The Dungeon

Tony, I like the new introduction. I'm no geologist but I thought the Dungeon was rhyolite. I will admit it seems a lot different than soccoro, which I also thought was rhyolite. Obviously it doesn;t matter much. Any geologists been to the Dungeon?

Really strange...
If I recall correctly from the late 80's/early 90's there was an older book for the North Shore areas of Lake Superior (IE Tetagouche- Palisade Head and Shovel Point) made a big deal about how tht rock was so rare...
That it was Rhyolite and that it was found there and in Turkey (yes, the country). Can someone confirm that there was such a book in print and that it did say that? My memory is usually very accurate for that sort of thing, but 20 years is a long time for minutia.

Tony S,

Let's not get into the habit of assuming the Falcon Guide is correct! ;) Surely, with highest per capita number of PHDs in the country in Los Alamos, somebody ought to be able to tell us what kind of rock it is...

...Tony B,

I know nothing of the book, but I'm sure there is rhyolite at Smith Rock in Oregon (the Smith rhyolite is the spitting image of Socorro rhyolite). Interestingly, Smith also has tuff and basalt. So it seems there is some cosmic geologic link between NM and OR, besides the common presence of the Monomaniac.

Ha ha, very true, those things have screwed me at least once. A black canyon trip comes to mind..... Guess I "assumed" that the guidebook would be more accurate and researched than my probably incorrect non-geologist impression that it was basalt!

Come on Geologists!

Although not terribly relevant at this point.. I looked some of this up this morning.
The rock at the Dungeon is rhyolite: Tschicoma Formation of the Polvadera Group (Pliocene age). The rock at Back Rocks is the same formation. The rock at 3 other Jemez areas (Cochiti Mesa, Las Conchas, Battleship Rock) is also rhyolite (but of different formations), so there's 4 different kinds of rhyolite that people climb on in the Jemez- and others that aren't developed for climbing (yet). Rhyolite refers to volcanic or shallow level rock with high silica content (actually compositionally similar to granite- it would have formed granite had it cooled really really really slowly under the ground instead of erupting).
The obvious textural differences between the rhyolite at these areas reflects the temperature and how quickly they cooled, how readily they "flowed", the water content, and more recent erosional/weathering processes (like amount of rain, frequency of freeze/thaw, which depends on aspect and elevation, etc., etc.).

Socorro Box I think is also rhyolite, but I haven't looked up the details on it. The reason it is much darker in color is probably because the area has seen much movement of hydrothermal fluids after the rock formed, which changes the chemistry by taking some elements and leaving others.

Killer explanation George! Thanks. Its hard to believe Cochiti has basically the same rock as the Dungeon. Where do "Volcanic Tuff" and or "welded tuff" fit into the picture?

Tuff describes the origin as having come from volcanic ash (all tuffs are volcanic), and rhyolite refers to the chemical composition. So a rock can be a 'rhyolite ashflow tuff' all at once.

For more geologic details of east Jemez rhyolites and related rocks, see: Singer and Kudo (1986)'s article in 'Contributions to Mineralogy and Petrology', v. 94, p. 374-386. Those guys call it 'andesite' at the Dungeon actually, which is sort of similar to rhyolite, but has a little less silica. I think I've also seen this formation called 'dacite' in some publication. I'm not really sure which of these is right.. it's really a distinction in chemistry that only geochemists would want to worry about.