Near Holbrook, Arizona – March 26, 2022
We made our first visit to Petrified Forest National Park on March 25, 2022, restricting our time to only the northern, smaller section of the park. That part doesn’t have much in the way of petrified wood, but it has plenty of awe-inspiring views. We returned the next day, with Gunther, to experience the southern side, and we did see some fossilized wood. And how!
Petrified Forest National Park, which measures about 350 square miles, receives about 600,000 visitors per year. That number, while impressive, makes it just the third-most-visited national park in Arizona, following Saguaro National Park in Tucson (1 million visitors per year; Nancy and I were two of those people a couple of weeks earlier) and the most-visited park in all the land, Grand Canyon National Park (4.5 million). Incidentally, Rocky Mountain National Park in north-central Colorado is just behind Grand Canyon, at 4.4 million visitors per year. If you’ve been to Rocky Mountain National Park in the last 20 years and felt a bit cramped, it’s probably due to 4.4 million other people visiting a park measuring 415 square miles.
But we’re here to talk about rocks. A piece of petrified wood isn’t really wood any longer: it no longer contains any organic material and it is most definitely a rock. The process of petrification takes several important factors, including a tree, water, sediment, and time. Lots and lots of time.
Let’s start at the beginning. The scientists believe that the trees in Petrified Forest National Park were alive between 210 and 227 million years ago. At that time, the Late Triassic Period, the current area of the park was just north of the equator – in fact, it was close to where Costa Rica is today. The land was much different then: covered with forests of immense trees as well as large rivers and other wetlands. Huge amphibians and early dinosaurs roamed the forests and dwelled in the rivers. (Although there were many dinosaur species in the ensuing years, famous dinosaurs such as Tyrannosaurus rex and Triceratops wouldn’t appear until the Late Cretaceous Period, almost 160 million years later.)
Many of these coniferous trees (there are nine species identified in the park; all are now extinct) grew to be enormous: some may have grown to 200 feet in height. When the trees died they lost their branches and bark, then eventually toppled over after being undercut by a river. If the tree fell into the river, it may have eventually been covered in sediment being carried by the waterway. This relatively rapid burial is critical to later petrification: the water sealed the dead tree away from both oxygen and bacteria, which helped prevent decay. That delay gave time for silicic acid in the rivers to percolate throughout the tree. This process chemically altered the wood into a mineral called opal that still retained the tree’s fine features, like the grain of the wood, or indications of where branches once sprouted from the trunk.
Converting the wood into opal took only a few thousand years. Further layers of sedimentation over millions upon millions of years would cover the logs with tons upon tons of soil and rock. This process recrystallized the logs, converting the opal into quartz and a few other minerals. Over many other ensuing millions of years, erosion and geologic upheaval brought the logs back to the surface of the earth to once again see the light of day – this time as petrified wood.
Now that you know the factors involved in creating petrified wood, can you name the states in our country that contain it? The answer is below – keep on scrollin’!
The visitor center at the southern end of the park, which is part of the original monument created in 1906 (it was made a national park in 1962), contains some interesting fossils of both trees and animals. The fossilized remains of many amphibians and some dinosaurs dating to the time that the trees were alive have been discovered in the park (and the process for creating animal fossils is much the same as that used to create petrified wood). The museum also exhibits some handwritten letters: apparently, some visitors over the years were unable to withstand the temptation (and federal law) to leave the petrified wood where it lay within the park. Upon their return home with a fossilized wood souvenir, some of them inexplicably fell into bad fortune, such as personal or business relationship issues, and returned the rocks via mail, with an apologetic letter, to the national park.
After going to the visitor center and museum, and walking the Giant Logs Trail behind the building, we decided to go on a longer walk to see some more rocks. The Long Logs Trail, located a short distance from the visitor center, is so named because some of the petrified wood is more than 180 feet in length.
About the states that contain petrified wood: were you able to name them? If you named all 50, you’re correct. Although each U.S. state contain some amount of petrified wood, northern Arizona is able to display one of the largest concentrations in the nation because of the geologic upheaval processes that brought the logs to the surface of the earth.