Boulder Origin?

Blocks litter the interior floor of an impact crater. NAC M187357438LR, image width 1.81 km [NASA/GSFC/Arizona State University].

Blocks littering the floors of impact craters are evidence that erosive processes continue to act on the Moon. Blocks are distributed along the boundary between the crater walls and the floor of von Bekesy F (52.800°N, 137.040°E, 20 km diameter) and also surrounding mounds located on the crater floor. Today's Featured Image highlights boulders >1 m across eroding out of a floor mound and boulders that fell from somewhere along the crater wall or rim (52.860°N, 137.094°E). Observations of geologic relations between features are integral to developing a geologic story for an area, so it is important to make careful study of the different features. If we were to tell a story about the different boulders observed in the opening image, where should we start?

LROC WAC monochrome mosaic centered on von Bekesy F crater, and asterisk notes location of opening image [NASA/GSFC/Arizona State University].

There are clusters of blocky, fragmented material located on and surrounding the mound in the lower left of the opening image, and the boulders range from ~10 to 15 m across. Observations of partially covered blocks on the mound support a mound origin. Similarly, the distribution of ~10 m wide boulders at the contact between crater wall and floor suggests that these boulders probably fell from higher up on the crater wall. But what about the boulders in between the mound and the floor-wall contact? At least one boulder is located at the terminus of a trail that can be traced back to the crater wall, but there are no other apparent relationships linking boulder to their origin. However, it may be possible that most of the boulders located in between the "boulder-rich" zones of the opening image originate from the crater wall side, simply because boulders falling from higher elevation will have a higher velocity component and will probably continue moving until the velocity and inertia components are lost (after the boulders hit the crater floor and begin to roll).

What do you think? Can you find evidence for boulder origin elsewhere in von Bekesy F in the full LROC NAC image?

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Published by Lillian Ostrach on 8 May 2013