Mercury appears to have a tectonic framework and diastrophic history not found on other terrestrial planets explored to date (earth, Mars, and the moon). On the part of the planet viewed by Mariner 10, only two localized areas show evidence of tensional stresses, both of which are apparently associated with the Caloris basin. Lobate scarps occur in the remainder of the explored region and appear to be primarily reverse or thrust faults which have resulted from compressive stresses acting on a global scale. The period of compression represented by these scarps occurred during the final phase of heavy bombardment on Mercury and was probably caused by crustal shortening due to a small decrease in the planet's radius. Stratigraphic, volumetric, and albedo considerations together with distribution indicate that the majority of smooth plains on Mercury were produced by volcanism which occurred at the close of the period of late heavy bombardment similar to that on the moon and Mars. Several generations of plains are evident; the oldest may have resulted in part from an early differentiation of the planet.

Several types of Martian impact craters have been recognized. The most common type, the rampart crater, is distinctively different from lunar and Mercurian craters. It is typically surrounded by several layers of ejecta, each having a low ridge or escarpment at its outer edge. Outward flow of ejecta along the ground after ballistic deposition is suggested by flow lines around obstacles, the absence of ejecta on top and on the lee side of obstacles, and the large radial distance to which continuous ejecta is found. The peculiar flow characteristics of the ejecta around these craters are tentatively attributed to entrained gases or to contained water, either liquid or vapor, in the ejecta as a result of impact melting of ground ice. Ejecta of other craters lacks flow features but has a marked radial pattern; ejecta of still other craters has patterns that resemble those around lunar and Mercurian craters. The internal features of Martian craters, in general, resemble their lunar and Mercurian counterparts except that the transition from bowl shaped to flat floored takes place at about 5‐km diameter, a smaller size than is true for Mercury or the moon.

A preliminary analysis of a global survey of Magellan data covering over 90% of the surface and designed to document the characteristics, location, and dimensions of all major volcanic features on Venus has revealed over 1660 landforms and deposits. These include over 550 shield fields (concentrations of small volcanoes <20 km in diameter), 274 intermediate volcanoes between 20 and 100 km diameter with a variety of morphologies, 156 large volcanoes in excess of 100 km diameter, 86 calderalike structures independent of those associated with shield volcanoes and typically 60–80 km in diameter, 175 coronae (annulus of concentric ridges or fractures), 259 arachnoids (inner concentric and outer radial network pattern of fractures and ridges), 50 novae (focused radial fractures forming stellate patterns), and 53 lava flood‐type flow fields and 50 sinuous lava channels (all of which are in excess of 102–103 km in length). The vast majority of landforms are consistent with basaltic compositions; possible exceptions include steep‐sided domes and festoons, which may represent more evolved compositions, and sinuous rules, which may represent more fluid, possibly ultramafic magma. The range of morphologies indicates that a spectrum of intrusive and extrusive processes have operated on Venus. Little evidence was found for extensive pyroclastic deposits or landforms, consistent with the inhibition of volatile exsolution and consequent disruption by the high surface atmospheric pressure. The large size of many volcanic features is evidence for the presence of very large magma reservoirs. The scale of resurfacing implied by individual features and deposits is typically much less than 125,000 km2. The areal distribution, abundance, and size distribution relationships of shield fields, arachnoids, novae, large volcanoes, and coronae strongly suggest that they are the surface manifestation of mantle plumes or hot spots and that the different morphologies represent variations in plume size and stage and thermal structure of the lithosphere. Maps of the global distribution of volcanic features show that they are broadly distributed globally, in contrast to the plate boundary concentrations typical of Earth. However, they are not randomly distributed on the surface of Venus. An observed deficiency of many volcanic features in several lowland areas of Venus may be due to an altitude‐dependent influence of atmospheric pressure on volatile exsolution and the production of neutral buoyancy zones sufficient to form magma reservoirs; this would favor lava floods and sinuous channels at low elevations and edifices and reservoir‐related features at higher elevations. A major concentration of volcanic features is observed in the Beta/Atla/Themis region, an area covering about 20% of the planet and centered on the equator. This region is unique in that it is the site of local concentrations of volcanic features with concentrations 2–4 times the global average, an interlocking network of rift and deformation zones, several broad rises several th...

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