>(no subject)
>Posted by Greg (203.45.205.153) on 2/15/2002 9:50:35 PM
>I have been collecting a few pictures like this crater lately. Are these likely to be volcanic or from >meteors? The ejecta like areas are usually higher off the ground.
>Photom1900946

>a few more
>Posted by Greg (203.45.205.153) on 2/15/2002 11:25:37 PM
>In Reply to: (no subject) posted by Greg on 2/15/2002 9:50:35 PM
>

>

>http://www.msss.com/moc_gallery/m19_m23/images/M19/M1901546.html
>

>

>

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

Maars could support the idea of subglacial volcanism that has been suggested to explain some of the features seen on Mars. Note the maar near the tuya or "table mountain" in Iceland on this page:

Geitlandsjökull
http://www.islandsvefurinn.is/pages/xs/si2.html

This might be analogous to the craters seen near the mesas in this MGS image:

MESA AND BUTTE IN CYDONIA MENSAE
http://ida.wr.usgs.gov/html/sp1220/sp122003.html

Maars can be difficult to distinguish from the craters caused by meteoric impact. This report suggests they aren't seen in Viking imagery:

Implications of the apparent absence of maars in Viking Orbiter Imagery
http://www.lpi.usra.edu/meetings/LPSC98/pdf/1589.pdf

However, I found this report that suggests subglacial volcanism explains some features seen in Ismeniae Fossae and describes some craters seen in Viking images of this region as maars:

Evidence for volcanism associated with Ismeniae Fossae fretted channels.
Workshop on Early Mars (1997)
http://www.lpi.usra.edu/meetings/earlymars/pdf/3003.pdf

An interesting feature of the craters Greg cited is that they sometimes have rounded rims. This might be analogous to the rims seen in this maar:

Espenberg Maars
Alaska Science Forum
http://www.gi.alaska.edu/ScienceForum/ASF2/245.html

Bob Clark