Three types of rille are found on the
lunar surface: •
Sinuous rilles meander in a curved path like a mature river, and are commonly thought to be the remains of collapsed
lava tubes or extinct
lava flows. They usually begin at an extinct
volcano, then meander and sometimes split as they are followed across the surface. , 195 sinuous rilles have been identified on the Moon.
Vallis Schröteri in
Oceanus Procellarum is the largest sinuous rille, and
Rima Hadley is the only one visited by humans, on the
Apollo 15 mission. Another prominent example is
Rima Herigonius. •
Arcuate rilles have a smooth curve and are found on the edges of the dark
lunar maria. They are believed to have formed when the
lava flows that created a
mare cooled, contracted and sank. These are found all over the Moon, examples can be seen near the south-western border of
Mare Tranquillitatis and on the south-eastern border of
Mare Humorum.
Rima Sulpicius Gallus is a clear example in southwestern
Mare Serenitatis. •
Straight rilles follow long, linear paths and are believed to be
grabens, sections of the crust that have sunk between two parallel
faults. These can be readily identified when they pass through
craters or mountain ranges.
Vallis Alpes is by far the largest graben rille, indeed it is regarded as too large to be called a rille and is itself bisected by a linear rille;
Rima Ariadaeus, west of Mare Tranquillitatis, is a clearer example. Rilles which show more than one structure are termed hybrid rilles.
Rima Hyginus in
Sinus Medii is an example, initially formed through a fault and subsequently subject to volcanic activity. ==Formation==