Re: Why doesn't Maxwell Montes fall over ?

There are several aspects that are relevant here.  

First, although the region includes peaks as high as 12 km elevation (above 
 average surface height) what really matters for the question you pose is 
the relief, the maximum differences in elevation over a local area.  The 
Maxwell Montes stand in the middle of a broad elevated region and their 
relief is apparently about 8 km (http://www.agu.org/pubs/crossref/1990/
89JB03628.shtml).  That's still a lot, but not as much as the entire 12 km 
of elevation.  Also, analysis of the radar mapping data reveals large (km-
scale) areas of terrain with slopes greater than the 30 degrees that is 
typically a maximum slope for granular materials on earth.

Venusian surface gravity is about 8/10ths of Earth's.  This has an 
important effect as well.  Mountain ranges with more then about 6-7 km of 
relief on earth start to spread apart under their own weight.  This is 
happening in the modern Himalayas, where big extensional faults are 
thinning the range and reducing its height at the same time that 
compressional faults are still pushing it together and higher.  The result 
is that Everest is not only the highest mountain on earth, it appears to be 
about as high as mountains can ever get on Earth.  On planets with lesser 
gravity, mountains can get correspondingly higher and steeper before they 
collapse under their own weight (Olympus Mons is at this limit on Mars).  
Some of this effect is also probably contributing to the steep slopes and 
high heights of Maxwell Montes.

Finally, some of the steep topographic slopes may be due to rocks that are 
significantly stronger than typical crustal rocks on earth.  This 
possibility cannot be addressed until better data are available to help 
identify what minerals might be present in Venusian rocks.

The bottom line is that although Maxwell Montes is impressive, it's not as 
far out of line as it might seem and there are several plausible factors 
that could explain the terrain, but more data would be required before we 
could know for sure.

Dave Smith