Maars and tuff cones are the leading explanations for the Home Plate and Pitcher's Mound formations seen by Spirit on Mars:
Spirit Slides into Home Plate as Opportunity Finishes Work at Erebus
By A.J.S. Rayl
February 28, 2006
http://www.planetary.org/news/2006/0228_Mars_Exploration_Rovers_Update_Spirit.html
Maars and tuff cones arise from volcano/water interaction. Some examples here:
Volcanic Features of Victoria's Western District.
http://members.iinet.net.au/~boxer/WesternDistrict.htm
What might be key is the "bomb sag" features given on the "Volcanic features in Western District of Victoria" page:
Volcanic bomb sag in tuffs at the Lake Purdiguluc maar.
Volcanic bomb sags in tuffs at the Tower Hill maar complex.
On the HYDROVOLCANIC ERUPTIONS page is also given an example of a bomb sag:
Bomb sag.
There was a discussion on Uplink.space.com about some Spirit images that might show a bomb sag, referred to as a "drop stone" on that forum:
Uncalibrated color image at Home Plate.
Taken from these Pancam images:
Panoramic Camera :: Sol 751.
http://marsrovers.jpl.nasa.gov/gallery/all/spirit_p751.html
Bob Clark
Mars' Maars.