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, Earth and Mars.as well as many of the solar system's 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. Pit craters are characterized by vertical walls that are often full of fissures and vents. They usually have nearly circular openings.

As distinct from impact craters, these craters are not formed from the clashing of bodies or projectiles from space. Rather, they can be formed by a lava explosion from a bottled up volcano, (the explosion leaving a shallow caldera), or the ceiling over a void may not be solid enough to prevent the collapse of the overlying material. A pit crater also could result from the collapse of lava tubes, dike swarms, or from collapsed magma chambers under loose material.

A newly formed pit crater has steep overhanging sides and is shaped like a cone inside, growing wider closer to the bottom. Over time the overhangs fall in and the crater fills with talus from the collapsing sides and roof. A middle aged pit crater is cylindrical but the rim will continue to collapse, expanding outward until the crater resembles a funnel or drain, narrower at the bottom than the top.

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