December 7, 2021

I, Science

The science magazine of Imperial College

Arizona Petrified Forest

The badlands of Arizona, once thriving with lush forests of the Late Triassic, are now littered with the reminder of greener times. Beautifully preserved wood logs have withstood the test of time and given scientists a glimpse into the complexities of a palaeo ecosystem. 225 million years ago the scene was very different from the dry eroded desert we see today.

In the Late Triassic, this region of Arizona was a part of the supercontinent Pangaea and situated near the equator. The sub-tropical environment and flowing streams facilitated the growth of thick vegetation. Over time sediment deposits buried the trees and animals that fell into the river. This was the initial condition for successful preservation. Burial prevented contact with oxygen rich atmosphere and inhibited aerobic decomposition. The process of permineralisation could begin.

The trees’ cells were slowly filled by mineral rich water flowing through the sediment. This replaced the organic matter in the trees’ cells with quartzite minerals whilst maintaining the structure of the tissue. These ancient trees still display annual growth rings 225 million years later. Moreover, the logs’ rich colour is the result of impurities in the quartzite crystals. Elements like manganese and iron produced a red colour while chromium and cobalt gave a blue/green tint.

It’s likely that there are many places on earth where this process has taken place, but few that rise to the surface for us to study. Fortunately, about 60 million years ago the Colorado Plateau underwent increased erosion due to tectonic uplift. As the continent rose and rains fell, the sediments were eroded away to expose the magnificently preserved trees.

Petrified Forest National Park consists of 380 km2 of land and an endless array of specimens. Among the geological history there lies 13,000 years of human history. From prehistoric peoples to early explorers, the petrified forest of Arizona is both an ancient hunting ground and scientific treasure. Each year new species of plants and animals are found in Arizona’s petrified forest. The immortalisation of organic material has deepened our understanding of palaeo-ecosystems. As the process of plate tectonics continues to shape the earth, there is no telling what buried treasures lie beneath.

Images: Stephanie McClellan