>Maars on Mars?
>Posted by Robert Clark (147.31.4.2) on 2/18/2002 11:38:24 AM
...
>Some great examples. These might be examples of maars, flat-bottomed volcanic >craters usually formed through interactions with water:
>Other Volcanic Structures
>http://pubs.usgs.gov/gip/volc/structures.html
>Another page though describes them as due to underground gas release which >would be consistent with the model Nick proposes:
>Maars and Tuff Cones
>http://vulcan.wr.usgs.gov/Glossary/Maars/description_maars.html

Ahh, both of these give the underground gas release explanation for the formation of maars. I was referring to a model of Nick Hoffman for some features seen on Mars:

>Re: a few more
>Posted by Nick Hoffman (144.132.82.122) on 2/16/2002 1:10:14 AM
>In Reply to: a few more posted by Greg on 2/15/2002 11:25:37 PM
>regarding:
>this one
>The mounds are suspiciously similar and closely packed. They're unlikely to be >craters and in fact are a good candidate for outgassing features from the fill of >the "Mud Ocean" That Ken Tanaka and myself have written about.

The formation of maars by interaction with water is described here:

HYDROVOLCANIC VENTS: Maars and Tuff Rings
http://www.geology.sdsu.edu/how_volcanoes_work/lava_water.html

and

Maar
http://volcanoes.usgs.gov/Products/Pglossary/maar.html

The first has a link to a great image of a maar in Saudi Arabia that has the type of rim seen in some of the images Greg gave:

Maar on Harrat Shamah, northwest Saudi Arabia
http://www.geology.sdsu.edu/how_volcanoes_work/Thumblinks/maar_saudi_page.html

Compare this to Greg's image:

M1900946

However, this MGS image also shows a fluidized crater ejecta which has been hypothesized to be produced by meteoric impact into a subsurface volatile such as water or ice or CO2. Are similar lobate flows produced during the volcanic activity that creates maars?

Bob Clark