As the summer wraps up, it is also time to wrap up my tour of the Cascade Range volcanoes as seen from space. The modern Cascades extend into Northern California and is the home of likely the largest volcano of the chain ... but the Earth's plates are plotting the end of the Cascades, starting from California.
The ocean plates that spawn the magma that forms the Cascade volcanoes are young and small. The Juan de Fuca plate is responsible for the volcanoes from Oregon to British Columbia (save for a sliver of the micro-micro Explorer Plate off Canada). However, once you cross into California, another micro-plate gets involved: the Gordo Plate. Sometimes lumped into the Juan de Fuca plate, the Gordo Plate is found off the very southernmost coast of Oregon and continues all the way down to area near Mendocino in northern California.
Triple the Fun
It is off the coast of Mendocino where something pretty unique exists: a triple junction. Here, three tectonic plates come into contact: the Gordo Plate, the Pacific Plate and the North American Plate. This triple junction means that south of Mendocino, subduction isn't happening anymore but instead the two plates (Pacific and North American) are sliding side by side - a motion called "strike-slip". This transform boundary is the reason the San Andreas fault system exists.
The triple junction isn't stationary, though. It is slowly migrating northward relative to North America, meaning subduction is being replaced by transform motion ... and with that comes the end of volcanism. The southernmost volcanism in the Cascades has been moving north at a rate of 15-20 kilometers per million years. Currently, the Lassen Volcanic Center is the end of the Cascade Range.
So, as the triple junction crawls towards Oregon and Washington, the Cascade Range will likely shut off until the entirety of the west coast of North America is one long transform boundary.
Thankfully for us volcanophiles, we got a long time to go before that happens. Let's celebrate the volcanoes that we have today, safe in the knowledge that they'll likely be around for hundreds of thousands to millions of years more.
Speaking of which, when we cross the border from Oregon to California, the monitoring of the Cascade Range changes as well. Instead of the Vancouver (WA)-based Cascades Volcano Observatory watching for restlessness, it is the relatively new California Volcano Observatory (CalVO). Formed in 2012, CalVO has purview over the California Cascades, the Long Valley Caldera area, the Salton Sea and Mojave volcanoes and anything else that happens in the Golden State.
Medicine Lake
Medicine Lake, like Newberry in Oregon, is a bit of an oddball. It sits behind (further east) the main alignment of the Cascade Range, nestled up against the Basin & Range province to west. The volcano is named for the lake that sits in the middle of the 7 by 12 kilometer caldera at its summit.
In many ways, Medicine Lake is a twin of Newberry. It, too, is a broad shield volcano capped with rhyolite lava flows. The eruptions that formed Glass Mountain, named because of the glassy (obsidian) nature of the lava were about 1,000 years ago. This is a few hundred years before Newberry's Big Obsidian Flow. Glass Mountain, unsurprisingly, is a common source of obsidian in tools across western North America.
And much like Newberry, Medicine Lake is surrounded by basaltic lava flows full of lava tube caves. These eruptions that comprise much of Lava Beds National Monument were made famous in the late 19th century by Native Americans that used them to hide during battles with American soldiers.
There are some unsubstantiated reports that Medicine Lake might have seen some explosions around 1910, but there is little to no physical evidence. However, the landscape is pockmarked with small craters from the dozens of eruptions that have occurred over the past few hundred thousand years.
Shasta
Mount Shasta is big. The volume of the volcano is over 350 cubic kilometers (~84 cubic miles). In fact, Shasta is better said to be a volcanic complex rather than a single volcano. There is the main Shasta edifice (the Hotlum cone) and then smaller Shastina built on its side. We could even lump in the barely separate Black Butte.
Everything about Shasta is impressive. It towards over the landscape of Northern California. If you have ever driven down I-5 from Oregon to California, you have driven through one of the largest volcanic debris avalanche deposits known that destroyed the original volcano. That avalanche from ~300,000-380,000 years ago moved 45 cubic kilometers (10 cubic miles) of material, or 18 times more than the avalanche from the May 1980 eruption of Mount St. Helens!
Shastina formed around 9,500 years ago with a series of explosive and dome-forming eruptions of andesite lava. Black Butte (west) is about the same age but the lava erupted there are more dacitic, meaning they are higher silica and were likely more silica rich. Both of these eruptions sent pyroclastic flows down the valleys around Shasta.
It has been a little less than 800 years since the last confirmed eruption of Shasta. If it were to erupt again, we might expect lava flows. They show up quite well on the satellite view of the volcano. We might also expect pyroclastic flows and lahars (mudflows) that have travelled tens of miles from the volcano in the past. The geologic record at Shasta suggests an eruption every 600-800 years. This does not mean we are "due" for a new one, but it does make it one of the longer periods of quiet at this giant volcano.
Lassen
Other than Mount St. Helens, the Lassen Volcanic Center is the most recently active volcano in all the Cascades. Lassen Peak erupted from 1914-17, creating what is called the "Devastated Area" where pyroclastic flows and lahars raced down the volcano. That eruption was pretty small, only ranking as a VEI 3 as a small dome of dacite formed at the summit of Lassen Peak.
A much bigger event in the Lassen Volcanic Center was the formation of Chaos Crags. These rhyodacite domes formed to the north of Lassen Peak only about 1,100 years ago. Likely these eruptions were explosive as well as producing the large lava domes that stand there today. There is also a large avalanche deposit (Chaos Jumbles) that came from the domes. You can see it to the north of Chaos Crags in the upper left hand side of the image.
In between these events was the eruption of Cinder Cone on the edge of the LVC in ~1666. You can see Cinder Cone in the top right hand corner of the Sentinel-1 image, with the darker lava flows standing out relative to the lighter ground that was scorched by fires a few years ago.
Due south of Lassen Peak near the bottom left corner of the image is Bumpass Hell, an area of vigorous hydrothermal activity. You can walk across Hell and smell the sulfur degassing and see mudpots and hot springs fed by superheated water. Close to Bumpass Hell is the ominously named Cold Boiling Lake where abundant carbon dioxide bubbles can be seen, again coming from the degassing of magma far below the surface.
So, there you have it, the Cascades from BC to California. If you want to revisit anywhere along the chain, here are the links:
California
As always, if you want to see these images at higher resolution, check out my depository of Cascade volcanoes from space.
