatures of magnetic minerals. The blocking of SRM to high temperatures suggests that SRM could persist without being eradicated by viscous relaxation over geologic timescales. However, SRM has been rarely observed in natural samples likely because of two factors: (1) other forms of impact‐related remanence (e.g., thermal remanent magnetization from impact‐related heating or chemical remanent magnetization from postimpact hydrothermal activity) are often acquired by target rocks that overprint SRM, and (2) low SRM acquisition efficiencies may prevent SRM from being distinguished from the underlying primary remanence or other overprints due to its low magnetization intensity.

Can the lunar crust be magnetized by shock: Experimental groundtruth

Since the first evidence of magnetized lunar crust, two mechanisms of magnetization have been suggested to account for lunar magnetism: thermoremanent magnetization (TRM), or shock remanent magnetization (SRM). We present here the first experimental acquisition of shock remanence by lunar rocks in the 0.1–2 GPa range, and discuss their implications for the interpretation of the paleomagnetic record of these rocks, as well as for the distribution of magnetic anomalies revealed by orbital data. Laser shock experiments in controlled magnetic fields performed on lunar mare basalts demonstrated that in the presence of an ambient field these rocks can be magnetized significantly starting at low pressure (~ 0.1 GPa). Hydrostatic loading experiments up to 1.8 GPa in controlled magnetic fields were used to impart piezo-remanent magnetization (an analogue for shock remanent magnetization) to mare basalts and highland regolith breccias. These experiments allow quantifying the shock remanence as a function of pressure and ambient field. Regarding the lunar antipodal magnetic anomaly model, our results show that lunar soils, regolith breccia and about 40% of lunar highland rocks (comprising regolith and impact-melt breccia) in the upper crust can be magnetized by low pressure shocks (< 10 GPa) to sufficient levels to account for the observed lunar antipodal anomalies. Therefore, the antipodal magnetization model appears to be plausible based on our experimental results, provided that several km of regolith and/or impact-processed rocks can be found at the antipodes of large impact basins. For typical lunar rocks dominated by multidomain FeNi with low Ni content, the maximum remanent magnetization that can be acquired during a low pressure shock (< 10 GPa) is about a third of what is expected for a TRM acquired in the same ambient field. Some mare basalts have identical coercivity spectra for their natural remanent magnetization and their SRM, leaving open the possibility that the NRM was imparted during an impact at the lunar surface. In that case, magnetizing fields of the order of 40 to 95 μT are required. SRM acquisition experiments appear necessary to ground the interpretation of lunar paleomagnetism, and should become a standard technique in lunar and extraterrestrial paleomagnetism.

Central magnetic anomalies of Nectarian-aged lunar impact basins: Probable evidence for an early core dynamo

A re-examination of all available low-altitude LP magnetometer data confirms that magnetic anomalies are present in at least four Nectarian-aged lunar basins: Moscoviense, Mendel–Rydberg, Humboldtianum, and Crisium. In three of the four cases, a single main anomaly is present near the basin center while, in the case of Crisium, anomalies are distributed in a semi-circular arc about the basin center. These distributions, together with a lack of other anomalies near the basins, indicate that the sources of the anomalies are genetically associated with the respective basin-forming events. These central basin anomalies are difficult to attribute to shock remanent magnetization of a shocked central uplift and most probably imply thermoremanent magnetization of impact melt rocks in a steady magnetizing field. Iterative forward modeling of the single strongest and most isolated anomaly, the northern Crisium anomaly, yields a paleomagnetic pole position at 81° ± 19°N, 143° ± 31°E, not far from the present rotational pole. Assuming no significant true polar wander since the Crisium impact, this position is consistent with that expected for a core dynamo magnetizing field. Further iterative forward modeling demonstrates that the remaining Crisium anomalies can be approximately simulated assuming a multiple source model with a single magnetization direction equal to that inferred for the northernmost anomaly. This result is most consistent with a steady, large-scale magnetizing field. The inferred mean magnetization intensity within the strongest basin sources is ∼1 A/m assuming a 1-km thickness for the source layer. Future low-altitude orbital and surface magnetometer measurements will more strongly constrain the depth and/or thicknesses of the sources.

Unraveling the simultaneous shock magnetization and demagnetization of rocks

In the natural case of a hypervelocity impact on a planetary or asteroidal surface, two competing phenomena occur: partial or complete shock demagnetization of pre-existing remanence and acquisition of shock remanent magnetization (SRM). In this paper, to better understand the effects of shock on the magnetic history of rocks, we simulate this natural case through laser shock experiments in controlled magnetic field. As previously shown, SRM is strictly proportional to the ambient field at the time of impact and parallel to the ambient field. Moreover, there is no directional or intensity heterogeneity of the SRM down to the scale of ∼0.2 mm 3. We also show that the intensity of SRM is independent of the initial remanence state of the rock. Shock demagnetization and magnetization appear to be distinct phenomena that do not necessarily affect identical populations of grains. As such, shock demagnetization is not a limiting case of shock magnetization in zero field. As a consequence, when it can be recognized in a rock, SRM must be considered as a reliable record of the direction and intensity of the ambient magnetic field at the time of impact. The natural process of hypervelocity impact where a rock carrying a remanent magnetization is shocked in the presence of an ambient field can be studied as the simple superimposition of shock demagnetization and shock magnetization. For this there are now a variety of techniques that allow experimental study of both phenomena separately or simultaneously as in this study. These results have potential implications for the paleomagnetic study of meteorites, and lunar rocks, and for the understanding of the magnetic signature (as studied through paleomagnetism and/or magnetic anomalies) of terrestrial, lunar and Martian impact craters.

Paleomagnetism of Lonar impact crater, India

Lonar crater, India, is the best preserved terrestrial impact crater formed in basalt and is a unique terrestrial analogue for small, simple craters on terrestrial planets and the Moon. We investigated the paleomagnetic and rock-magnetic properties of the 1.88 km diameter crater in order to understand the effect of impacts on magnetization in target rocks. The magnetization in the Lonar basalts consists of an original 65 Ma Deccan magnetization and a recent overprint. We constrained the timing of magnetization acquisition at Lonar using a combination of conglomerate tests on ejecta deposit clasts and fold tests on the overturned and jumbled rim fold. In some areas, the recumbent rim fold is preserved and can be approximated as a horizontal cylindrical fold. In other areas, substantial vertical axis rotation may have occurred where tear zones developed during folding. We observed only subtle effects from the impact on the rock-magnetic properties of Lonar materials, which include a slightly elevated coercivity in shocked ejecta blocks. We show that paleomagnetism can provide a constraint on shock heating in the absence of petrographic evidence of shock (in this case, < 187 ± 15 °C). At Lonar, viscous (and/or chemical) remanent magnetization acquired in the < 50 kyr subsequent to crater formation has obscured any evidence of shock remanent magnetization. We also find no evidence of shock demagnetization or the presence of intense impact-induced or impact-amplified transient magnetic fields that have been proposed around larger impact structures.

Acquisition of shock remanent magnetization for demagnetized samples in a weak magnetic field (7 μT) by shock pressures 5–20 GPa without plasma‐induced magnetization

Abstract— Demagnetized samples of cobalt precipitates in a copper matrix were shocked to 5, 10, and 20 GPa in a weak magnetic field of 7.7 μT to elucidate the origins of the natural remanent magnetization of meteorites and the magnetic anomalies of impact craters on the moon and Mars. The samples placed in the target acquired shock remanent magnetization (SRM) whose intensity increased up to 21.3 times compared with the demagnetized state, but SRM intensity and shock intensity were not correlated. The SRM direction was in most cases approximately perpendicular to the shock direction. The samples placed 4.8 mm from the impacted surface did not acquire significant magnetization, suggesting no plasma‐induced remanent magnetization (PIRM) up to 20 GPa. When the samples were divided into 8 sub‐samples, the SRM intensities of sub‐samples increased up to 40 times compared with bulk ones and their directions were scattered. Higher coercive force grains were magnetized perpendicular to the shock direction for shocks of 5 and 10 GPa, but at 20 GPa the directions were less systematically oriented. These results suggest that the proposed plasma‐induced magnetization of impactites should be reconsidered.

On the efficiency of shock magnetization processes

The acquisition of remanent magnetization caused by hypervelocity impacts may be a major contribution to the remanent magnetization of planetary surfaces and meteorites. In this paper, we investigate the properties of shock remanent magnetization (SRM) and in particular the relative efficiency of this phenomenon with respect to thermoremanent magnetization (TRM), a parameter of interest for the understanding of the magnetic signatures of impact basins on extraterrestrial bodies. We imparted shock remanent magnetization on basalt and microdiorite samples using laser shock in controlled magnetic field. Shock modeling indicates a pressure range of 0.6–1.5 GPa for our experiments. The results confirm that the SRM is parallel to the ambient magnetic field. SRM increases linearly with the ambient field in the 1–2500 μT range. The intensity of SRM is independent of the angle between the shock direction and the direction of the ambient field. The coercivity spectrum of SRM is shifted towards lower values compared to the coercivity spectra of TRM and isothermal remanent magnetization. For the two studied lithologies, the efficiency of SRM versus TRM acquisition is ≥17% for the basalt and is 36% for the microdiorite. This efficiency depends on the magnetic mineralogy and increases with decreasing coercivity. The rather high efficiency values that we obtained show that a significant remanent magnetization can be acquired by the crust during an hypervelocity impact on a planetary surface in the presence of an ambient field.

Antipodal effects of lunar basin-forming impacts: Initial 3D simulations and comparisons with observations

3D simulations of basin-scale lunar impacts are carried out to investigate: (a) the origins of strong crustal magnetic fields and unusual terrain observed to occur in regions antipodal to young large basins; and (b) the origin of enhanced magnetic and geochemical anomalies along the northwest periphery of the South Pole-Aitken (SPA) basin. The simulations demonstrate that a basin-forming impact produces a massive, hot, partially ionized cloud of vapor and melt that expands thermally around the Moon, converging near the basin antipode approximately 1 h after the impact for typical impact parameters. In agreement with previous work, analytic calculations of the interaction of this vapor–melt cloud with an initial ambient magnetic field predict a substantial temporary increase in field intensity in the antipodal region. The time of maximum field amplification coincides with a period when impacting ejecta also converge near the antipode. The latter produce antipodal shock stresses within the range of 5–25 GPa where stable shock remanent magnetization (SRM) of lunar soils has been found experimentally to occur. Calculated antipodal ejecta thicknesses are only marginally sufficient to explain the amplitudes of observed magnetic anomalies if mean magnetization intensities are comparable to those produced experimentally. This suggests that pre-existing ejecta materials, which would also contain abundant metallic iron remanence carriers, may be important anomaly sources, a possibility that is consistent with enhanced magnetic anomalies observed peripheral to SPA. The latter anomalies may be produced by amplified secondary ejecta impact shock waves in the thick SPA ejecta mantle occurring near the antipodes of the Imbrium and Serenitatis impacts. Together with converging seismic compressional waves, these antipodal impact shocks may have produced especially deep fracture zones along the northwest edge of SPA near the Imbrium antipode, allowing the ascent of magma with enhanced KREEP concentrations.

Magnetic properties of a freshly fallen LL ordinary chondrite: the Bensour meteorite

A comprehensive rock magnetic, magnetic anisotropy and paleomagnetic study has been undertaken in the brecciated LL6 Bensour ordinary chondrite, a few months only after its fall on Earth. Microscopic observations and electronic microprobe analyses indicate the presence of Ni-rich taenite, tetrataenite and rare Co-rich kamacite. Tetrataenite is the main carrier of remanence. Magnetization and anisotropy measurements were performed on mutually oriented 125 mm 3 sub-samples. A very strong coherent susceptibility and remanence anisotropy is evidenced and interpreted as due to the large impact responsible for the post-metamorphic compaction of this brecciated material and disruption of the parent body. We show that the acquisition of remanent magnetization postdates metamorphism on the parent body and predates the entering of the meteorite in Earth’s atmosphere. Three components of magnetization could be isolated. A soft coherent component is closely related to the anisotropy of the meteorite and is interpreted as a shock remanent magnetization acquired during the same large impact on the parent body. Two harder components show random directions at a few mm scale. This randomness is attributed either to the formation mechanism of tetrataenite or to post-metamorphic brecciation. All components are likely acquired in very low (≈μT) to null ambient magnetic field, as demonstrated by comparison with demagnetization behavior of isothermal remanent magnetization. Two other LL6 meteorites, Kilabo and St-Mesmin, have also been studied for comparison with Bensour.

Dating of thaw depths in permafrost terrain by the palaeomagnetic method: experimental acquisition of a freezing remanent magnetization

SUMMARY The acquisition of a freezing remanent magnetization (FRM) has been studied in controlled magnetic and thermal environments by successive freezing and thawing (- 18 to + 20 C) of samples of natural sediments from a frost polygon near Ny Alesund, Spitsbergen. Successive freeze-thaw cycles cause a significant decrease in the intensity of the initially induced shock remanent magnetization (SRM), associated with directional trends towards the ambient magnetic field direction during the freezing phase. A slow increase in intensity commences after seven to 10 freeze-thaw cycles. The acquisition of a FRM in samples carrying an isothermal remanent magnetization shows a significantly smaller reduction in intensity and only minor directional variations. This result indicates that only a fraction of the magnetic grains in a natural sediment contributes to the natural remanent magnetization. Insignificant changes in lengths and directions of the principal susceptibility ellipsoid axes also indicate that magnetic fabric and remanent magnetization are carried by partly different populations of magnetic grains. The acquisition of a FRM in nature has yet to be explored. If such a process is confirmed, however, it has the potential for obtaining age estimates of ancient thaw depths and for providing insights into material transport processes in frost polygons.

Palaeomagnetism of the Lappajärvi impact structure, western Finland

Palaeomagnetic and rock magnetic results are presented for rocks from the Lappajärvi meteorite impact structure, western Finland. Oriented samples were collected from the melt rocks in the central island of the crater and of the Palaeoproterozoic (Svecofennian) target rocks at the crater rim. The natural remanent magnitization (NRM) of the melt rocks is generally weak and metastable during demagnetization. The characteristic NRM component, carried by pyrrhotite, yields a palaeomagnetic pole (latitude 55.4°N, longitude 152.6°E, dp = 6.2°, dm = 8.7/ dg, 6 sites) suggesting an age of ~ 195 Ma for the impact. The results from a deep core drilled from the impact melt rock display similar palaeomagnetic directions but with slightly shallower inclinations. The palaeomagnetic age differs from the 40Ar- 3939Ar age of 77 Ma for the impact. Assuming that the 40Ar- 39Ar age is correct, the discrepancy between palaeomagnetic and radiometric ages is best explained by post-impact (and post-cooling) tilting of the melt layer. The tilting (15°) may be due to isostatic rebounds along joints in the melt. Impact remanence has also been isolated in a few target rocks at the rim. This poorly defined overprint could be a mild shock remanent magnetization superimposed on the 1.9 Ga old Svecofennian direction. This interpretation is supported by the observation that the NRM intensity is enhanced in specimens having the impact overprint. The palaeomagnetic results are consistent with earlier interpretations based on geophysical, morphological, mineralogical and geochemical data that the Lappajärvi structure is a complex meteorite impact site less than 200 Ma old.

Formation of magnetic anomalies antipodal to lunar impact basins: Two‐dimensional model calculations

The production of large‐scale magnetic fields and associated crustal magnetization in lunar basin‐forming impacts is investigated theoretically. Two‐dimensional numerical models of the partially ionized vapor cloud produced in such impacts show that the low‐density periphery of the cloud expands thermally around the Moon and converges near the antipode in a time of the order of 400 to 500 s for silicate impactor velocities of 15 to 20 km/s. Fields external to the impact plasma cloud are produced by the magnetohydrodynamic interaction of the cloud with ambient magnetic fields and plasmas. For the most typical case in which the Moon is immersed in the solar wind plasma and its embedded magnetic field, an MHD shock wave forms upstream of the cloud periphery separating the shocked solar wind from the free‐stream solar wind. For impacts occurring on the downstream (antisunward) hemisphere, convergence of the impact plasma cloud and associated MHD shock waves occurs on the upstream side and results in a large antipodal field amplification. For impacts occurring on the upstream (sunward) hemisphere, some antipodal field amplification is still expected due to the finite electrical conductivity of the lunar interior (requiring an induced external magnetic field) and the likely presence of some residual plasma in the wake of the impact plasma cloud. During the period of compressed antipodal field amplification, seismic compressional waves from the impact converge at the antipode resulting in transient shock pressures that have been calculated to be as large as 2 GPa (20 kbar). This is near to the range of 50–250 kbar at which stable shock remanent magnetization has been found experimentally to occur in lunar soils. Significant crustal magnetization anomalies antipodal to lunar impact basins are therefore expected, consistent with orbital mapping results. Weaker magnetization observed peripheral to the Imbrium basin may also be explained by shock effects together with compressed ambient fields in a surface boundary layer. Although other processes such as cometary impacts and a former core dynamo may have contributed significantly to the observed paleomagnetism, meteoroid impact plasmas appear capable of explaining a major part of the large‐scale magnetization detected thus far from lunar orbit.

Lunar surface magnetic field concentrations antipodal to young large impact basins

We find that the major regions of strong surface magnetic fields detected by planetary electron reflection magnetometry (PERM) measurements of the Apollo 15 and 16 subsatellites are located near the antipodes of four young large ringed impact basins: Orientale, Imbrium, Serenitatis, and Crisium. Statistical analysis of the 26 impact basins whose antipodes fall within the 35°S to 35°N latitude band covered by PERM measurements shows that the antipodal regions of older basins have median surface magnetic fields of ⪅0.2 nT compared with ∼0.5–1.5 nT for the four youngest basins. Although the mechanisms for producing the surface magnetic field concentrations antipodal to impact basins are not well understood, these results indicate a period of strong lunar magnetic fields between ∼3.85 and 3.6 aeons, consistent with lunar sample paleomagnetic data. The strong surface magnetic fields near the basin antipodes may have resulted from concentration of a preexisting ambient magnetic field at the antipode by the partially ionized vapor cloud produced in hypervelocity basin-forming impacts and acquisition of shock remanent magnetization from the impact of secondary ejecta.

Additional experimental evidence for the domain-wall theory of piezo- and shock-remanent magnetization of rocks.

Additional experimental evidence for the domain-wall theory of piezo- and shock-remanent magnetization of rocks.

The Slate Islands meteorite impact site: a study of Shock Remanent Magnetization

Summary. A detailed palaeomagnetic study of the Slate Islands meteorite impact site, northern Lake Superior, has successfully isolated a secondary Component of remanence which on the basis of field relationships, models of impact crater formation and laboratory investigation appears to be a Shock Remanent Magnetization (SRM). Evidence for this hitherto rare type of naturally formed remanence is that: (a) the component is confined to rocks having experienced intense shock (50 to more than 100 kb) on the basis of shatter-cone development and the presence of planar features in quartz and felspar (b) the component appears to have been acquired virtually instantaneously in terms of normal geological processes as a combined palaeomagnetic—shatter cone analysis suggests that it was formed within a time interval (perhaps several minutes in duration) between the moment of impact and the formation of the central uplift (c) the extent of magnetic resetting decreases with increasing distance from the centre of the impact site, and (d) the degree of resetting increases with the abundance of low coercivity magnetic grains as given by Hcr, a relationship found by other investigators for experimentally-produced SRM. Intrusive breccias found on the islands yield a secondary component of TRM or TRCM origin which was frozen in after central uplift formation. Increased convergence of shatter cone axes to a central focus after rotation of host rock blocks according to palaeomagnetic data, suggests that the SRM had the same direction as that found in the breccias and hence was aligned with the ambient field at the time of impact.

The effect of shock on the magnetism of terrestrial rocks

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.