Tree Molds at Craters of the Moon National Monument


Volcanic tree molds form when hot, fluid lava surrounds a tree and the tree either decays or burns away after the lava cools and hardens.   Since a'a lava is very blocky, the more fluid type of lava, pahoehoe lava, generally forms the best tree molds.   As lava advances and surrounds the tree, it usually lights the tree on fire.  

Here are some pictures taken in Hawaii by staff at the Hawaiian Volcano Observatory that show how advancing lava sets trees on fire.  

As advancing lava surrounds the tree, the thin, partially cooled crust of the pahoehoe lava may stack up against the tree trunk in rope-like forms.   These ropes often accumulate on the upstream end making the tree mold larger on that side.   Often, tree molds are vertical.   However, trees may fall over and leave behind horizontal molds too.   The height of the tree mold indicates the maximum height of the lava flow.   Sometimes geologists can tell which direction the lava was flowing long after it cooled and hardened by examining how the ropes of pahoehoe wrapped around the tree. These tree molds show how pahoehoe ropes stacked up against the tree.

Notice how the pahoehoe ropes wrap around and adhere to the tree.

A rectangular pattern resembling a tire tread may form as the tree cooks to charcoal.   Lava may enter the cracks in the charcoal exterior of the tree and after the tree decomposes or burns completely, "tire tracks" are left in the lava.

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