Lunar geologists describe the maria as vast lava flows, many of which fill large shallow basins excavated by impacts. After a basin forms, millions of years may pass before lava flows into it. In the meantime, impacts can form craters on the basin floor. Look at Archimedes in Mare Imbrium—its walls are intact but its floor is flooded by lava.
When lava floods a basin, it flows easily and for long distance, being about as runny as motor oil at room temperature. Mare lavas are made up of several layers that may form a total thickness ot less than 2 miles (3 km)— paper-thin in relation to their large extent.
Full Moon is a good time to look for differences in hue that reveal separate episodes of flooding. In places, the channels that fed the flows are visible as thin, winding grooves, known as rilles, that meander and then disappear. The Apollo 15 astronauts landed near the Hadley Rille, a sinuous rille at the foot of the
Where the basin floor-buckles under the weight of the lava, the mare surface pushes up in what is known as a wrinkle ridge. Where mare lava bends over a rise, the brittle lava cracks in straightish lines to form long troughs, called grabens. Several sets of grabens are visible on the eastern shore of Mare Humorum, from about 11 days after New