Planet Earth - Lava tree molds: How molten rock preserves forest skeletons

When slow-moving lava courses through a forest, it can leave behind unusual hollow cylinders known as lava tree molds. As molten rock wraps around a tree trunk, its outer edges chill rapidly against the cooler wood, creating a hardened crust. Over time, the living tree trapped inside incinerates, leaving a distinct cylindrical cavity in the solidified stone. Depending on whether trees were standing or uprooted during the flow, these unique volcanic landforms can be preserved vertically or horizontally.

Source: Wikipedia