Heavily cratered terrain is characteristic of the older surfaces of the tèrrestrial planets; this type of terrain is older than 3.9AE on the moon. Lunar highland rocks, the only samples other than meteorites from such terrain, consist of about 10 percent primitive crust lithologies, 60 percent impact-produced fragmental breccias, and about 30 percent impact-melt rocks with abundant clasts or xenoliths. Both the impact-melt breccias and the fragmental breccias consist of intimate mixtures of superheated shock melt derived from near the point of impact (a small fraction of the crater volume excavated) and less shocked, colder clastic debris from farther out but mostly within the crater. This initial bimodal temperature distribution is due to the extremely rapid attenuation of the shock wave away from the point of impact, and the fact that rock can be crushed and excavated although it may have been shock-heated to <200°C. The distinction between the melt rocks and fragmental breccias is the fraction of melt in the mixture, the former resulting from mixtures with over 50 percent melt component. There is a complete spectrum of melt-clast mixtures, but lithologies with between 1/4 and 2/3 melt seem to be relatively rare in the lunar highland collection. Sheets of impact melt typically occur as a lining on the floor of craters and pools or tongues that spill out a short distance over the rim; the fragmental breccias tend to overly the melt sheets and form most of the deposits outside the crater; however, glass fragments and bombs of melt are mixed into many of the fragmental breccias.
The shock wave accelerates the melt to velocities of several km/sec downward and outward, allowing the melt to overtake the slower moving fragmental debris. The mixing must be extraordinarily complete, because melts commonly have clasts within every 1 mm2 area of thin sections. Once the hot and cold components are mixed, thermal equilibrium is achieved in less than 100 sec due to the sub mm scale of mixing. In the fragmental breccias small particles of melt sintered to act as a binder for the fragmental material; in some mixtures the melt quenched so rapidly that glass remains in the matrix. In other cases the melt was sufficiently abundant or the clastic material sufficiently warm that the matrix melt particles cooled more slowly, crystallized, and reacted with the rims of small mineral clasts. In the crystalline impact-melt rocks the mixtures initially contained over 2/3 melt, a substantial fraction of the clastic debris was digested, and the liquid quenched into the interval between liquidus and solidus, initiating nucleation and accounting for the fine grain size of most impact melts. Because heat represents 23-35 percent of the impacting projectiles’ kinetic energy, and melting takes up a substantial fraction of the heat, the heat transfer and phase transformations driven by melt-clast interactions are a significant fraction of all the reactions which can take place in the anhydrous, refractory materials that made up the lunar highlands during the period of intense meteorite bombardment.