A series of craters approximately in line. They may be overlapping, touching, or separate from one another. Crater chains are usually of either secondary impact or volcanic origin. Secondary impact craters are normally oriented more or less radially around a much larger impact crater. Volcanic crater chains occur when several vents are active along the same volcanic fissure. They may be formed either mainly by collapse, when a series of calderas or collapse pits forms in a line, or by explosion, in which explosive activity occurs at several places along the fissure. Some crater chains on Mars appear to have formed by collapse along a fissure without volcanic activity being involved.