Petrified Forest National Park - Legacy of a Lush World

Some 200 million years ago this was a subtropical land. It was a vast, low-lying floodplain fringed on the south and west by active volcanoes -- a swampy world overgrown with lush tangles of green and populated by an assortment of giant reptiles and amphibians. Across the floodplain spread an emerald shag of dainty club moss. Spreading ginko trees, palmlike cycads, tree ferns and their low-growing cousins, all crowded the banks of rivers that meandered northward toward a sea that has long since vanished. Giant ancestors of the modern horsetail rose 30 to 40 feet above the mud-clouded waters of marshes. Tall conifers dotted the highlands and mantled the slopes of distant mountains.

The titan of these upland forests was an extinct relative of the Norfolk Island pine. When it lived, there were no humans to give it a name; today's scientists call it Araucarioxylon arizonicum. A mushroom-shaped tree, it commonly topped out at 100 feet and attained a diameter of 6 to 8 feet -- some grew to double that size. The species accounts for about 90 percent of the petrified wood in the park, and nearly all of the most brightly colored specimens. The remainder of the wood comes from two other extinct trees: the pinelike Woodworthia and the water-loving Schilderia, with a slender trunk that was fluted and buttressed at the base like that of a modern bald cypress. Neither grew much taller than 50 to 75 feet.

Some of the trees died of old age; fire, flood, and disease probably killed many others. Whatever the cause of death, the conifers periodically fell where they stood, and most of them rotted away -- but not all. Intermittent floods carried some of the toppled giants along rain-swollen rivers and through swirling rapids -- an arduous journey that stripped them clean of bark, branches, and roots. When at last the naked trunks arrived on the lowlands, many lodged at bends in stream banks; others came to rest atop sandbars or in shallow pools. Logjams formed and were quickly covered by thick layers of mud, sand, and volcanic ash that cut off oxygen and retarded decomposition.

Beneath their airless shroud, the logs began to change. Water leached silica from the volcanic ash and carried it down through the sediments, picking up traces of other minerals along the way. As the solution soaked into the wood, molecules of silica were drawn inside the cell walls, where they crystallized in the form of quartz. Gradually the trees became stone. Sometimes the process culminated in exact look-alikes, in which the wood's cellular structure was preserved in microscopic detail. More often, the mineral simply replaced woody matter and formed gigantic chunks of quartz that masqueraded as trees. In wide cracks and hollows, where the quartz crystals were unconfined by the structures of woody cells, they were able to assume their natural hexagonal shape, and here they crystallized as jewels. Some logs became warehouses in which multitudes of semiprecious gems were hidden.

Other minerals in the silica-laden waters gave the petrifying trees their colors. Pure quartz resulted in white and gray; iron supplied reds, yellows, browns, blues, and greens; carbon and manganese produced black. At times the colors were true and well defined. At other times, shades swirled and blended into tints without names and patterns without edges -- blurred color wheels frozen in time.

As the transformations continued, flood after flood, logjam upon logjam, the strange burial ground reached a depth of hundreds of feet. The climate changed. Deep waters and dry landscapes came and went, and for millions of years new sediments piled up, thousands of feet thick, on top of the prehistoric graveyard. Beneath the great weight, the mud, ash, silt, and sand of that vanished world hardened into the layers of shale and sandstone that geologists call the Chinle Formation. There it remained until, beginning about 65 million years ago, a series of slow geologic convulsions split much of the Southwest asunder. Over tens of millions of years, the Rocky Mountains were thrust skyward, the Colorado Plateau was uplifted, and the buried floodplain rose at least a mile. As it did so, the younger sediments were peeled away by erosion and sluiced to the Colorado River. Eons after their soggy interment, the ancient trees were once again exposed to the sky.