Hollow volcanic landform left by a tree trunk burnt in a lava flow
A horizontal lava tree moldA horizontal lava tree mold in a Tseax Conelava flow
A lava tree mold, sometimes erroneously called a lava tree cast, is a hollow lava cylinder formed around a tree trunk. These hollow volcanic landforms are produced when lava flows through a forest, coating the trees' trunks. The lava cools just enough to create a solid crust around the trunk, but the tree inside burns away, leaving a cavity.[1] Molds of trees may be vertical (tree still in place) or horizontal (uprooted tree).[2] In many cases, the mold formation requires slow-moving lava, as well as enough time for the mold to chill.[2]
A unique phenomenon may occur during the formation of vertical tree molds. As the lava-encased tree burns, its roots are heated, emitting a so-called "producer gas", such as methane (CH4). If the roots penetrate a cavity, such as a lava tube or tumulus crack, it may come into contact with atmospheric oxygen. Because a source of heat is already present, the charred root or the lava itself, a methane explosion may follow if the oxygen and "producer gas" mixture is between 5 and 15% (volume-percent fuel).[3]
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