These tree casts formed during the 1965 eruption, when lava flowed through forest. Where the lava came in contact with a tree, it chilled to form a solid rind surrounding the trunk, while the rest of the flow remained molten.

Later, as the flow spread laterally, its surface subsided, leaving the solidified crust around the trees standing high. The 1965 flow surface is now buried in cinders from the hgh-fountaining episodes of Pu`u `O`o.