Karen Nolkemper, age 10, of Ballwin, Missouri, for her question:

How does wood get petrified?

The word "petrified" means turned into stone. And this is just what happens in the case of petrified wood. Once it was ordinary wood, growing in a forest of tree trunks. Then, perhaps 100 million years ago, the forest was destroyed. Perhaps it died of thirst, perhaps a volcano covered it with ashes or perhaps a flood washed it away. In any case, the tree trunks fell down and were buried in sandy mud.

Through millions of years, rainwater seeped through the ground, dissolving the rocky minerals. It also seeped through the fallen logs and dissolved their woody materials. But as it carried away fragments of wood, it left behind fragments of dissolved rock. Slowly, all the wood was replaced with stone minerals, such as opal and agate. The lines and cells in the original wood were copies in fine detail and usually tinted with pretty colors. But at last the petrified logs were made entirely of stone.