What are these "ship wrecks" (pictured) in the old volcano crater? They consists of stone, of course, but have strange U cross section, as if they were the remaining of some ships without the head and tail section. Could anyone tell how have they formed? They almost perfectly in the center of the old, flat volcano (Strytur) near Hveravellir, Iceland. They are quite high uphill and I am surprised how lava could get into that place and then made into such a shape as it probably only flows down.
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.