Petrified Wood – how the trees turned to stone

Hundreds of millions of years ago, a massive flood ravaged what is now Arizona. Trees were sent crashing from stream banks.

When groundwater passes through buried wood, it deposits minerals, usually silica, inside the tiny cell spaces. Over millions of years, these deposits replace almost all of the original organic material, turning the wood into stone.

You need silica to petrify wood. Silica binds to cellulose, a major ingredient in wood cell walls. When it crystallizes, it preserves the microscopic structure of wood. So scientists can often identify ancient trees — even to species — from their petrified remains.

Volcanic eruptions are thought to be key to the formation of petrified forests. In the Petrified Forest National Park in Arizona, a massive flood left trees buried in mud for millions of years.

Volcanic eruptions to the west spewed ash that blanketed the park — released silica — and slowly caused the trees to petrify. In our time, scientists believe the forests buried by the 1980 eruption of Mount St. Helens may already be beginning the long process of petrification.

Relevant Web Links

• Petrified Forest National Park (U.S National Park Service)

When minerals like manganese, or iron-rich hematite or pyrite are deposited along with silica, the result is a rainbow spectrum of colors in the petrified wood. Silica in the form of opal is usually whitish, and quartz is generally clear. The addition of hematite will give shades of red and pink, goethite will give yellows, browns and oranges, pyrite will give blacks, and manganese is responsible for blues and purples. It all depends which minerals are picked up from the soil and deposited by the circulating ground water.

Silica is not the only petrifying agent — calcite, hematite and pyrite can also petrify. But silica is by far the most common, in part because of its strong attraction to cellulose. For this reason, petrified wood is also called silicified wood.

When it first crystallizes, the silica is mixed with water in a deposit of soft, rough opal. This whitish opal, called potch opal, is a far cry from the iridescent opal one sees at a jeweler’s, although structurally, they are very similar. The iridescence of gem-quality opal is caused by the presence of water trapped in the silica. In the process of petrification, over the next 10-40 million years, the opal dehydrates and changes to chalcedony (kal-SED-nee) and finally to quartz. Petrified wood is mostly quartz, but traces of opal and chalcedony may be visible as well.