Mt Shasta

On my field trip to California, I visited Mt Shasta. I learned a lot about this beautiful mountain and have several questions that I would like to help me interpret the history of the area.

What I found out is that Mt Shasta is an active stratovolcano in a subduction zone with several volcanic cones surrounding it. Mt Shasta is on a convergent margin, formed by the Juan de Fuca Plate subducting along the Cascadia Trench.

Mt Shasta is on the southern part of the Cascade Mountain Range, which is 37 million years old. The Cascade Range is relatively young, however, the oldest rocks found in the area of Mt Shasta are only 600,000 years ago, so Mt Shasta is quite a very young mountain in geological terms. It formed primarily over the last 100,000 years through four cone building volcanic events. There are primarily 4 volcanic cones that make up Shasta, varying in age. Some cones are older than others, which geologists are able to determine because they are glaciated and further eroded. Other cones are misshapen because of earthquake shaking and avalanches.

Something that was very interesting to me is the hummocks that look like spotted mounds spread across the lowlands of Shasta's southern section. These hummocks were a result of a massive avalanche that occurred 300,000 years ago. The debris from the avalanche covered 675 square kilometers and filled the valley at a depth of 50 meters, which was discovered through investigated old water wells. What is interesting is that geologists didn't know what the hammocks were or what caused them until after the Mt St Helens eruption. Hummocks made up of avalanche debris also covered the valley after the St Helens' eruption and this revealed the cause of the Shasta hummocks!

I would like to know what other unanswered questions there are that geologists have about Mt Shasta and the formation of its geological history. Are there aspects to the formation and the dating that are still unknown? And how can other similar volcanos located along ocean-continental converging boundaries inform us about the geological history of Shasta?

Further, I would like to know what technique was used to date the volcanic rock and debris. And what other signs of geological disturbance are found on Mt Shasta that could further help us understand it's history.

The catastrophic debris avalanche at Mount Shasta. 2011. Retrieved from https://volcanoes.usgs.gov/volcanoes/mount_shasta/mount_shasta_hazard_24.html

Geologic Field-Trip Guide to Mount Shasta Volcano, Northern California. 2017. Retrieved from https://pubs.usgs.gov/sir/2017/5022/k3/sir20175022_k3.pdf

Lopez, R. Mount Shasta, Cascade Province. 2015. Retrieved from https://www.youtube.com/watch?v=X5ZO9l2StZw