Lava tree mold

The lava cools just enough to create a solid crust around the trunk, but the tree inside burns away leaving a cavity.

[2] A unique phenomenon may occur during the formation of vertical tree molds.

As the lava-encased tree burns away, the roots are heated up and generate a "producer" gas, such as methane.

If the roots penetrate into a cavity, such as a lava tube or tumulus crack, it may come into contact with oxygen.

Because there is a source of heat 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).