Petrified wood at Pepperwood


Pepperwood and the Santa Rosa Junior College partner to offer a two-semester Bio 85 – Natural History of Pepperwood course that certifies participants as UC California Naturalists. Students have the option to submit an article for the Pepperwood Field Notes Blog as part of the course. The following is a student-submitted article.

Petrified Wood at Pepperwood

By Cate Steane, Pepperwood member and Fall 2016 Bio 85 participant

Look carefully at the rocks on the trail as you stroll along Roller Coaster Ridge at Pepperwood. Here and there you may see something that feels and smells like a rock and has the weight of a rock. It looks like a rock, but at the same time it doesn’t–it looks very much like a piece of wood. The outside looks exactly like bark. It may have shapes that look precisely like the base of a branch coming off of a trunk. And if there’s a cross-section, you can see annual rings exactly like those of a tree. What you have found is actually a piece of petrified wood.

What is petrified wood, and why is it found at Pepperwood?

Petrified wood could be an answer to the riddle, “When is a stone not a rock?” It is not igneous, sedimentary, or metamorphic, but it is made up of minerals. It is a fossil – the preserved remains or traces of a tree from the remote past.

Much of Pepperwood is a part of the Sonoma Volcanics geological formation. It was created about 3.5 million years ago by volcanic eruptions.  When the volcanoes erupted, layers of lava and volcanic ash were deposited on what is now Pepperwood. Mount St. Helena was probably the vent of this volcanic system responsible for the deposits on Pepperwood.

Trees growing in the area were buried by ash. As the ash cooled and hardened, it sealed off the trees from oxygen and microbes, which would normally decompose them. Instead, a process called permineralization took place. Permineralization is a type of fossilization involving deposits of minerals within the cells of organisms. Water rich in dissolved solids seeps into the pores of organic tissue and forms a crystal cast with deposited minerals. Crystals begin to form in the porous cell walls. This process continues on the inner surface of the walls until the central cavity of the cell, the lumen, is completely filled. The cell walls themselves remain intact surrounding the crystals. Most dinosaur bones are permineralized.

Permineralization is the first step in petrification. In petrification, the cellulose cell walls are completely replaced by minerals. So the petrified wood you may find along Roller Coaster Ridge is an exact duplicate of a tree that lived there 3.5 million years ago, except that all of the organic material has been replaced by silica. In this area, the form of silica in petrified wood is opal. Opal is extremely common in the Sonoma Volcanics. Pliny the Elder, the famous Roman scholar of natural history who succumbed to ash and smoke while rescuing people during the volcanic eruption of Mt. Vesuvius in 79 C.E. was thought to have coined the term “opalus” for this gem.

To learn more about the geology of Pepperwood watch the events calendar for upcoming lectures and outdoor explorations.

Want to learn more about petrified wood? Visit our friends and neighbors at The Petrified Forest!

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