Pit crater

A pit crater (also called a subsidence crater or collapse crater) is a depression formed by a sinking or collapse of the surface lying above a void or empty chamber, rather than by the eruption of a volcano or lava vent. Pit craters are found on Mercury, Venus,^{[2] [3]} Earth and Mars.^[4] as well as many of our solar systems terrestrial moons. Pit craters are often found in a series of aligned or offset chains and in these cases, the features is called a pit crater chain. Pit crater chains are distinguished from catenae or crater chains by their origin. When adjoining walls between pits in a pit crater chain collapse, they become troughs. In these cases, the craters may merge into a linear alignment and are commonly found along extensional structures such as fractures, fissures and graben. Pit craters usually lack an elevated rim as well as the ejecta deposits and lava flows that are associated with impact craters.^[5][6] Pit craters are characterized by vertical walls that are often full of fissures and vents. They usually have nearly circular openings.^[7]

• http://en.wikipedia.org/wiki/Pit crater
• http://www.dartmouth.edu/~volcano/texts/DekHawaii.html