Origin of Home Plate, Columbia Hills, Mars: Hydrovolcanic Hypothesis
Rice, JW, et. al.

Home Plate, a roughly circular shaped plateau structure 2 to 4 m high and 90 m diameter, is located on the floor of the Inner Basin of the Columbia Hills. The Inner Basin is an amphitheater shaped lowland which opens westward toward the basaltic plains. Spirit reached Home Plate on sol 744 and found this feature to be composed of inward dipping layered rock capped with scoriaceous basaltic rocks. Home Plate is composed of two major rock units. Pancam and MI imagery show that the lower unit is a massive, fairly coarse, poorly sorted ribbed rock with wavy undulations, containing subrounded to rounded coarse granules up to several mm in size and rounded voids. These granules are interpreted to be accretionary lapilli, spherical balls of volcanic ash that form from a wet nucleus falling through a volcanic ash cloud, and the voids are interpreted to be vesicles formed by the entrainment of vapor in wet ash or cavities created as lapilli eroded out of the rock. However, the most impressive feature in this lower unit is a bomb sag. This was created by the ballistic emplacement of a clast into wet sediments (volume of pore water ~15-20% based on terrestrial observations of similar features). The upper unit is a finer grained, moderately sorted, finely laminated, matrix supported cross bedded clastic rock. This package of layered rocks is interpreted to be a base surge deposit. Terrestrial base surge deposits generally contain a coarse massive lower unit capped with thinly laminated cross bedded material. All of the above features suggest that Home Plate may have been created by hydrovolcanic explosions forming a tuff ring or maar which was subsequently eroded.

From http://www.agu.org/meetings/fm06/fm06-sessions/fm06_P41B.html via the AGU Fall 2006 meeting.

Okay, some think the inward dipping layers are indicative of a vent. Me, I'm not so sure. The dips don't seem steep enough for a vent facies.