Bright and dark regions on Mars can actually be distinguished from Earth by telescope. They differ by composition and particle size: bright regions are mostly covered by fine dust from silicate minerals, whereas dark regions are dominated by mafic minerals found for instance in basalt, which represents the most common volcanic rock found on the Red Planet. Layers of basaltic sands, as seen in the northern (right) part of the color image, probably originated from volcanic ashes and elsewhere form imposing dune fields in various places on Mars. Five large volcanic provinces and countless small, scattered volcanoes have distributed ash layers all over Mars, later concealed beneath other rocky material. When exposed at slopes, such as in impact craters, the ash can be carried out by wind and deposited over wide areas.