But there exist numerous other lunar volcanic features that are interesting to study. Lunar samples returned by the Apollo missions proved that the chemical composition and mineralogy of mare basalt are similar to those that characterize basalts on the Earth. Radiometric ages and relative geologic dating show that the flows probably occurred several hundred million years after the formation of the impact basins, thereby indicating that they were not impact related melt caused by the craters. Perhaps some great thermal event occurred within the Moon between 4 to about 3 billion years ago.
Some mare flows fill their craters completely and even spill out onto the surrounding landscape.
Lobes and flows of lava indicate high rates of basaltic extrusion on the Moon. Some of these flows have been measured at over 600 kilometers long, a value that is consistent with major flood basalt eruptions on Earth. These long flows are useful in determining the relative ages of the tectonic features on the Moon, such as ridges or troughs. Meandering, sinuous rilles are found along the flat edges of the maria. The Hadley Rille, an example of a sinuous rille, is seen at the top of this image.