Rilles are long, narrow depressions on the surface of the Moon that resemble channels. They can be several kilometers wide and hundreds of kilometers long. The term is also used to describe similar features that have been found on a number of planets in the Solar System, including Mars, Venus, and several moons.
They are classified into three types : Arcuate (smoothly cuving) Sinous (meandering) and Straight.
The conditions that produced the rilles are unknown, but are generally assumed to be a combination of different geological effects :
Precise formation mechanisms of rilles have yet to be determined. It is likely that the different types are formed by different processes. Common features shared by lunar rilles and similar structures on other bodies suggest that common causative mechanisms operate widely in the solar system. Leading theories include lava channels, collapsed lava tubes, near-surface dike intrusion, nuée ardente (pyroclastic cloud), subsidence of lava-covered basin and crater floors, and tectonic extension. On-site examination would be necessary to clarify exact methods.
Source : Wikipedia
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