The surface of Callisto is heavily cratered and extremely old. The surface does not show any sign of processes occurring below the surface such as plate tectonics, earthquakes or volcanoes, and is thought to have evolved mainly under the influence of meteorite impacts. Notable surface features include multi-ring structures, impact craters in different shapes, and chains of craters (known as catenae) and associated scarps, ridges and deposits. On a smaller scale, the surface is varied. It is made up of frost on hill tops, surrounded by a smooth blanket of dark material in valleys. This is thought to result from the degradation of small landforms, caused by sublimation. This is supported by the few small impact craters, and the presence of numerous small knobs which are thought to be their remains. The absolute ages of the landforms are not known.