Samples from a suite of 10 low porosity igneous rocks were shock loaded to low pressures (5 to 10 kbar). The specimens were recovered and analyzed with differential strain analysis (DSA) in order to characterize the microcracks and thus determine the influence of initial sample properties and shock pulse characteristics on shock-induced microcracks. The DSA results reveal the following: The duration of a shock pulse is an important parameter; those samples that had been subjected to a shock of longer duration had a higher postshock crack porosity. If a sample initially contains a high concentration of cracks closing at a particular pressure, then such a concentration is also present in the postshock sample in addition to a concentration of cracks closing between 100 and 300 bars which is generally produced by the shock process. The shock-induced cracks in samples with low initial crack porosity close over a wider range of pressures than those in samples with higher preshock crack porosity. Neither the grain size nor the high pressure (2 kbar) compressibility of the samples exerted any influence on the postshock crack distributions. Thus, the distribution of crack porosity with closure pressure of shock-induced cracks reflects only the shock history and the intial crack distribution of a rock.