Natural bridges
Posted: Wed Sep 08, 2010 2:53 pm
http://www.planetary.org/blog/article/00002652/ wrote:
Two natural bridges on the Moon (now with 3D!)
The Planetary Society Blog By Emily Lakdawalla
Sep. 7, 2010 | 11:28 PDT | 18:28 UTC
<<Imagine this landscape: you're walking across an unusually smooth lunar surface, an impact melt sheet on the floor of a relatively recently formed crater. Suddenly, a pit opens before you, leading to a floor six meters below you. Crossing the pit is a thin, arching bridge of solid lunar rock. Taking a chance on an unusual walk, you venture out onto the platform, which is 7 meters wide and 20 meters long -- just a short walk across, but you feel nervous about doing your usual astronaut skip, knowing there is open space passing beneath the bridge under your feet. So instead you shuffle along, raising little puffs of dust, and breathe a sigh of relief as you finish your crossing. That scenario is now known to be possible, as the Lunar Reconnaissance Orbiter Camera team has discovered a natural bridge within one of the "Constellation Regions of Interest" within King crater, on the lunar far side.
Two orbital views of a natural bridge spanning a collapse pit in King crater on the lunar farside have been combined into a 3D anaglyph. The bridge is about 7 meters wide and 20 long; the pit is about 12 meters deep on the left side and shallows to 6 meters on the right.>>
http://earthobservatory.nasa.gov/IOTD/view.php?id=44127 wrote: <<At the far southern end of Lake Powell, on the arid, earth-toned Colorado Plateau, a massive sandstone bridge straddles a river valley. May 30, 2010, marks the centennial of this dramatic rock formation—Rainbow Bridge—as a United States national monument. President Taft set the area aside in 1910 to preserve the rare geologic feature.
On May 23, 2010, the Advanced Land Imager (ALI) on NASA’s Earth Observing-1 (EO-1) satellite captured this natural-color image of the Rainbow Bridge area. Red, brown, and beige rocks predominate in this scene. In the past, the water level in Lake Powell has been high enough to submerge the canyon, but if any water trickled through the streambed below the bridge at the time of this image, the stream was too narrow for the satellite sensor to see. Clouds and canyon walls cast dark shadows toward the northwest. The bridge also casts its own slim shadow.
The bridge is made of ancient sandstones—the base laid down more than 200 million years ago by inland seas, and the bridge itself left over from massive dunes that once marched across a shifting-sand desert. In the millions of years that followed, newer rock layers were deposited on top of these sediments, hardening them into tough sandstones.
About 5.5 million years ago, tectonic forces began uplifting the region. The uplift led to erosion of overlying rock layers, and also steepened the gradient of local streams, increasing their ability to eat through the remaining sandstones. A tall, thin structure of sandstone projected skyward on this uplifted plateau, and for many years, a local stream curved around it. Because the structure’s sandstone wasn’t strong enough to resist erosion indefinitely, the water eventually wore through the rock, forming a natural bridge. The same geologic forces that made the bridge will eventually destroy it. Water will likely continue to wear away at the bridge’s base until it collapses.>>
