[Geysers] Grotto's cone

[jacross]

I was looking at a photograph of Grotto's cone today, and it occurred to me 
that Grotto is about the only geyser I can think of where the main mass of the 
cone is penetrated by large hollow spaces.  The inner parts of the cone seem 
to have eroded out at some point, possibly following a long dormancy.  Rocket 
has a few thinner arches.  Also, Grotto's cone is really a half-cone, or a 
2/3s cone--it doesn't completely encircle the vent.  The only other cone I can 
think of where this kind of thing is happening is Union, where the large 
(unnaturally caused) hole on the east side is expanding slowly, eroding softer 
material out from under a harder shell.

How could this happen?  A long dormancy in Grotto's past?  Perhaps that's when 
the tree grew in front of the vent?

Jeff Cross
jacross at lamar.colostate.edu