Matt Bieker, age 7, of Portland, Ore., for his question:

HOW LONG DOES IT TAKE FOR PETRIFIED WOOD TO FORM?

Petrified wood is made up of tree trunks that were buried in mud, sand or volcanic ash ages ago and have since turned to stone. This action was caused by water seeping through the mud and sand into the buried logs. The empty cells of the decaying wood were filled with mineral matter until the structure became solid stone. The stone today shows every detail of the original wood structure, including the grain.

Scientists figure it may have taken about 150 million years for a once living tree to turn into petrified wood.

In the Painted Desert of northern Arizona is Petrified Forest National Park. The greatest concentration of petrified wood known in the world is found here.