How Yellowstone’s Petrified Forests Were Created

Large "petrified forests" or fossil forests cover much of the Yellowstone National Park's northern portion.
A petrified tree in northern Yellowstone.

A petrified tree in northern Yellowstone. Photo by Ben Wadzinski

Petrified forests are among Yellowstone National Park’s many claims to fame. The largest fossil forests in Yellowstone cover much of the park’s northern portion.

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Petrified trees

What are Petrified Forests?

Petrified forests are of course composed of petrified wood, which is the name given to any terrestrial vegetation that has been fossilized. The tree, or tree-like plant, undergoes the process of permineralization in which minerals replace the organic materials; however, while this process occurs, the original, three-dimensional structure remains intact.

The petrification process can only occur when sediment completely buries the tree, blocking oxygen flow and preventing standard decomposition. Water flowing through the sediment deposits minerals in the tree’s cells, and over thousands of years, these built-up minerals form a perfect replica. Even the tree’s cell structure and fibers are preserved.

In the case of Yellowstone’s petrified forests, volcanic materials buried the living. Softer rock that surrounded the petrified tree has eventually worn away, leaving it standing as it originally did thousands of years ago.

The fact that these forests feature upright trees is unique to Yellowstone’s petrified forest. Most other forests of this kind have petrified logs strewn about, with the specimens having been carried along by rivers or having rolled down steep slopes. Typically the “wood” is fragmented and broken.

In large part, the petrified trees of Yellowstone stand tall, covered by moss and lichen, and blending in with their living brethren. The one significant difference is that petrified trees do not have limbs or small branches, as the falling volcanic matter that initially rained down on the tree to preserve it broke off its more fragile parts.

Learn more about Yellowstone's petrified forests with this video from the Yellowstone Foundation: