AS17-1672(M)               AS15-0181(P) & AS15-0176(P)
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                                                            Figure 233

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The enlarged view provided by this stereogram shows that there are at least three different types of material within the floor of the D-shaped depression. A brighter annulus parallels the wall, and darker material fills the inner floor. Within both areas numerous bulbous and slightly raised comical structures are easily distinguishable. There are craters on the summits of many of the structures, suggesting that each one is probably an extrusive dome with a summit crater. Many similar features on Earth are caused by the subterranean drainage of lavas following extrusion and release of pressure following the upward movement of a central plug.   -F.E.-B..

Diagram Figure 234  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.


Report Source: NASA SP-362, Page 219, Figure 233

This web page was created by Francis Ridge for The Lunascan Project
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