Lunar Reconnaissance Orbiter Camera

Zebra Stripes

Boulders decorate the slopes of this wrinkle ridge in southern Oceanus Procellarum. Image number M135507772R, incidence angle 56°, image scale is 0.5 m/pixel [NASA/GSFC/Arizona State University].

This portion of a wrinkle ridge in southern Oceanus Procellarum, located at about 2.18°N, 48.55°W, has many boulders gathered on its slopes. These boulders are eroding out of the wrinkle ridge. How can we tell? In this image there are no fresh impact craters that could have thrown the boulders all over the wrinkle ridge. Even if the boulders did come from a far away impact crater, we would expect the boulders to be distributed more randomly. Instead the boulders neatly line the ridge. What is the erosive mechanism? There is no wind or rain on the Moon. Most likely micrometeorites are slowly blasting loose regolith particles, leaving behind bedrock that was fractured by the faulting that resulted in the ridge formation. Many other wrinkle ridges observed by the LROC NAC also have boulders most likely caused by wrinkle ridge erosion.

Wrinkle ridges are fascinating tectonic features found in nearly all the lunar mare. They commonly have a distinct broad, low-relief arch with a more steeply-sloped ridge superposed on the arch. One theory of wrinkle ridge formation is described as a simple chain of events and physical forces. Scientists think that wrinkle ridges form due to the forces created when large amounts of mare basalt erupt on top of existing rock. Basalt is much denser than the anorthositic crust on which the mare basalts are deposited. As the basalt fills in low areas in the crust, the increased weight causes sagging in the crust. As the crust sags and changes shape, the forces due to the shape change act on the basalt above the crust. The basalt deposit ends up compressed. The stress in the basalt due to the compression produces faulting within the basalt. When a fault forms in the mare, the effect we see on the surface is a wrinkle ridge. When you think about it, the wrinkle ridges in the basalt are caused by the basalt itself!

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