Pit craters at Masaya volcano
Each of the four pit craters (Masaya, Santiago, Nindiri and San Pedro) is a nearly vertical cylinder whose walls may be slightly overhanging. They cut through all types of cone material without any apparent response to lithology. The bottoms of the craters are funnel shaped unless they have been refilled by lava lakes. Sections though Nindiri indicate that collapse occurred mainly along outward-dipping faults. Such fault scarps are also present in Santiago. Vertical faults and fractures around Santiago indicate that the steep walls of the pit craters are probably formed by collapse above the outward-dipping fault set. Debris from collapsed slices of vertical faults forms large scree fans in the craters. (Excert from Rymer et al., 1998).