[Geysers] Grotto's Cone

[Gordon Bower]

Some good questions, to which we don't have answers of course - but when did that ever stop geyser gazers from spending the whole month of February speculating about it anyway?

I have had a theory for some time that geyser cones are primarily erosional features, not depositional ones. That is, the valley floor has been dropped some tens of feet since the end of the last ice age, as topsoil and loose gravel have gotten washed away, and the areas adjacent to the mouths of hot springs were armored somewhat by the mineralization.
Quietly flowing hot springs have some actual deposition, aided by the bacteria, mostly loose and crumbly. Inside the crater of a geyser or intermittent spring, or on the rims of very hot springs like Crested and Ear Spring, some hard rock gets formed - but mostly growing inward rather than upward. But Old Faithful's mound is being eroded. And Great Fountain's. And Giant's. And F&M's. And Lion's. And Giantess's. Maybe the top of Sponge's cone is healing/growing, but the sides are being eaten away. 

"Grotto is about the only geyser... where the main mass of the cone is penetrated by large hollow spaces." How sure are we of this? There's a cavity of some size under Fan's main vent, for instance. I haven't ever looked into Jet.  Super Frying Pan excavated a sizable underground cavity and then peeled the roof off it. Beehive's second/third/fourth indicators haven't sealed themselves off in the course of a few decades. The holes have gotten bigger and more numerous. So have Aurum's.

I would say that Grotto's uniqueness lies just in the fact that it happens to be in a stage  where the consequences of being-eroded-from-the-inside-out and being-eroded-from-the-outside-in are equally visible. The formations are hard enough and the eruptions gentle enough that the appearance can be retained for a few hundred years, rather than passing through that stage in a couple decade like Fan is doing. I would suggest that the trees fell into the crater, and the proto-Grotto of a few thousand years ago looked a lot likea less violent Buried Geyser in the mid-late 1980s. Now the combination of silicified fallen tree and old-crater-wall partially remains, while the hillside has retreated to the other side of the road / general ground level has been stripped down 8 or 10 feet.

Just idle speculation, really. But fun.