interior is cooling and contracting. This contraction creates planet-wide compression. Wrinkle ridges on Mercury are thought to come from material loaded on top of the volcanic plains, in addition to the cooling. When plains have volcanic material deposited on top, the lithosphere can flex and create more compressional stresses. Temporal relationships Scientists used cross-cutting relationships in order to determine the sequence of events that formed ghost craters. Most likely, an impact struck Mercury. Then volcanic material flooded over the surface and filled the inside of the crater. Next, the graben or wrinkle ridges formed. When the graben are all included within the wrinkle ridges, then they are interpreted to have formed after the wrinkle ridges. If a graben cuts across a ridge and appears to be modified, then it is interpreted to be older, and to have formed before the ridges. Importance The presence of both graben and wrinkle ridges within ghost craters is a feature unique to Mercury. This combination is because of the tectonic processes that formed the graben and wrinkle ridges. The graben and wrinkle ridges came from rapidly accumulating lava flows that cooled within impact craters while the planet was experiencing global contraction from interior cooling. Other planetary bodies such as the Moon and Mars do not have both tectonic features within their ghost craters, possibly due to the accumulation rates of volcanic material on the Moon and Mars being slower than the rates on Mercury. References Impact craters on Mercury Geology of Mercury