skip navigation

Geology of the Petrified Forest

 

Excerpted from the work "Volcanoes in Eruption" by Terry Wright, PhD.

Imagine that you are in a grove of redwoods, some millions of years ago, with strange creatures scurrying around under a dense growth of trees and ferns. Smoke has been sighted coming from the towering volcano to the northeast, and a few earthquakes have rumbled through the forest in the past days, but nothing else is unusual. Suddenly, a puff of smoke arises from the side of the volcano, growing larger and larger in seconds and towering upward, shooting streamers of ash and bombs of lava outward.

A thundering noise starts, slowly at first then deeper and deeper as the cloud rises. A shock accompanies the sound and you realize that the cloud is moving down the slope, faster than an avalanche. A wind starts also and the trees lean towards you, away from the oncoming cloud. You hear cracking of tree trunks and limbs are falling all around. You feel and smell hot sulfurous gas. Suddenly things around you start bursting into flame. The sky darkens and you are blasted by the wind and tiny shards of ash. Trees are leaning toward you, cracking at the base and crashing to the ground. In an instant, all is hot, flames surround you and the forest is laid out on the ground, trunks pointing towards the volcano. The wind slows down and a fine rain of water and ash follows the cloud. Small and large fragments of cooling lava fall from the sky, burying the trees with a blanket of ash.

Then all is still, the violence of the explosion gone, and more ash is falling, smaller now, from a high cloud from the rumbling distant volcano. For days and weeks, the eruption continues, with more layers of ash burying the prone forest. Streams erode the surface, depositing gravel in their channels. Glassy lava flows erupt from domes of sticky lava. For perhaps a million years, more eruptions lay down ash, lava flows and ash with glass so hot that it is welded into hard layers.

Time passes and you remain buried with the rotting logs of the forest. The ground is now saturated with water containing dissolved silicon and oxygen, or silica, from the overlying ash. This silica is present in molecular form, as any submicroscopic particles which are carried down by water through the ash toward the buried forest. One by ones the molecules of silica replace the molecules of the wood, turning wood to solid silica, solid quartz, solid stone.

Slowly, over a million years, the- mountains are pushed upwards, the Coast Range is built, by pressures along the San Andreas Fault to the west. The flank of the volcano starts to erode and rivers and countless streams wash broken rock into adjacent valleys and to the sea. Steams cut deeper and deeper until the forms of the volcanoes are gone, mere stumps of their former selves. You watch the landscape change to rugged hills and a high mountain forms to the east on the hard remains of welded ash flows. Native Americans roam the woods and start to pick up the stone remains of trees to use as tools,but find the glassy obsidian from Clear Lake to the north is better suited to make arrowheads and spears.

Settlers come and build cabins, noting and collecting the stone wood as a curiosity. One man known as "Petrified Forest Charley" unearths a whole tree and becomes guardian of this natural wonder. He charges people admission to view the petrified trees and meets Robert Louis Stevenson, who writes about him and the trees. The distinguished scientist O.C. Marsh arrives in 1870 and takes samples back to Yale University for study. Then an unusual woman, Ollie Bockée, comes in 1914 and stays with her husband to develop the area as a public attraction. She advertises the forest and soon people from all over the world are flocking to see this unique area. Her heirs, the Hawthorne family, continues to improve the property in the same proud tradition.