Accretionary Wedge #55: Blood on the Rocks, and Bad Karma on the Plateau

This month's Accretionary Wedge is hosted by Maitri at her Vatulblog, and it concerns our injuries incurred in the field: "Show me your injuries! ... You don’t have to be a geologist to play; simply having been injured by a rock or while doing something geological is sufficient. I will put the entries together and post them here as well as at the AW site at the beginning of March. The winner (one with the most spectacular wound or story) gets their choice of two bandage tins from this website mailed to them."

Actually, in twenty-five years of leading field trips, I have been remarkably (some would say eerily) free of serious injuries. Of course, by simply saying this I am bringing on the threat of geo-karma, and I will no doubt get bit or fall down a steep slope on my next trip. On the other hand, I have taken somewhere around 1,500 students on multi-day field trips, and some of them have managed to inflict a bit of self damage. Luckily, only a few required a trip to the emergency room, including a split-open chin, and a hairline fracture of a wrist caused when someone caught their fingers in a van door and wrenched an arm trying to pull them out.

The chin involved stitches after a fall on some rhyolite boulders in the Mojave Desert. From my point of view, it was my discovery of the accident site that was intriguing. I was trailing behind the rest of the crew in a deep cleft in the rocks, and I noticed the blood on the rocks (see the picture above). I stopped, looked around, looked up. In my ignorance, I assumed an owl had made short work of some rodent that night and was a sloppy eater. I continued on down the trail until some of the students came back and let me know we needed to make a stop in the emergency room in Needles, California.

The most famous injury during my time at the college was not quite so serious, a turned ankle, but it set in motion events later that day that have since become legend in our department. It involved rattlesnakes, and a suicidal deer (I originally blogged this story in 2008).

The rattlesnakes were simply an omen for the day. I and my students had set out on a cross-country trek to look for Ancestral Pueblo cliff-dwellings in eastern Utah. I stopped a moment and heard under my feet that buzzing sound that field workers don't usually like to hear: a rattlesnake. Only it seemed like it was in stereo. I looked down and saw not one, and not two, but three rattlers between my legs. I should have known right then to cancel the day and just go home, as I jumped through the air in some random and probably poorly chosen direction. No bites, but a very high heart rate for awhile....

Meanwhile, a deer had decided to cross the highway near Blanding and Monticello. That was still four hours in the future, though.

One of my students (who shall remain nameless for the moment) walked into the wilds, lacking water, companions, and common sense. He wandered off the mesa, climbing down a 4oo foot cliff, and climbing onto the adjacent mesa. He hadn't heard anyone for several hours and finally realized he was lost, and was long overdue at the vans. He tripped and fell, wrenching his ankle badly. Back in the parking area, concern was growing and search parties were organized. Finally, two hours later, as we prepared to contact the local Search and Rescue, our lost student stumbled out of the pinyon forest, having heard the honking of horns. We were now two hours behind schedule. We set out on the highway, headed to Blanding and Mesa Verde. And a few minutes after that, a deer, hidden completely by a concrete embankment, chose that moment to cross the busy highway.

We should have passed the embankment along the highway two hours previously, but our student  had been lost. And so we were selected, involuntarily, to introduce the deer to the hereafter. And the front of our van would never again be the same. We were, in fact, alive only because of the expert emergency driving skills of our professional volunteer, who managed not to turn us headlong into the big truck in the opposite lane as the collision occurred.

That's geo-karma...

A final note:

This wedge entry and others like it are a light-hearted look at what turned out to be fairly minor injuries. Such injuries have a potential to become major injuries in the isolation of the field, and major injuries have the possibility of becoming life-ending situations. There is no getting around the necessity of being as safe as possible when conducting field work. I can think of no situation worse than having a student die while under my supervision.  Someone passed away during a recent conference (from a long-term medical condition) we sponsored in an isolated mountain location. The staff at the conference center and our volunteers acted professionally and quickly, but nothing could be done. It was tragic beyond words.

If you are responsible for the students or fellow researchers, hold them to the highest standards of safety, and always have first aid kits available, and the training to use them. Don't allow a moment of fun taking a bit of a chance turn into tragedy.

Hunting for Fossils in the Sierra Nevada...Wait a Minute, What Fossils are there in Granite?

Photo by Mrs. Geotripper

Last night I attended a great presentation on the dinosaurs and other (more interesting) Mesozoic reptiles found in California. It was given by Dick Hilton, a former prof at my school, who is currently teaching at Sierra College in Rocklin. I was invited to head up to the Sierra Nevada to look for fossils with Dick and fellow prof Noah Hughes, and I jumped at the chance.

But...fossils? In the Sierra Nevada? Isn't the Sierra Nevada composed of granitic rock? Granite and other plutonic rocks develop from cooling magma deep in the Earth's crust, an environment that is neither conducive to life, nor to the preservation of fossils. A quick look at a geologic map reveals that the Sierra is only about three-quarters exposed granitic rock. Most of the remainder is composed of metamorphic slate and metavolcanic greenstone, with a fair amount of serpentinite (California's state rock).

Metamorphic rock is the product of taking pre-existing rock and subjecting it to extreme heat and pressure. The resulting rocks, with names like slate, phyllite, schist, marble and quartzite, often bear little resemblance to their previous form, their protoliths. Any fossils that might have been part of the original rock are often destroyed in the process. There is a rich record of tectonic events leading to the formation of the Western Metamorphic Belt, a story too complex to even summarize in a short blog post. Long story short, fossils shouldn't be found in the Sierra Nevada. The rocks have been too distorted and altered. For the most part...

The Mariposa formation is a deposit that formed on the bottom of a deep sea off the coast of California in Jurassic time. The shoreline lay east of where it is today, and the Sierra Nevada was a different place: a series of active volcanoes led to a coastal forest. Dinosaurs, pterosaurs, and primitive mammals roamed the forests and floodplains. In the sea, large swimming reptiles including plesiosaurs and ichthyosaurs were to be found. The Mariposa is somewhat less altered than many other rocks of the metamorphic belt, and because of this, a few fossils have been found, fossils that enabled geologist to figure out the age of the rocks, an important step in unraveling the complex geologic history of the region.

The Mariposa formation is exposed along Highway 49, and we were searching for fossils in the vicinity of Don Pedro Reservoir. To my great delight, after a bit of sweating and slipping down wet grassy slopes we found some interesting specimens.

The most common fossils include the bivalve Buchia, an important diagnostic fossil indicating a Jurassic age for the unit.

I almost missed a small belemnite, which looks a bit like a fossilized cigar. The cylindrical fossil is the internal shell of a squid-like creature.

The prize find for me on this day was a small ammonite. Ammonites are relatives to the pearly nautilus which lives in today's seas. They can be thought of as an octopi with a shell. I have a spotty history with ammonites in the sense that for the last quarter century, I haven't been able to find any, and it hasn't been for lack of trying. I once stood in front of a productive outcrop with a noted paleontologist, and I watched him walk up and pull an ammonite out of the cliff face. I've been back to that spot many times over the last twenty years, and I have yet to find another. But today, I flipped a rock over, and there it was, a little tiny ammonite.

I didn't make that best find of the day, though. That honor belonged to Noah, my fellow prof at Modesto Junior College. The sample below shows a sprig of a species of redwood tree. This is an astounding find to me. Redwoods survive today in just three places in the world, on the northern California coast, in the Sierra Nevada, and in a small grove in China. But the trees once ranged across the northern hemisphere, and as Noah's find shows, they have been around for more than 160 million years. Think of it this way: dinosaurs once roamed through forests of redwood trees. And here in the rock was a distant ancestor to the Sequoia trees that grow just a few miles up the hill from our fossil site.

We were reminded of dinosaurs one more time today as we were driving home. We passed a large herd of modern dinosaurs who were displaying the kind of herd behavior that we think the large plant-eating dinosaurs displayed during the Mesozoic Era. Luckily they didn't attack!

Dick Hilton wrote the guide about the history of Mesozoic reptiles in the California region, and the book remains the best source of info about mosasaurs, ichthyosaurs, plesiosaurs, and the handful of dinosaur species found in the state. You can get the book at http://www.ucpress.edu/book.php?isbn=9780520233157 or any other online seller. I give it my highest recommendation!

Cheating Death One More Time: Home from the Field

Sunset in Death Valley near Badwater

Hey, I'm back, after five days in the wilds with no wi-fi access, and no television. Did anything happen while I was gone? What about that asteroid that was making a close pass...did it cause any problems? I sure wouldn't want to be around if a big chunk of space rock actually entered the atmosphere...

Okay, I actually did hear about the Russian near miss, and because our media access was really limited, we first thought we were hearing that 400 people had DIED. It just goes to show how stories can get out of hand. What an extraordinary event however, and what a relief that no one actually was killed.

Meanwhile, I've had a delightful weekend doing what I love: teaching geology in the field, this time mostly in Death Valley National Park. There is such a difference between drawing lines on a chalkboard to represent rock layers, and standing in front of the real thing.

Our group included a fair number of geology majors, but most were new to the science, with only a few weeks of a lecture class completed before we tossed them, kicking and screaming, onto an outcrop of rock (actually, this was a stand-alone class, and all the students had chosen to be there).

To get our new students up to speed, we started in the Renaissance, reviewing the basic rules of stratigraphy as described by Nicolas Steno in the 1600s: superposition, original horizontality, and lateral continuity (I described these principles in detail last year, in this post). These principles allow us to start discerning the sequential story of these rocks. We did some basic rock identification so the students could start determining the ancient environments that were responsible for these sediments. Note the preponderance of reds and browns; these rocks were exposed to oxygen at the time of their formation and burial. The region was also subject to occasional volcanic activity, as shown by the white ash layers (which also provide a means of accurately dating the rock).

The sedimentary sequences were cut by a series of minor faults, providing us with an introduction to tectonic processes, as well as offering a fine example of cross-cutting relationships. Ultimately, the students were able to work out a reasonably complete sequence of events that produced the spectacular red cliffs.

Although we couldn't see any at this locality, we talked about the rich fossil record at this site, a group of animals that included early species of horses and camels, large predatory ancestors to wolves and bears, and large grazing species similar to rhinos.

All in all, Red Rock Canyon State Park in the Mojave Desert of California is a great place to introduce basic principles of geology. When we finished up here, we were ready to take on the much more complex landscape of Death Valley National Park.

The Answer to Sunday's Mystery Quiz...And Where I'm Headed Tomorrow

The answer is: Death Valley National Park!

I admit that I was trying to be a bit deceptive with my pictures. Many people have a stereotypical image of Death Valley that includes salt flats, sand dunes, and miners with their burros tramping over barren desert mountains. Those things are part of what Death Valley is, but there is so much more.

I mentioned in Sunday's post that Death Valley is possibly the most geological park in the entire National Park System. The reasons for saying this include what I pointed out before: "the park contains rocks from every major era, ranging in age from as much as 2.3 billion years ago, including perhaps the most complete Paleozoic sequence of rocks found anywhere (something like 20,000 feet of Paleozoic rocks, including formations from each period), rocks from the Mesozoic (including plutonic granitic rocks), and a sequence of early Cenozoic sediments known for their mammalian fossils. The youngest rocks in the park may be only a few hundred years old."

There are other superlatives that can be added: the lowest point in the western hemisphere, the hottest temperature ever recorded on planet Earth, the driest place in North America, and features like the mysterious sliding stones of Racetrack Playa, the strange "turtleback" faults and metamorphic core complexes, Lake Manly (the freshwater lake that filled Death Valley 600 feet deep during the ice ages), and four species of fish (!!) including possibly the most endangered vertebrate species on Earth, the Devils Hole Pupfish.

There's more: the Ubehebe Craters, classic maar volcanoes that may have exploded only a few centuries ago, the badlands topography of Zabriskie Point, the Badwater salt pan, evidence of previous Death Valley type grabens that formed 15-20 million years ago, Neogene tuff deposits recording a wave of rhyolite eruptions across the American West, and the iconic sand dune fields.

As I said before, the park is so extensive and intricate, one could explore it for years and always find new treasures. If you are curious, the top picture is a view towards northern Death Valley from the lower end of Titus Canyon. The road through Titus is an epic adventure where one drives not over, but through a mountain range. The second picture is from the deepest part of the canyon, where the walls rise 3,000 feet, providing a Grand Canyon kind of feeling.

The third picture is one of the unexpected pleasures of exploring Death Valley. In the newer part of the park on the western side of Panamint Valley there is a permanent spring system and a 20 foot high waterfall. Darwin Falls is only a mile of hiking, and is a pleasant contrast to the barren desert.

The last picture is Mosaic Canyon, a better known part of the park, but a place with strange secrets. The mosaics are breccias and conglomerates that have been plastered to the sides of the canyon, but structural geologists seek out the exquisite structures revealed in the yellowish Noonday Dolomite of latest Proterozoic age. Callan Bentley of Mountain Beltway recently posted some neat shots of the folds in the lower canyon.

So that's where I'm taking 25 or so students tomorrow. It's hard for me to think of another single park or region that offers so much geologic diversity in one place.

Of course, you may have a different opinion...is there a place in the world that has more geologic diversity than Death Valley? I would love to know what you think!

The Winged Messenger: Appearing Now on Your Western Horizon!

I saw something I've only seen once before in my life last night: the planet Mercury. It's not that hard to see it, but being so close to the sun, it must be at the highest point in its orbit to avoid being lost in the glare of twilight. Basically, I've missed it over the years from sheer laziness.

On the other hand, it's there. It doesn't look like much in a telescope, especially the kind of scopes I've had access to over the years. And satellites had only visited the planet once while I was growing up, and the satellite was only able to map a bit less than half the planet. It doesn't have showy rings or moons.

Even last night, I took only a few pictures of Mercury, but spent most of my time capturing the beautiful crescent moon (below). Mercury is basically the poor stepchild when compared to the other terrestrial planets of Venus (bathed in cloudy mystery), Earth, and Mars (we are leaving tire marks all over it right now, looking for signs of water and/or past life).

This is the joy of living in the times we are, though. We are in the midst of a voyage of discovery that humans can only do once in our history: to explore our Solar System in detail for the first time. Mercury actually is an interesting planet, and it has offered up a few surprises, like complicated surface structures, including large fault scarps, and a magnetic field. It even has a few wispy bits of an atmosphere and possible ice deposits in the polar regions. We are learning about these things because we've had a second satellite, Messenger, that has been orbiting the planet for the last two years (it arrived in 2008, but it took a complicated series of passes to establish an orbit in 2011).

http://www.nasa.gov/mission_pages/messenger/multimedia/detail_first114075114076.html

All our planets have interesting stories, and it's more meaningful if you have actually made the effort to see the planets with your own eyes. Steven Schimmrich at Hudson Valley Geologist has a nice post on how to see Mercury over the next week or so (the post is what got me off my couch to have a look). To learn more about NASA's Messenger mission, check out this website.

A Maybe Not So Hard Quiz...Where Are We?

We are in a national park...it is perhaps the most geological park in the entire National Park System. The park contains rocks from every major era, ranging in age from as much as 2.3 billion years ago, including perhaps the most complete Paleozoic sequence of rocks found anywhere (something like 20,000 feet of Paleozoic rocks, including formations from each period), rocks from the Mesozoic (including plutonic granitic rocks), and a sequence of early Cenozoic sediments known for their mammalian fossils. The youngest rocks in the park may be only a few hundred years old.

The park includes gorges 3,000 feet deep that cross mountain ranges, and a waterfall (I know "waterfall" in a national park might seem not unusual, but it is unusual in this park). The relief of the park (the difference between the highest and lowest points) is more than 11,000 feet.

The park is so extensive and intricate, one could explore it for years and continue to find new treasures. So where are we? And what is giving away the answer?

Time Beyond Imagining on the Colorado Plateau: You can imagine it too, on June 1-7, 2013

Antelope Canyon, on the Navajo Reservation near Page, Arizona

The Colorado Plateau, the region encompassing large parts of Arizona, Utah, New Mexico and Colorado, is one of the great geologic showplaces on planet Earth. Within the region, one can see in vivid colorful detail nearly two billion years of Earth history, from the ancient Proterozoic crust exposed at the bottom of the Grand Canyon to the Cenozoic lake sediments that formed the strange hoodoos of Bryce Canyon. The plateau country has been central to many of my blogs over the last five years, including three major series: Time Beyond Imagining, Vagabonding Across the 39th Parallel, and The Abandoned Lands. In addition to being a bountiful source of information about the past, it is one of the most beautiful landscapes in the world.

The Great Unconformity, the erosional boundary between the Proterozoic rocks of the Yavapai Orogeny and the Cambrian Tonto Group exposed in Diamond Creek on the Hualapai Reservation.

On June 1-7, 2013, I will be conducting a tour through the heart of the Colorado Plateau under the auspices of the American Association of Petroleum Geologists. The trip will begin and end in Las Vegas, Nevada, and will be a 1,000 mile loop through Grand Canyon National Park, Glen Canyon National Recreational Area, Grand Staircase-Escalante National Monument, Bryce Canyon National Park, and Zion National Park. The purpose of the journey is to provide an introduction to this fascinating landscape to anyone with an interest in geology, including geologists, teachers, students, and their family members. One does not need to be a member of the AAPG to participate. We will travel in rental vehicles (friendly drivers provided!), and stay in hotels at or near the parks. The fee, including all transportation costs during the trip, accommodations, tour fees, park entrance fees, and the trip guidebook, is $2,850 ($3,050 after 5/3/13). The fee doesn't include food, or travel to and from Las Vegas, where the trip will originate.

The Colorado River at Diamond Creek on the Hualapai Reservation

What will you see and learn? Our route will begin in Las Vegas. As we leave town, we will have a first look at the Colorado River at Hoover Dam, and then drive southeast on Highway 93 to Kingman Arizona. We will have a close look at the Peach Springs Tuff, remnants of a vast explosive eruption that blanketed thousands of square miles, providing some evidence of the origin of Grand Canyon.

From Kingman, we will head northeast on the longest remaining stretch of Route 66 to Peach Springs. At this point we expect to make our way down Diamond Creek, the only place where one can drive to the bottom of the Grand Canyon. We will have a close look at the Proterozoic and lower Paleozoic rocks of the canyon, formations not easily accessed in most parts of the Grand Canyon. If we are lucky, we may run across a herd of bighorn sheep.

We will then drive to the south rim of Grand Canyon, and spend a day exploring one of the most spectacular gorges in existence. Some free time will be available for a hike into the canyon, or for an optional canyon overflight. Relatively short (but steep) hikes from the rim provide access to the upper Paleozoic rocks of the plateau country, such as the Coconino Sandstone, Toroweap Formation, and Kaibab Limestone.

The following day we will work our way east to the canyon of the Little Colorado River and the Navajo Reservation. We will be exploring the Mesozoic formations of the plateau, including the Moenkopi, Chinle, Kayenta, Wingate and Navajo formations. Along the way we will stroll out to Horseshoe Bend on the Colorado River (below) and the incredible Antelope Canyon (the top photo), one of the most dramatic slot canyons to be found anywhere. We will spend a night in Page, Arizona, next to Lake Powell.

Horseshoe Bend on the Colorado River below Glen Canyon Dam.

From Page, we expect (weather conditions permitting) to follow Cottonwood Canyon to Grosvenor Arch and Kodachrome Basin State Park in Utah. The road follows the Cockscomb monocline, one of the major Laramide folds on the plateau (the southern extension of the fold forms the eastern edge of the Grand Canyon) through the heart of Grand Staircase-Escalante National Monument. The spectacular road exposes much of the Mesozoic stratigraphy found in the region, including the Tropic Shale, the Entrada Sandstone, and the Dakota Sandstone. Kodachrome Basin is a small gem of a park containing unusual sedimentary "pipes" that formed in the Entrada Sandstone.

The Cockscomb monocline near Kodachrome Basin State Park in Utah.

From Kodachrome Basin, we will climb through the Cretaceous sediments of the plateau country, including the Tropic Shale and Mesa Verde Group. We will arrive at Bryce Canyon National Park, which exposes one of the youngest formations on the Colorado Plateau, the Claron Formation. The hoodoos of Bryce are some of the most photogenic rocks to be seen anywhere. There will be time to hike below the rim for a completely different perspective on the unusual spires.

Wall Street Canyon in Bryce Canyon National Park

Leaving Bryce, we will head south along the Sevier fault and then turn west at Mt. Carmel Junction to drive into Zion National Park. Zion Canyon provides the best possible look at the incredible Navajo Sandstone, a Jurassic deposit that preserves the evidence of a vast sand sea that once covered a large part of the western United States.

Our route will take us on a little-traveled road through the western and northern part of the park to Lava Point. Along the way we will traverse a unique inverted stream, and pass through some rarely seen lava flows and cinder cones.

Leaving Zion, we will head southwest back to Las Vegas.

There are a host of other wonders along the route! I've been writing about this country for a long time, introducing you, my readers, to one of the most beautiful and geologically rich corners of our planet. We've traveled together in words and pictures, and I would love the opportunity to travel with some of you in person this summer. Join us!

Detailed information and registration forms can be accessed at http://www.aapg.org/education/fieldseminars/details.cfm?ID=76. I would be pleased to answer any questions you might have by emailing me at hayesg (at) mjc.edu.

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.

Living in the Shadowlands (for the time being)

"We live in the Shadowlands. The sun is always shining somewhere else. Round a bend in the road. Over the bough of a hill..."

C.S. Lewis (movie quote from "Shadowlands")

This quote from the movie Shadowlands is packed with religious meaning and metaphor concerning the necessity of suffering and the nature of the world we inhabit and the promised paradise to follow. It and the rest of the movie gives one a lot to think about. On the other hand I am neither a movie critic (I liked this one), nor a theologian, so I read the lines above and see instead a literal description of my home in the Great Valley of California. In my two decades in the valley, I have come to see the months from November to February as the shadow months, when a fog siege could come at any time, bringing a veil of darkness and limited visibility over everything for weeks at a time. Driving to work becomes a potential destruction derby. The constantly gray sky even effects the psyche, bringing about depression in many people.

What makes it worse is the knowledge that the sun is close by. It is shining on the cloud tops only a few hundred feet overhead, and on the slopes of the Sierra Nevada foothills only a few short miles to the east. Why is it that we have to spend all these weeks in the shadows and darkness?

To a climatologist, there is no mystery. The Great Valley is surrounded by mountain ranges, and unless a storm system is moving through, the air stagnates. In the cold months of winter, heat from the ground radiates into the atmosphere above, bring air temperatures at ground level below the dew point. Water vapor condenses into microscopic water droplets too small to sink to the ground. The landscape is hidden in ground-hugging clouds. The so-called Tule Fogs of the Great Valley are the stuff of legend. When I was growing up in Southern California, I lived in the delusion that fog was a sort of overcast thing. Things in the distance might be a bit indistinct, but you could see them. The fogs of the Great Valley cause visibility to drop to mere feet. Driving in such conditions is a crapshoot. Most people know to slow down, and even keep the windows down in the freezing weather so they can hear oncoming traffic. Unfortunately there are morons too, and deadly chain reaction accidents are disturbingly common.

Living in a place for decades gives one a perspective on changes, however gradual. It's dawned on me recently that I haven't experienced as many week-long fog sieges like those that plagued my first few years in Modesto. A recent study supports the perception that the fog has not been as pervasive as it once was. The number of foggy days in some areas has decreased by a third (20% versus 30%) in the last thirty years.  Much of the change is no doubt related to global warming, as warmer overnight temperatures mean that the dew point is reached less often. The growing population of the valley no doubt has an effect, since cities give off a great deal of heat. One more factor is the decrease in particulate air pollution, since the water droplets need a bit of dust (condensation nuclei) to coalesce around. Stagnant air basins are a great place for accumulating air pollution, but we have made great efforts to decrease the particulate levels over the decades (we are still the worst in the nation, but progress is progress).

While the lower number of foggy days and cold nights is a welcome change for commuters, it is a problem for many farmers. Many of the fruit trees grown in our valley require a series of cold nights in winter to increase production of fruits in the spring. The lack of cold days is having an effect on crop yields.

As dark and depressing as the fog can be, there are times when it provides a bit of beauty. Driving to work the other day was a shadowy journey through the pasture-lands, but the layers of fog were only a few feet off the ground, giving rise to treetops that seemed to be floating in a gray ocean. The sun peaked out over the distant Sierra Crest, and soon the fog was receding and it was the start of a beautiful sunny day.