What are these "ship wrecks" (pictured) in the old volcano crater?
If U-shaped in cross-section, it is plausible that these are the partial remnants of lava tubes. See, for example, Morphology and mechanism of eruption of postglacial shield volcanoes in Iceland (Rossi, 1996). It is difficult to propose more detailed formation mechanisms in the absence of significantly more information about the structure.