Earth as Inspiration: Mt. Shasta for Earth Science Week, October 14-20

Mt. Shasta and Shastina from the north

I usually don't need an inspiration to write about Mt. Shasta, but I realized (two days in) that October 14-20 is national Earth Science week, and the theme of the celebration this year is "Earth as Inspiration". In case you have not yet realized this yet, but I am in fact inspired by the Earth. It started early on in my life with family vacations to some stunning places in California and the American Southwest, continued with scouting experiences nearly every weekend in my teens that included extensive work with topographic maps, and right into my college major and subsequent career as a geology professor. And then I found the form of story-telling called blogging in 2008.

Panther Meadows at the 8,000 foot level on Mt. Shasta

Mt. Shasta is an earthly inspiration. We visited two weeks ago as part of our field course on California's volcanoes. The stops included a drive to the end of the highway at Panther Meadows at 8,000 feet on the side of the 14,179 foot high volcano, and another stop on the north flank where we could see the glaciers and one other astounding feature of the volcano (mentioned below). Shasta is the second tallest volcano (behind Mt. Rainier) but is the most voluminous composite cone (stratovolcano) in the range (I'm parsing words here; there is a larger volcano, but it is of a different kind and will be discussed in a future blog). As a classic composite cone, it is composed of the remains of at least four previous incarnations, capped by the current active vent, Hotlum Cone. The oldest lava and ash dates back to around 600,000 years ago, but the youngest is a mere couple of hundred years. It likely erupted in 1786.

As the highest mountain in Northern California, it supports seven glaciers. Whitney Glacier, with a length of two miles, is the longest in the state (below). The glaciers are responsible for one of Shasta's unique hazards: jokulhlaups! These are floods caused when meltwater is sealed underneath the glacial ice which then breaks out in a catastrophic manner. Although caused by eruptions under the glacial ice in places like Iceland, those that occur on Shasta can happen almost any time. They are generally more of a nuisance, messing up roads and bridges, rather than a killer.

The biggest dangers of a volcano like Shasta are volcanic mudflows (lahars), and hot ash flow eruptions. Lava flows, unless they interact with the ice, are of a lesser concern. Andesite lava has a pasty consistency and is not likely to flow overly far from vents on the mountain. Lahars are of the greater concern, as they are capable of producing massive casualties and structural damage. The towns of Mt. Shasta, Weed, and McCloud are constructed on old lahar deposits. Major events could cause the closure of Interstate 5. Hot Pompeii-style ash eruptions are somewhat less likely, based on the previous history of the volcano.

Whitney Glacier, the largest in California

One of the most outrageous landscapes to be found anywhere on the planet lies to the north of Mt. Shasta. When compiling geologic maps of the region in the 1970s, geologists weren't sure how to interpret the vast region extending north from the mountain reaching 28 miles to the edge of the village of Yreka. It was a hummocky landscape, made of hundreds of small hills and cones of lava fragments with intervening hollows, some containing lakes and ponds. Was it some kind of odd field of cinder cones? It didn't make a lot of sense until the eruption of Mt. St. Helens in 1980.

The St. Helens eruption was initiated when an earthquake caused the entire north flank of the volcano to collapse in a gigantic debris avalanche that traveled for twelve miles down the Toutle River valley. Humans had never witnessed a landslide so large. It uncapped the volcano, leading to the very explosive ash eruption that followed.

To the geologists studying the region around Shasta, it was a revelation. The avalanche at St. Helens formed hundreds of hummocks similar to those found at Shasta. It quickly became clear that the deposit on the north flank of Shasta was the remains of a gigantic debris avalanche. The St. Helens avalanche involved less than a cubic mile of material (0.67 cubic miles), but the now-apparent debris avalanche at Shasta was ten times larger (about 6.5 cubic miles), and it traveled twice as far. It seems to have happened between 360,000 and 300,000 years ago. It's stunning just to image seeing something like this happen.

What aspects of the Earth do you find inspiring?

The Other California: Five for the Price of One

The Other California is the story of the places in the state that aren't found on all the postcards. I'm finding some subject material in the familiar places that turn out not to be so familiar. Mt. Shasta is an example. The mountain is one of the most famous sights in the state, and people come to the mountain for many reasons: contemplation, climbing, skiing, hiking, and searching for errant Lemurians and Atlanteans. Everyone seems to refer to Mt. Shasta as a single mountain, but a brief look at the peak shows it to be actually five distinct volcanoes.

Mt. Shasta is often referred to as a stratovolcano, due to the alternating layers of volcanic ash and andesitic lava flows making up the structure of the mountain. It is often compared with symmetrical cones elsewhere in the world, such as Fujiyama in Japan and Mayon in the Phillippines. A different term, composite cone, is probably more descriptive. At least five major cones make up the edifice, erupted at different periods over the last 600,000 years.

The two youngest volcanoes, Hotlum Cone and Shastina are the most obvious of these multiple cones. Shastina, at 12,330 feet, is technically the third tallest volcano in the Cascades even lumped as it is with Shasta (it is the prominent peak on the left side of the photo above). It erupted during a series of eruptions around 9,800 years ago.

Hotlum Cone, the highest summit of the volcano, began erupting about 9,000 years ago, and has erupted numerous times since, including a possible eruption only 200 years ago. It is the most likely source of renewed activity on the mountain.

A drive to the end of the Everitt Memorial Highway at Panther Meadows provides a look (above) at two of the older cones. From the parking area Hotlum Cone is not visible. Instead, the upper flank is Misery Hill, and the jagged peaks to the right are Sargents Ridge. The Misery Hill Cone began erupting around 50,000 years ago, while Sargents Ridge is closer to 200,000 years old. Both cones are deeply scored by glacial erosion, and retain little of their original shape. I've outlined the cones in the photo below, including an approximate guess about the location of the Sargents Cone summit (note the slope of the layers).

The fifth volcano is the most enigmatic of all. It pretty much doesn't exist. The mountain developed prior to 360,000 years ago, but for a long time, no one knew where it went. Erosion could explain why it is gone, as could a massive explosion. It wasn't until 1980 that the actual explanation became apparent.

There is a vast area of knobs (called hummocks), small valleys, and ponds extending for 28 miles north of the volcano, almost to the town of Yreka. Geologists didn't quite know what to make of it, and it was mapped as a cinder cone field, which strangely had a consistent composition. Then Mt. St. Helens happened.

The St. Helens eruption was precipitated by an earthquake that shook the north side of the mountain loose into the largest debris avalanche ever witnessed by human beings. It traveled twelve miles down the Toutle River valley, and produced a unique topography: hummocky. As the dust settled at St. Helens, geologists knew they had an explanation for the strange landscape north of Shasta. Between 360,000 and 300,000 years ago, about 11 cubic miles broke loose from the volcano and flowed 28 miles, more than twice the distance of the 1980 event. So far as I know (I'm open to corrections), it remains the largest landslide known in North America.

The hummocky avalanche is visible in the aerial photo below (another view can be seen in one of my earliest blogs here).

We have a Winner!

That didn't take long, Ron

Many of the details of Mt. Shasta's complex history are summarized nicely by Bill Hirt at http://www.siskiyous.edu/Shasta/geo/index.htm. The small hills and ponds are part of the hummocky topography produced by a giant debris avalanche that broke loose from the north flank of Mt. Shasta Version 1.0 or 2.0 (Note that the modern Mt. Shasta is something like version 5.0; Sargents Ridge, Misery Hill, and Shastina are all earlier volcanoes that preceded the young summit called Hotlum Cone). The topography, seen from the air in Picture of the Day - The Airliner Chronicles Part 11, was an enigma to geologists until 1980 when the debris avalanche thundered off of Mt. St. Helens, producing a similar topography in the Toutle River Valley. The Shasta avalanche was much larger, traveling some 55 kilometers, making it one of the biggest ever documented.

Here is one more photo showing Shasta (the two summits are Shastina and Hotlum Cone), and a portion of the debris field along Interstate 5. Good job, Ron!