I created a new world (seed: 990498698) and I observed a lava lake. As you can see in the picture, a part of the tree (wood) are turned into stone.

Edit: I have an another device uses the 0.11.0 Alpha Build 14 but I am not able to recreate the same world since the spawn of lava lake is done randomly.

^{(You can see some grass on the stone!)}