Chapter 7: Unusual Features (1/2)
Included in this chapter is a pot pourri of features that defy easy classification. Many craters with unusual shapes are represented that may be volcanic rather than impact in origin. Other craters have irregular hummocky deposits on their floors; still others may be covered by lava flows. Other features depict collapsed lava tubes or depressions formed by drainage of fragmental surface material into cracks. Together they constitute a fascinating group of features, and they are grouped in this chapter because, at the resolution of the Apollo imagery, more than the usual uncertainty exists in attempting to interpret their origin. Perhaps later Moon landings or a comparison of these pictures with orbiter photography of Earth, Mars, and other planetary bodies will provide additional insight into the geologic processes that formed them.-H.M.
FIGURE 224 [above] .- On January 16, 1973, the Soviet unmanned roving vehicle Lunokhod 2 was landed by Luna 21 in or near this area in the southeastern part of the crater Le Monnier. This crater is a large (61 -km) pre-Imbrian crater cut into terra at the eastern edge of Mare Serenitatis before Serenitatis was flooded by mare laves. Part of Le Monnier's southern wall fills the lower part of the picture. A conspicuous chain of elongate depressions has formed in the lava-filled floor of the crater. The chain trends 22 km northward and its pattern is quite surely controlled by an underlying fracture system. Regionally, the inferred fracture system is concentric to the grossly circular Serenitatis basin, and in this area trends northward. No comparably young structural features having the same trend cut the terrae surrounding Le Monnier. However, older structures having this trend occur in the southern and northern walls and rims of Le Monnier. The alined depressions on the mare are mostly 300 to 400 m wide and 30 to 60 m deep. The three deepest stretches are 1 to 2 km long and about 50 to 65 m deep. These depressions probably were the locus of fissure eruptions of mare basalt. Withdrawal of the last lava back into the fissure may have created subsurface voids into which collapse took place, causing the depressions and accounting for the absence of raised rims on the depressions.-R.E.E.
FIGURE 227 [above].- The largest crater in this picture is Aratus D in western Mare Serenitatis. Its circular outline, high rim, and deep bowl-shaped interior are characteristic of many impact craters of comparable size and age. The next largest crater is tear shaped and nearly rimless. It is, furthermore, associated with a rille (arrows). These differences strongly suggest that it was formed or at least highly modified by structural collapse.-D.H.S.
 FIGURE 228 [above].- The very young rimless crater near the center of this picture is located near the area where Oceanus Procellarum and Mare Imbrium join. The crater apparently formed in regolith-covered mare basalt. It differs from lunar impact craters of comparable size and age by its lack of a raised rim, surrounding ejecta deposit, or associated secondary impact craters. In addition, its interior walls do not show the steep slopes with craggy outcrops of rock in their upper parts, nor the streams of debris-avalanche deposits and talus that are usually seen in the walls of impact craters of comparable age and size.
Judging from the clear and sharply formed pattern of concentrically curved grooves and scarps that surround the hole, the material near this depression has apparently subsided into a subsurface void. Because of the extreme rarity and inferred short lifetime of steep slopes on the Moon, the latest subsidence must have taken place very recently, after most of the 50- to 300- m diameter craters that densely pepper the nearby mare surface were formed. Movement of the regolithic debris layer during subsidence apparently smoothed out most, if not all, of the craters that must have existed near the depression. Now the depression is surrounded by low, curved fault scarps and narrow, curved grooves that may be fault troughs (grabens) or may represent drainage of regolithic debris into cracks that opened in the underlying sagging basalt rock. The few craters that have formed on the subsided surface compare in density to the craters formed on the cluster (arrow) of Aristarchus secondary impact craters and associated herring- bone ridges; comparable ages for the Aristarchus secondary features and the depression are thus indicated. The subsidence was triggered either by the ground shock or seismic wavetrain generated when Aristarchus was formed 300 km to the west, or by the impacts of the secondary features.
The subdued depression in the upper left may be a similar older feature that was flooded by a later lava flow that now covers the area. The density of craters within the depression and the density on the surrounding lava are comparable. Alternatively, the subsidence there may have been incomplete; however, there is no sign that this subsidence is as young as that in the deeper crater.-R.E.E.
 FIGURE 234 [above].- This diagram shows a postulated sequence of events leading to the formation of the D-shaped structure; the events are presented in order of occurrence: (a) Numerous faults were generated in the crust and a thick blanket of debris was deposited (upper layer) as a result of the gigantic impact event that formed the Imbrium basin. (b) Basaltic lavas migrated to the surface along fractures to form a small, probably thin layer of mare material. Higher areas escaped inundation. (c) Vertical displacement then occurred along major faults, and the inundated block was displaced upward relative to the surrounding blocks. (d) A broad, gentle volcanic dome formed. It is recognizable as a younger eruptive stage because its surface is less densely cratered than the surrounding mare surface. (e) The center of the dome collapsed to form the caldera. This segment is outlined in (d). (f) Many small extrusions of lava formed the bulbous structures on the caldera floor. The centers of some of these later collapsed to form small summit craters, thus, on a much smaller scale, repeating the earlier collapse caldera sequence.-F.E.-B.