There are many knob formations is the southeastern Acidalia region of Mars. All show a hilltop crest except one which has a summit crater that resembles a cone volcano.
Knobs like these likely formed from interactions between water ice and lava. For instance, the crater at the top of the center knob is likely from a steam explosion when lava vaporized ice. As such, this summit crater is not an impact crater.
This caption is based on a public target suggestion from HiWish .
Written by: HiRISE Science Team (audio: Tre Gibbs) (14 January 2015)
More info and image formats at http://hirise.lpl.arizona.edu/ESP_038877_2135
Image: NASA/JPL/University of Arizona