I am no geology expert. When I see a rock, I think “rock.” I don’t usually think about the subduction of the oceanic plate or rocks melting or an explosion of gas and ash. And I certainly don’t notice that the entire range of the Western Cascades (including Pilot Rock) are slowly tilting towards the east. Then I attended the hike and lecture hosted by Friends of the Cascade Siskiyou National Monument. Jad D’Allura, our geologist expert leader, was a passionate professor filled with knowledge and humor. I hope to pass on just a pinch of what he filled our brains with:
Pilot Rock was so named because travelers in the 1800s would use it as a guide to find their way through the mountains. There was no freeway. They had to push and pull their wagon up and down every hill.
The formation of Pilot Rock. I will keep jargon to a minimum. The oceanic plate subducted (slid) under the continental plate. At some depth, the rock began to melt and expand. The magma (melted rock) made its way to the surface and erupted in a small volcano. Some of the magma cooled inside the neck of the volcano. Many geologists believe that Pilot Rock is this “neck” that has been exposed. Today, the oceanic plate is still sliding under the continent and causing the monument to tilt 20 degrees to the east.
When Pilot Rock cooled, it cracked and created columnar joints (shown above). As the rock cooled, the cracks began at the top and bottom then met in the middle to make a column. The columns were easy to see as we climbed to the top of Pilot Rock!
Hornblend (above) is black and shiny. Apparently rare to find in the area, Hornblend was created when magma (underground) mixed with water. Where this water came from (ocean? ground water?) is still a mystery.
When the rocks melted, they expanded. When they cooled, they contracted – and cracked! In these cracks, other rocks and crystals fill in. Over time, the rock weathers away and leaves only the cracks like this quartz (above), this other green rock (below), and others. Notice how flat and smooth they are.
This rock I call “Grape Rock.” Jad D’Allura calls it “Botryoidal.” Botryoidal is a greek word that means a cluster of grapes. Obviously, the name describes the texture of the rock shown above. When this rock filled in a crack, some space was left. In that space there was room to make the bubbly globs.
Most of the rock surrounding Pilot Rock (the road and slopes) is Breccia (a fancy Italian word for bigger rocks glued together by smaller rocks – similar to a conglomerate). When the gases and ash exploded from the ground, much of it poured down the sides of the volcano picking up boulders and trees and eventually settling. In the two boulders blocking the path to Pilot Rock, you can see bits of tree in the rock! When eroded into soil, it is a very sticky clay. The BLM must “armor” the road with rocks to make the road drivable (“drivable” is a relative term). I won’t give you a picture of this one – you’ll have to drive out there and see it for yourself!