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This picture, showing an area in central Mare Serenitatis 200 km from the nearest outcrops of terra rocks, is an oblique view of a 4-km diameter crater form about 200 m deep. The crater floor is similar to the surface of the surrounding mare, which is presumably mare basalt covered by 3 to 6 m of regolith. The rim of the crater is raised, although subdued; from this we infer that the present form was developed when mare lava flooded a normal impact crater that had formed on a lower, preexisting surface. The flooding basalt then subsided, more or less in proportion to its thickness, which was greatest inside the crater. The subsidence may have resulted from escape of bubbles from the lava while it was soft, thermal contraction of the lava, and compaction of an underlying relatively loosely packed regolith. Regolith compaction may have occurred when the load and heat of the flooding lava crushed irregular fragments of regolith into more compact shapes and plastically deformed the glassy components of the regolith into more compact shapes or into voids between other particles.
R. J. Pike's (1972) data on the rim heights of lunar craters 90 m
to 10 km in diameter indicate this crater probably had a rim about 200
m high when it was buried. The lava plain around the depression presently
stands about 25 m below the rim crest, giving a minimum thickness for the
flooding lava of 175m. -R.E.E.
Report Source: NASA SP-362, Page 228, Figure 242
This web page was created by Francis Ridge
for The Lunascan Project
Section Directory 24