America Has Other Volcanoes: Airliner Chronicles Visits Mt. Hood

The tragic and yet fascinating activity on the Big Island of Hawai'i has focused attention on volcanism in the United States, and has served to remind us that Hawai'i isn't the only place in the country that has to face up to the hazards of living in the shadow of dangerous mountains. I traveled to Washington by plane last week and was lucky enough to capture images of several of the volcanoes of the Cascades. We looked at Mt. Rainier first, and then at California's largest yet little-known volcano, Medicine Lake Highland. I really wanted to show some shots of St. Helens but we flew right over it, so I cheated and used some shots from 2006. But I wasn't disappointed by Oregon. The face of Mt. Hood was still illuminated by the rapidly setting sun.

Mt. Hood is the headache for emergency planners in Portland and the small villages south of the Columbia River. It is the highest mountain in Oregon at 11,249 feet (3,429 meters). The upper slopes are extremely rugged and steep and as such present a serious threat of debris avalanches similar in nature to that which destroyed the summit of Mt. St. Helens in 1980. The scar of the most recent avalanche can be seen on the right side of the summit in the picture below. The slide took place about 1,500 years ago. The prominent spike of rock in the alcove is called Crater Rock, and it is the remains of a lava dome that erupted around 1781. An earlier avalanche around 100,000 years ago removed the north flank of the mountain and flowed down Hood River Valley and across the Columbia River.

The thick mantle of snow presents the other serious hazard, that of lahars, or volcanic mudflows. The fluid masses have reached the outskirts of Portland in the past, and some lahars have occurred in recent years even though no eruption took place.

Living near volcanoes doesn't and shouldn't mean living in constant fear, but it is important to be aware of the potential threats where you live, and an understanding of what you will need to do in the event of an eruption. And because of all the crap roiling around on the internet, get your information from the geologists who work for the U.S. Geological Survey or state surveys in your area. Always be aware of the potential of exaggeration in the media, because even if they present good information, it will be cloaked in clickbait-style headlines that they utilize to get attention these days.

Meanwhile, the plane continued southward, and another volcano or two could still be discerned in the fog and mist. My mention of the Airline Chronicles refers to my first blog series that started way back in 2008. Some more information on Mt. Hood is available at  https://pubs.usgs.gov/fs/2000/fs060-00/.

Airliner Chronicles: Do You Know California's Biggest Volcano? You May Be Surprised at the Answer

The summit complex of Medicine Lake Highland, with Glass Mountain at the top, the Medicine flow center-left, and Medicine Lake at center-right.

It's a real problem for a geologist to be flying through cloud cover. One loses track of time and location so when an opening in the clouds occurs, one cannot know for sure where one is. I can pick out the essential geologic details a lot of the time, but without temporal or spatial context, I can't recognize even some of the most familiar places that I've visited many times. That's what happened last Thursday when I was flying to SeaTac. I lost sight of the ground only a few minutes out of Sacramento, and when the ground reappeared again, I was totally fooled by the scenes in these pictures. I thought I might be over Newberry Crater in Oregon, but I couldn't place Paulina Lake with the steep ridge that should have been to the right in the picture above. So I figured it must be some place in the Three Sisters area that I wasn't familiar with. But then I got home and looked at the pictures on the computer instead of the camera. I was wrong.

The Medicine Flow dates to about 2,000 years ago. It is composed of thick dacite lava flows. Frozen Medicine Lake is to the right.

It turns out I was flying right over the top of California's biggest volcano. Mt. Shasta is California's most prominent volcano, rising to 14,162 feet above sea level. It's the largest stratovolcano in the entire Cascades chain, with more bulk than even Mt. Rainier, which we looked at in our previous post.

[At this point there is a bit of uncomfortable shuffling in the back of the room as a few of the geologists look at each other and start mumbling something unintelligible.

"Ahem, is there something you would like to add to the discussion?" I say.

"But we've seen Mt. Shasta, and it looks nothing like that".

"And you would be right" I say, "but give me a chance to finish". They relent.]

But this isn't Mt. Shasta. It may be the tallest volcano in California, and it may be the biggest stratovolcano in the Cascades, but we are forty miles northeast of Shasta, looking at an entirely different kind of volcano. It's a shield volcano called Medicine Lake Highland. Although it is not even 8,000 feet high, it is much wider, with a total volume of around 130 cubic miles, compared to about 108 cubic miles for Mt. Shasta.

The view from the ground shows a gently sloping edifice covered by a variety of cinder cones and plug domes. Medicine Lake Highland (MLH)is shield-shaped like the giant volcanoes that make up the Hawaiian Islands, and is composed in large part of basalt, the fairly non-viscous lava that can flow for long distances before congealing. But MLH is different in some important ways. Some of the magma chambers beneath the volcano originate in the crust rather than the mantle (dacite or rhyolite), and the silica-rich lavas that erupt at the surface are sticky and barely flow at all.

MLH is an active volcano. There have been at least 17 eruptions in the last 11,000 years, and the Glass Mountain flow (below) erupted only about 850 years ago. There are areas of geothermal activity that have been proposed for energy development, but these have been rebuffed so far (thank goodness).

Glass Mountain at the top of Medicine Lake Highland. The flow dates to 850-900 years before present.

The top of the shield complex is a caldera (a sunken crater-like depression caused by the inward collapse of the top of the volcano). It is about the size of the Crater Lake Caldera, but is not nearly as rugged, as it is many thousands of years older and has been partially filled with subsequent lava flows. Medicine Lake sits within the caldera, but is not volcanic in origin. During the ice ages, glaciers scoured the summit area of MLH and left behind some impermeable clay deposits that prevent water from sinking into the ground (there are very few streams or rivers on the volcano, as it is highly porous; massive springs are found around the base of the volcano.).

One of America's most interesting national monuments is found on the north flank of MLH. Lava Beds National Monument preserves more than 75 miles of underground lava tubes, one of the highest concentrations of such caves in North America (and maybe in the world). The park also preserves the memory of the last stand of the Modoc people in their struggle against the U.S. military in the 1870s. They held out for months in the barren lava flows but were finally betrayed and captured. The Modoc people survive, but much of their cultural history was lost when they were banished to Oklahoma.

The clouds soon closed in again and I saw nothing more until we were over Washington.

Dear Washington State: All is Forgiven...The Airliner Chronicles Returns

The Pacific Northwest has this problem. It has two particular things in great abundance: trees and clouds. It also has volcanoes, but much of the time the first two obscure the latter. At Christmas I drove to the Seattle area and spent three days, and never laid eyes on Mt. Rainier, the gigantic stratovolcano that looms over the Puget Sound region. In the last week I had occasion to fly to the Seattle area, and being the month of May, I figured I had a pretty good chance of seeing some of the Cascade volcanoes. It started out pretty well as we left Sacramento, since I could see Lassen Peak off in the distance and then it disappeared. The clouds obscured the views all the way to Washington with the sole exception of a brief look at Mt. Adams, and the view below of Mt. Rainier.

Yeah, somewhere in there is the summit of Mt. Rainier.

That was okay. I had four days of walking and driving around Kent and Renton, and figured there would be a view once in awhile of the massive volcano. Nothing doing. There were tantalizing glimpses while driving around, but there were always trees in the way. Washington was disappointing me.

The flight home was scheduled to take off at 7:50PM so I figured there were too many things that would happen to either delay the flight until after sunset, or the clouds would be there again. But we boarded the plane on time and while taxiing down the runway I got the best ground-based picture of the mountain on the entire trip. It wasn't much, but it was something. 

And then we took off. It was hazy, and we took off towards the north and did a long turn towards the south, long enough to make me wonder if we were going to pass Rainier on the wrong side. But as the plane banked there was suddenly a very big mountain in the window, and Washington with all her trees and clouds were immediately forgiven. The mountain was stunning in the evening light.

Mt. Rainier from the runway at SeaTac.

At 14,411 feet, Mt. Rainier is the tallest volcano in Cascades Range, and is exceeded in volume only by Mt. Shasta in Northern California. It was once even higher, but glaciers have removed 1,000 feet of rock or more from the summit.  Because it is by far the tallest mountain in the Pacific Northwest it is completely covered by the largest mass of glacial ice in the lower 48 states, about a cubic mile (I read somewhere that it contains half of the all the glacial ice in the lower 48, but I can't find the source and would welcome any corrections from those who know such things). Aside from the "normal" threats that volcanoes might present to a given region (lava flows, ash flows, and that sort of thing), the snow makes the mountain far more dangerous. It's not hard to imagine why: any small eruption would melt a vast amount of ice, forming volcanic mudflows called lahars that are capable of flowing for many tens of miles, and threatening many of the cities along the southern part of the Puget Sound. The entire city of Tacoma is built on a mudflow that thundered down the mountain 5,000 years ago. The last major eruption occurred around a thousand years ago, although minor activity may have occurred several times in the 1800s. 

We flew south as the sun was sinking below the horizon and several other volcanoes appeared out of the gloom. Those pictures will come later. By way of explanation, the term "Airliner Chronicles" refers to the very first blog series I ever put together way back in 2008. One of my earliest posts in the series was also an evening shot of Rainier. By some strange convergence, that post received its first and only comment just two days ago!

The Airliner Chronicles: The San Francisco Peninsula and the San Andreas Fault

Crystal Springs Reservoir is on the upper left, while San Andreas Reservoir is on the lower right

As has been no doubt obvious, I was in Hawai'i last week, and there have already been several posts about some of my adventures. I started through the many pictures in a more chronological manner, and realized there were some neat things I saw before I even left California. I always notice on a flight that some people do such things all the time and are not particularly impressed that they are in a large metal tube traveling at hundreds of miles an hour at 35,000 feet. Others have a sense of history, realizing that for the first few million years of hominid existence, this was an unnatural thing to be doing. I don't get to fly all that often, so I tend to fall into that second category. As soon as we are off the ground, the camera that I surreptitiously placed at my feet swings into place and I start snapping pictures as if I were the first human being ever to see such incredible sights. My photo habit led to my first blog series in 2008, the Airliner Chronicles. Consider this a new entry...

It is the geologist's perspective that provides some insight as to the nature of the landscape below. We left from Oakland Airport and flew over the north end of the San Francisco Peninsula, which provided an outstanding view of the San Andreas fault. Often such faults are indicated by linear valleys owing to the ease with which crushed rock in the fault zone erodes. But on the peninsula the fault is even easier to pick out because of the presence of two reservoirs filling the linear valley, Crystal Springs reservoir, and San Andreas reservoir.

It's quite a coincidence, the fault and the lake both being called "San Andreas", so the question naturally arises, which was named first? The fault may be more famous to most people, but the lake came first. It was constructed in 1868. In 1895, the geologist Andrew Lawson was mapping in the area and discovered the fault, which he named after the lake. He had no idea that the fault was 600 miles long, or that it was particularly active, or that it was the boundary between the Pacific and North American tectonic plates. And he most certainly didn't know that it would shift in 1906 in a most tragic way.

The city has grown in the years since 1906, to say the least. We know a great deal more about fault zones and earthquakes, and thus we have a better understanding about the threats of future quakes in the Bay Area. Both by law and by subsequent experience (Loma Prieta in 1989 and the Napa Valley in 2014), the cities of the peninsula are better prepared for large earthquakes, but no matter how ready they may be, the next large earthquake will cause major damage on the peninsula and many will die or be injured. Seeing the proximity of the fault and the cities from above provides a stark reminder of the need to be prepared.

We flew out over Half Moon Bay, and soon land was left behind. There was more than 2,000 miles of open ocean ahead of us, a five hour flight. I settled in and thought of how the islands were inaccessible to humans of any kind until only a thousand or so years ago, and even a hundred years ago it took weeks or months to cross the ocean. How even more wondrous the number of bird and insect species that survived the journey over the millennia.

Airliner Chronicles: Stuck on a Plane with a Proselytizer...

And really, I felt sorry for the poor guy who was stuck sitting with me on the plane flight from St. Louis to LAX. Oh, I wasn't trying to convert the poor guy into some religion. No, he got the full-court press from me about the importance of understanding what was going on 35,000 feet below us on the ground. He was being proselytized into the world of geology.

People who fly a lot for whatever region may be forgiven for not paying attention to the grand panorama unfolding below them, but to a geologist, the extra dimension is pure gold. Seeing a large swath of the Earth's surface grants a whole new perspective to understanding geological processes.

I didn't have a working GPS on the flight, so I had to guess our location for the first two hours of the flight, somewhere over Missouri, Oklahoma, or Texas. But it was unmistakable that we were over oil and gas country. The drilling rigs and their connecting roads could not be missed. Some politicians once described the "footprint" of oil and gas drilling on a landscape as just a few acres being torn up. Seeing the scene from above suggests that the footprint is "small" in the sense that a spider web is a few strings of dragline silk.

I was lost until the mesas and plateaus appeared. I knew at that point that we were in New Mexico, and I correctly figured out that we passed Sante Fe and Las Vegas, New Mexico. My seatmate, a Pittsburgh resident, got a bit confused about the Las Vegas part; "We're in Nevada already?". I crushed his hopes (that is NOT the way to proselytize, by the way).

The landscape turned into a rainbow of color, and I suddenly knew our precise location better than a GPS unit. We had reached the Painted Desert area of Petrified Forest National Park in Arizona. A plateau covered by basalt flows was breached by erosion, exposing the brightly colored layers of the Triassic Chinle Formation. The Triassic rocks reveal the beginnings of the dinosaur domination of our planet, and the floodplain and river deposits contain some of the earliest dinosaur species known. There is the wood, of course, and a stunning variety of amphibians and reptiles, including Phytosaurs, huge crocodile-shaped creatures that exceeded 30 feet in length.

Just west of Petrified Forest, I got the finest treat of the day, a perfect view of Meteor Crater. I put the best of the pictures up in yesterday's post.

A short time later, more colorful rocks came into view, but they were older than the Chinle of Petrified Forest. We had reached the Pennsylvanian and Permian rocks of the Supai Group. These rocks are the same ones exposed in the walls of the Grand Canyon, about eighty miles to the north. This is the edge of the Colorado Plateau, where the high flat landscape gives way to the deep fault valleys of the Basin and Range Province. Because the valleys are so deep, erosion eats away at the edge of the plateau, forming scenic deep gorges like Oak Creek Canyon, north of Sedona.

I was distracted by some wildfires burning in the thick forests of the plateau. I finally realized I was missing one of the more extraordinary features of the Colorado Plateau, the San Francisco Peaks Volcanic Field. The field is a vast basalt lava plain populated by hundreds of cinder cones, and an immense stratovolcano that reaches more than 12,000 feet in elevation, the highest point in all of Arizona. The edifice of the San Francisco Peaks has been altered somewhat by erosion; it was once 4,000 feet higher. It would have been the highest point in the lower 48 states.

The existence of the volcanic field is somewhat of an enigma. There's no obvious reason for it being here. There are suggestions that it is the result of an incipient hot spot, but the idea is not wholly accepted. The field is active; an eruption took place less than a thousand years ago.

Somewhere near the end of the flight, my poor beleaguered seatmate asked a geological question. He was wondering why there was a gigantic hole in the ground that wasn't a meteor impact crater. It was an open pit mine, probably for copper. I almost had a convert to the ages of rock!

Airliner Chronicles: When Disaster Arrived from the Heavens

I thought briefly of making this one of those "What is it?" kind of posts, but it seemed kind of obvious. We were flying home from our weekend in St. Louis, and without a GPS, I was trying to get myself situated correctly into the geography that was drifting by slowly far beneath us. I was not too particularly successful at orienting myself while over the "flyover" states of Oklahoma, Kansas or Texas, but the fracking rigs were obvious, as were the irrigation rings, where farmers are pumping up fossil water from the Great Plains Aquifer and letting it evaporate in the intense sunshine.

Once we reached lands with actual topography, I began to suspect where we were...was that Las Vegas, New Mexico? What about that town? Santa Fe? It turned out I guessed pretty well, and also recognized the holy peak of Mount Taylor in New Mexico, and the Ambrosia Lake Mining District. But it wasn't until I saw the intense magenta of the Painted Desert and the unique road loop near the northern visitor center at Petrified Forest National Park that I knew precisely where I was. And then I realized that Meteor Crater was just ahead. I was sure that the plane would go right over it and it wouldn't be visible, but luckily I was wrong. I had a box seat view (although metal cylinder seat view would be a better description). I was, in a word, thrilled. I'd never seen it before from above.

Meteor Crater is justly famous as one of the best preserved meteorite impact sites in the world. Roughly 50,000 years ago, when camels, horses and mammoths were grazing the grasslands, the sky lit up with fire and a chunk of space rock about fifty feet long impacted the surface at a speed of 30,000 mph. The ensuing cataclysm produced a crater about 1,200 m (3,900 ft) in diameter, and 170 m deep (570 ft).

Meteor Crater is privately owned, but the company has done a good job of protecting this unique site. They are especially good at accommodating school groups on tours. You can check out their website at http://meteorcrater.com/.

The "Airliner Chronicles" was one of my first web series, and whenever I fly somewhere, I add to it. I got a few other shots, so if I am not too distracted with Hawaii preparations, I will post some.

Airliner Chronicles: Finding Fault by Flying

Flying can be no fun whatsoever, what with the lines, the waits, the security checks, surly attendants, and the tarmac delays. It would be intolerable were it not for window seats. So while I'm in connecting flight limbo, here is a sight from my flight between Oakland and LAX today. We generally flew right over the San Andreas fault for most of the flight, but in Southern California, the fault takes a more southeasterly trend and it suddenly appeared below me. This section of the San Andreas is just west of Gorman at the summit of the Grapevine, the point where Interstate 5 crosses the Transverse Ranges on the way into Los Angeles.

The "Airliner Chronicles" is a throwback to the very first blog series I did way back when. I don't fly all that often, but if there is good geology below, you can be sure that I'm trying to capture it in pictures.

The Airliner Chronicles: the San Andreas Fault in Southern California

The San Andreas fault at Cajon Wash and Lone Pine Canyon at the eastern end of the San Gabriel Mountains

Oh how I love flying. I don't get all that many chances to do it, and flying on days with clear weather happens even less often, but sometimes it works out. One of my first blog series was called the Airliner Chronicles, and in it I featured geology from an aerial perspective. I haven't flown in a number of years, but I had a brief trip to Southern California last month, I scored a window seat, and managed to chose the correct side of the plane, so I got to see some marvelous perspectives of the San Andreas Fault.

Mount San Antonio is the highest peak in the San Gabriel Mountains at 10,069 feet (3,069 meters). The highway in the foreground is Interstate 15 heading towards Cajon Pass.

What made the views so memorable was that they were in the San Gabriel Mountains, which just a few weeks ago were made into a National Monument. These were the mountains of my youth, the peaks and valley where I learned to camp, hike, and climb. I've seen these mountains from down in the canyons, and from the tops of the peaks, but never so well from high above in a plane. The flight was too short!

The San Bernardino Mountains are higher than the San Gabriel Mountains on average, but not generally as rugged. I was on the wrong side of the plane for good pictures, though.

The San Andreas fault slices through 600 miles of California, but the most rugged stretch of its path is along the northern edge of the San Gabriel Mountains. The fault actually crosses the Transverse Ranges, offsetting the San Gabriels from the San Bernardino Mountains at Cajon Pass. The presently active stretch of the fault has shifted at least 130 miles in a right lateral sense (when standing on one side of the fault, the other side shifts to the right; a left lateral moves the opposite direction). The grinding of the rocks along the fault produces a powdery material called gouge that is easily eroded. The fault reveals itself as a series of linear valleys that cut across the mountain ridges. I've provided labeled versions of most of the pictures showing the trace of the fault.

The San Gabriel Mountains are moving northwest along the fault.

I passed the small mountain village of Wrightwood, a town with a lot of geological problems. Not only does it sit directly on the San Andreas fault, it also lies at the base of a steep mountain ridge composed of deeply sheared Pelona schist. When wet, the schist is prone to failure, and there have been a number of serious mudflows over the years including an event in 1941 that buried 190 acres. The scars of the landslides were easily visible from above.

The village of Wrightwood on the San Andreas fault. The Heath Canyon Mudflow started at the scars on the left side of the photo.

The San Andreas has produced a number of very large earthquakes in Southern California, most notably in 1857, an event with an estimated magnitude of 7.9. The quake fractured the ground for a distance of 225 miles (350 kilometers) from Cajon Pass to Parkfield in the Central California Coast Ranges. The ground shifted as much as 30 feet (9 meters), with an average of 15 feet (4.5 meters). Trees that were damaged in the quake can still be seen in Wrightwood. The average recurrence interval over that last few thousand years has been about century, but it varies widely from a few decades to more than 300 years. It is considered one of the more likely faults to produce large earthquakes in coming decades.

Wrightwood on the San Andreas fault in the San Gabriel Mountains

A bit further I could pick out the Mountain High Ski Resort and the deep gorge of the East Fork of the San Gabriel River. The heart of the newly established San Gabriel Mountains National Monument, the East Fork is around 4,000 feet deep, almost as deep as the Grand Canyon.

Mountain High Ski Resort and the East Fork of the San Gabriel Canyon

The canyon is unique. It was the focus of a huge political battle in the 1970s over a wilderness designation that brought much of the canyon under protection from mining and development. There are herds of Nelson Bighorn Sheep and natural trout fisheries in the depths of the canyon tributaries. Development was always a dicey proposition. There is little in the way of flat terrain, and terrible floods have scoured the canyon bottom on numerous occasions. There is a paved "bridge to nowhere" in the lower canyon. The road it was once connected to was washed away long ago.

Further west there is a fine example of a shutter ridge. These are mountain ridges that have been moved laterally by the fault in such a way as to block of the river drainages from the other side of the fault, deflecting the streams sideways.

A shutter ridge along the San Andreas fault in the San Gabriel Mountains

The winding road in the picture is the Angeles Crest Highway, which was opened for traffic in 1956. It is an anachronism, built in a time when development was encouraged in the national forests. It goes for 66 miles from La Canada in the L.A. Basin to Wrightwood. In such steep terrain, keeping the road open is a challenge. Landslides and slope failures are commonplace along the highway.

Shutter ridge on the San Andreas fault

 A special geological treat from the air is Devils Punchbowl, a Los Angeles County Park. The park lies on an ancient strand of the San Andreas fault. The sandstone and conglomerate of the Punchbowl Formation was twisted into a plunging syncline, a downward pointing fold with an axis that slopes to the west (to the right in the picture below).

The Devils Punchbowl, a west plunging syncline in resistant layers of conglomerate and sandstone.

Devils Punchbowl is a marvelous little park with memorable hiking trails that explore the steep fins of sandstone. You can see some pictures from ground level of the region in this post from last year.

Offset stream near Palmdale. The dark line is the California Water Project.

As we neared the town of Palmdale, I could see examples of offset streams. These are stream channels which have been shifted and separated by the lateral motion along the fault.

My flight also included a stretch of the fault system a little farther north. I'll try to include it in the next post.

Down the River: Geotripper Goes Geo-Rafting

I'm on the verge of one of my great life adventures. Sunday morning, I'll be rolling over the Paria Riffles (below) on the first day of a 226 mile journey down the Colorado River through the Grand Canyon. I've never done it, and I've always wanted to, and after 40 years of waiting, it's happening. I'm excited to have the chance to explore the depths of the canyon pretty much for the first time. I've hiked to the river on three or four occasions, at Hance Creek and Bright Angel Creek, but those trips were decades ago, and had little to do with the river itself.

Of course, there is no communicating with the outside world, so Geotripper will be on hiatus for a couple of weeks. If you are new to this blog, this is a great chance to catch up on some of the past Geotripper series on the Colorado Plateau, the Sierra Nevada, and other wonderful parts of the world. They're listed below...

See you all in a few weeks! With pictures, I hope, assuming I don't drop the camera in the river!

The Other California: what to see when you've seen all the really famous places in the Golden State (in progress).

Vagabonding Across the 39th Parallel: A journey through the geological wonderland in central Nevada, Utah and Colorado in 2011.

A Convergence of Wonders, a compilation of posts on our journey through the Pacific Northwest and Northern Rocky Mountains in 2011.

The Abandoned Lands, a compilation of posts on our journey around the margins of the Colorado Plateau in 2012.

Time Beyond Imagining: A "Brief" History of the Colorado Plateau - this was an extended exploration of the geology of one of the great geological showplaces on planet Earth

Under the Volcano and Into the Abyss: Yosemite National Park - Exploring a few of the lesser known corners of Yosemite Valley, from below and from above

The Airliner Chronicles: My First Blog Series - Seeing geology from the perspective of seven miles above.

The Airliner Chronicles: Glory in the Skies (not a religious post, though)

Pilots and frequent fliers are no doubt familiar with the phenomena, but for infrequent fliers like myself (I will qualify for a free flight sometime in the middle or late Anthropocene epoch), this was new. We were headed east in the late afternoon approaching Las Vegas when I saw a strange optical effect in the clouds directly opposite from the sun. I could see a spectrum of colors, but it was most certainly not a rainbow, although some of the same optical physics principles turn out to be involved.

I zoomed on the colorful arc for a better look. I vaguely remembered hearing about the phenomenon but for the life of me I couldn't remember the name. It's called a "glory", or in more modern times, the "glory of the pilot". It happens when the observer has the sun behind, and clouds in front. Light is backscattered in the water droplets of the cloud and diffracted, and as such is similar to the formation of a rainbow, but is different in size and pattern. According to (the always dependable) sources on the Internet, the dynamics of the light pathways are not fully understood. It was first noted by mountain climbers, where it was associated with an apparently magnified shadow of the observer called a Brocken spectre. The climber will see the colorful arc surrounding their head like a halo.

Luckily, we were descending for our landing at Vegas, and we passed right through the clouds, so I was able to catch a few shots of the airplane version of a Brocken spectre: the glory around the shadow of the plane we were in. It was small at first, but then we passed right next to a tall cumulus cloud, and the image was much larger (below). The placement of the glory actually reveals what part of the plane I was sitting in!

The Airliner Chronicles is one of my on-again/off-again serial features, which is usually updated whenever I fly somewhere.

The Airliner Chronicles: A Broken Land of Unspeakable Violence

That's an inflammatory title, and yet in the context of geology it is quite literally true. The land in today's picture is indeed broken, and was the scene of unspeakable violence 760,000 years ago. If such an event were to recur, it's a fair bet that civilization could end up in danger of collapsing into unrecoverable chaos.

Yellowstone National Park gets a lot of attention for being a "supervolcano", and numerous TV documentaries fan the flames of concern about the possibility of a giant eruption in the midst of our country. It's quite true that a full scale explosion like those that have happened three times in the last 2.1 million years could wreak havoc on a wide scale, but Yellowstone hasn't actually had an eruption in 70,000 years.

The proper geological name we geologists use for a "supervolcano" is rhyolite caldera. This works a little better because these calderas are not volcanoes in the normal sense of the word. They are gigantic holes in the ground caused by the collapse of the crust after vast amounts of magma are blasted into the atmosphere. Some calderas were singular mountains at one time (like Mt. Mazama, which collapsed to form Crater Lake in Oregon). Others weren't. They were volcanic centers that included a collection of smaller cones and lava flows. The collapse of giant calderas seem to begin with smaller eruptions that grow in intensity over a time scale measured in decades or centuries. That's why Yellowstone caldera is of somewhat less concern to geologists (that's not to say that lesser eruptions aren't impossible). What is less known to many is that we have more than one recently active rhyolite caldera in the United States; we actually have three. I got a close look at the other two on my plane flight in December.

My flight took a more southerly route than past trips, and as we crossed the crest of the Sierra Nevada, I realized I was flying directly over the north end of the Owens Valley and the Volcanic Tablelands. The Tablelands are the southeastern flank of the Long Valley Caldera, California's version of a "supervolcano". And if anything, it's more dangerous than Yellowstone, if for no other reason than the fact that volcanic activity is ongoing, even though gigantic eruptions like the one that rocked the region 760,000 years ago are unlikely.

The caldera itself was obscured by clouds, but I had a perfect view of the Tablelands, a region adjacent to the caldera that was covered by 400 feet or more of volcanic ash that was so hot when it landed that it welded itself into solid rock. The pink colored rock is called rhyolite tuff. The tuff from this eruption is called the Bishop Tuff.

The scale of the eruption is hard to imagine. For perhaps a week, huge explosions blew ash into the stratosphere. Not a little bit of ash; it amounted to around 150 cubic miles of ash. The ash buried the local landscape under hundreds of feet of hot steaming rock, while some ash deposits have been found as far away as Kansas and Nebraska.

The collapse of the caldera was a stupendous event as well. The hole was around 20 miles long and 10 miles wide, and as much as 1-2 miles deep (much of the caldera was filled with ash during the eruption itself as the ash column collapsed downwards). After 760,000 years, it is still plainly visible in satellite imagery and topographic maps (below).

After the eruptions ceased, water started to fill the caldera, eventually forming a 1,000 foot deep lake. The lake ultimately breached the margin of the caldera and rapidly carved a gorge hundreds of feet deep across the western edge of the Tableland. The walls of the Owens River Gorge expose a fine cross-section of the rhyolite tuff, allowing geologists to work out the sequence of events during the eruption.

Map courtesy of GeoMapApp

The Tablelands provide a window into the tectonic environment of the region. By covering the slope like a thick blanket, the tuff provided a blank slate on which post-eruption faulting can be easily seen and analyzed. There are dozens of faults, mostly trending north or northwest, with scarps that face west (the sunlit terraces) and east (the shadows). Geoblogger Callan Bentley provided a marvelous grounds-eye view of these faults in this post on his old NOVA Geoblog (Callan is now blogging primarily at Mountain Beltway under the auspices of the American Geophysical Union; he's always posting something interesting).

There is a much more extensive example of the broken nature of this landscape. The prominent mountain range on the upper right of the picture below is the White Mountains. In any other setting this mountain range would be a national park, but lying east of the Sierra Nevada, it sort of loses out. It tops out at 14,242 feet, rising as much as two miles above the adjacent Owens Valley. It is a gigantic fault block that formed when the land that is now the Basin and Range province began stretching and collapsing, forming a series of horsts (mountain blocks) and grabens (faulted valleys). The mountains lie in the rain shadow of the Sierra and never formed glaciers of any great extent. The highest arid slopes play host to the most ancient life on planet Earth: the Bristlecone Pines. The oldest Bristlecone is 5,062 years old. The Whites are a fascinating (and lonely) place to visit.

The Airliner Chronicles is one of my on-again/off-again serial features, which is usually updated whenever I fly somewhere.