A petrified forest is a forest made out of fossil or petrified wood. In other words, it is a forest made out of stone trees. Petrification is a natural process that occurs when all organic material in a tree dies and is replaced by a combination of quartz, copper, iron, and other minerals. Once the process has been finalized, petrified wood is no longer considered wood, and it becomes classified as a stone. In fact, petrified wood is Washington's official stone. All petrified forests are national monuments and are carefully protected.
A petrified forest becomes such over the course of million of years. Some of the world's largest ones are an estimated 100 million years old. Volcanic ash is sometimes a key step in the process of petrification. In fact, it's not uncommon for a petrified forest to have been covered by mud made out partially by volcanic ash at some time. Volcanic ash produces a chemical reaction in the wood, which in turn accelerates the process of petrification.
A petrified forest can take different colors, depending on the type of material once absorbed by the living trees. Cobalt and copper cause a greenish blue tint, while manganese turns the wood pink, and carbon produces gray. Some trees have more than one color, while others are basically composed of crystal quartz, which are transparent and allow for a view of the grain wood.
The world's most impressive petrified forest is in Santa Cruz, Argentina. With trees that are over 10 feet (3 meters) in diameter, the Patagonian petrified forest is one of the best giant examples of wood-stone in the world. As a comparison, the Petrified Forest National Park in northeastern Arizona, considered the best example of petrified wood in North America, boasts trees that barely reach six feet. Canada has the largest forest of this kind in the world, covering an area of thousands of kilometers and dating back to the Eocene period.