The Airliner Chronicles: Wrinkles Tell a Lot About a Person (and the Earth, too)

My grandfather had wrinkles. Lots of them. He was a man who spent a large part of his life working outdoors in the days before sunscreen lotions. You could see in his face the rigors of the Great Depression and the tough choices that came with trying to farm in Oklahoma and Texas in the Dust Bowl years.

In the same way, the Earth tells a story through the pattern of wrinkles on the surface of the planet. Some rocks are softer, others are harder, and some are in horizontal layers while others are tilted. Some aren't in layers at all, such as plutonic intrusive rocks, so the type of rock exposed is fairly uniform, unless it has been broken up by jointing. Water and other agents of erosion attack the rocks in a differential manner, and reveal something of the underlying structure in the way that streams cross the landscape (the most commonly recognized drainage patterns are shown in the diagram below).

The top left frame should be labeled "Dendritic", and the top right "Parallel". From NASA (http://daac.gsfc.nasa.gov/geomorphology/GEO_4/GEO_CHAPTER_4.shtml)

In the case of my last airline trip, it was a quick way of determining what part of California's Coast Ranges I was flying over. The Coast Ranges are about as schizophrenic as can be: crossed by the iconic San Andreas fault and dozens of others, numerous tracts of rock are juxtaposed against each other. These rock groupings, called terranes, include rocks related to the Sierra Nevada batholith (the Salinian Block), the Franciscan Complex (rocks formed within a trench-subduction zone during Mesozoic time), the Great Valley Group (rocks that formed in a forearc basin between the trench and the Sierra Nevada volcanic arc), and the Coast Ranges Ophiolite (rocks derived from the oceanic crust of the Pacific/Farallon plates and the underlying mantle).

The first photo above doesn't reveal a lot of rock, as most of the surfaces are covered by grass and brush. If you concentrate on any one drainage, you will notice that many small tributary channels gather into larger and larger channels in the way that a tree has multitudes of branches that end up at a single trunk. This is an example of dendritic ("tree-like") drainage pattern, which indicates that the underlying rock is fairly uniform. There is no evidence of sedimentary layering, and thus the rocks must be part of the Salinian Block, the Coast Range Ophiolite, or the Franciscan Complex (layers are found over short distances in the latter two, but not enough to effect the overall drainage pattern). Of course, this part of the Coast Ranges is right in my backyard, so I knew I was seeing mainly the ophiolite found in the upper part of Del Puerto Canyon above the town of Patterson.

On the other hand, look at the picture above, from the easternmost part of the Coast Ranges (the town of Patterson is the light colored area in the upper right hand side). The shadows on the right half of the picture run parallel to each other, but at right angles to the eastern margin of the mountain range. Streams that would be expected to run from left to right down the slope of the mountains are deflected into northwest trending strips. Clearly some of the rock here is more resistant to erosion, and the pattern suggests sedimentary layers that have been turned on edge. This is the hallmark of the Great Valley Group. These rocks were laid down in a shallow sea that subsided and collected more sediments and subsided under the weight of the overlying sediments again, over and over. Eventually the sediments reached a thickness of 20,000-25,000 feet. Later deformation turned this incredibly thick sequence on end forming a vast homocline, indicated by the trellis drainage pattern.

I saw one other really distinctive drainage pattern on my trip, but far removed from these first two photos. If you look at the photo below, you might pick out that many of the smaller channels are dendritic, but that the larger channels are roughly parallel to each other. This is called a parallel drainage pattern, caused by streams eroding uniform, but slightly sloping rocks. I was lost at the moment because we had been flying over clouds for several hours, but the moment I saw these channels, I knew exactly where we were: on the flanks of the Valles Caldera (sometimes called the Jemez Caldera). There is no other place in New Mexico (and indeed most of the southwest) quite like it.

The caldera is a vast volcanic feature which formed when a gigantic eruption about 1.15 million years ago coated the entire region with around 70 cubic miles of volcanic ash. The volume of volcanic ash blown out of the volcanic vent was so great that the center of the volcano collapsed inwards, forming the caldera. Popular media outlets call these calderas "supervolcanoes" (Yellowstone being the most famous example). The NASA image below gives a wider perspective of the volcano. The circular mounds in the central area are volcanic cones that erupted years later. From this perspective (and distance), the drainage could be considered more of a radial pattern.

Image courtesy of NASA (http://earthobservatory.nasa.gov/IOTD/view.php?id=50666)

The Airliner Chronicles is one of my on-again/off-again serial features, which is usually updated whenever I fly somewhere. These pics were from a flight to and from St. Louis during the Christmas break.

The Abandoned Lands...A Journey Through the Colorado Plateau: Calderas and Inside-Out Caverns

Our journey through the "Abandoned Lands" of the Colorado continued over the flank of one of the larger geological features of New Mexico, the Valles (or Jemez) Caldera, a huge volcanic edifice whose summit collapsed during a catastrophic rhyolite ash eruption about 1.15 million years ago. The volcanic center produced the incredible scenery that we observed at Kasha-Katuwe Tent Rocks National Monument, and the setting for the strange ruins we saw at Bandelier National Monument. It was a highly unusual sight for us after a week on the road in the southwest in that it was also lush and green. I guess it helps that we were at 9,000 feet, so that even in a drought year there had been a bit more precipitation.

The highway skirts the outer edge of the twelve-mile wide caldera so there is no one place that one can see the full scale of the feature. The meadows in the picture above are only 2 miles across.We'll have to depend on radar imagery to get a full sense of the shape of things (below).

Image of the Valles Caldera courtesy of the New Mexico Bureau of Geology and Mineral Resources.

The eruptive activity of the caldera coated thousands of square miles with ash, and being an eruption that was hundreds of times larger than St. Helens, destroyed all life over a vast region. At first the  caldera basin contained a lake. Eventually the edge of the caldera was breached, the lake drained, and Jemez Creek eroded the deep gorge of San Diego Canyon. Numerous other smaller canyons have been carved into the margins of the volcano as well, including Frijoles Creek which we explored in Bandelier.

It has been around 60,000 years since the last volcanic activity at the caldera, but magma still simmers in the heart of the volcano. Heat flow under the caldera is high, and hot springs can be found around the margins. We found one of them while driving down San Diego Canyon, something so out of place that I hit the brakes and came to an unexpected and unannounced stop (to the chagrin of the five vans following me...). It has to do with the "inside-out" cavern of today's post title. We had happened upon Soda Dam, an outstanding example of a travertine bridge.

Caves usually form because limestone layers are dissolved as acidic groundwater moves through fractures and crevices. The familiar speleothems (cavern features) like stalactites, stalagmites and flowstone result later on when carbonate-rich water drips into the cavern and a bit of evaporation causes the calcium carbonate (called travertine) to precipitate.  In this case, hot acidic water has certainly dissolved limestone, but faulting forced the hot carbonate-rich water to the surface at springs. Instead of filling caves with features, the features form on the surface of the Earth instead. An inside-out cave.

The travertine built up around the springs and the deposits ultimately built up a dam-like feature that crossed the canyon floor, forming a natural bridge. The travertine bridge has numerous cave-like features, including flowstone and rimstone pools. I suspect it once had stalactites and stalagmites in the small cave openings, but the site is too accessible from the highway for such features to survive the inevitable vandalism.

The hot springs have been active for nearly a million years, as evidenced by travertine deposits high on the canyon walls. Other deposits nearly the valley floor are around 60,000 years old, giving an idea of how rapidly San Diego Canyon was carved. The Soda Dam itself is about 7,000 years old.

The springs flow at a small fraction of their original volume. The dam was breached during highway construction so that the main flow of the springs no longer reaches the travertine bridge, other than a small trickle. The travertine is disintegrating, both naturally and from vandalism.

Jemez Creek forms a nice waterfall at the Soda Dam, and a sumptuous-looking swimming hole can be found at the base.

A nice source on Soda Dam and the many other geologic features of northern New Mexico can be found in The Geology of Northern New Mexico's Parks, Monuments, and Public Lands from the New Mexico Bureau of Geology and Mineral Resources.

The Abandoned Lands...A Journey Through the Colorado Plateau: Migrations, Calderas, and the Middle of the Story at Bandelier

My ancestors migrated. My great-grandfather's family was struggling to survive in the hill country of Kentucky, and with eight hungry children it wasn't easy. In the early 1900s they heard of farmlands and jobs in the Indian Territories north of Texas, the land that would soon be known as Oklahoma. They arrived, and although life was still hard, the family thrived.

After a generation, things began to change. There were a series of drought years, and the Great Depression added to a hard existence. There were the horrific dust storms, and in 1936 a tornado destroyed my grandfather's house, nearly killing him. But they hung on. They found jobs here and there, and always kept up a vegetable garden in their spare hours. They canned food when they could, and sold fresh vegetables off their wagon in town.

World War II was well underway when the family made a momentous decision. Relatives in California said that things were better out there, and that they should move west. The family sold most of their things, loaded up their car, drove through New Mexico, and got as far as Kingman, Arizona before their transmission failed. The repairs took six days and nearly all their money, but they labored on, traveling at night through the Mojave Desert and arriving the next morning in the Central Valley. We're still here several generations later.

Reading about the journeys of the Ancestral Puebloans made me think of my own family history. How intolerable did things have to become to cause a person, a family or a village to give up and move on? Something happened on the Colorado Plateau that caused entire societies to abandon the region. Many of the displaced groups headed southeast, presumably because of stories of more secure water supplies along with relatively unpopulated but arable lands near the Rio Grande River.

I've been telling the story backwards. I've had a couple of posts about the pueblos at Acoma, Pecos and Taos where the Ancestral Puebloans ended up and are thriving today. Today's post is about the middle of their journey, a place that was a population center for several centuries, but which was ultimately abandoned as the inhabitants moved down into the Rio Grande Valley. It is Frijoles Creek on the flanks of the Jemez Caldera which is preserved as part of Bandelier National Monument.

Bandelier has one of the stranger geologic settings of any archaeological park. The walls of the canyon are full of holes, not unlike a giant chunk of swiss cheese. We had seen a place like this on the first morning of our trip, at Hole in the Wall in the Mojave Desert. It resulted from differential erosion of a thick layer of rhyolite tuff. Here at Bandelier, the origin is the same, only from rocks much younger.

It was less than 1.2 million years ago that a vast eruption blanketed the region with a thick layer of hot ash, more than a hundred cubic miles worth. The source of this incredible explosion was a vent to the northwest. The erosion was so huge that the crust collapsed into a vast hole 12 miles across. The Jemez (or Valles) Caldera, as it is called, is one of the three active supervolcanoes in the United States. Yellowstone is the best known, with the California's Long Valley Caldera at Mammoth Lakes being the other. The ash at Bandelier is soft, but readily forms cliffs, so the inhabitants of Frijoles Canyon took the holes and enlarged many of them into rooms. Other rooms were built out from the cliffs, reinforced by logs embedded in the ash.

People have lived in Frijoles Canyon off and on for several thousand years, but the majority of the structures and villages were constructed after 1150 AD, when the Colorado Plateau was beginning to be depopulated. The villages at Bandelier were abandoned by the middle of the 1500s as the Puebloans moved closer to the Rio Grande River. Their descendants live on in the present-day pueblos.

The canyon is a fascinating place to explore. The large village of Tyuonyi is easily accessed from the visitor center. A huge excavated kiva can be viewed next to the village. The most spectacular ruins are those which are scattered along the base of the cliffs for a long distance up the canyon. A trail winds up and through the ruins, and ladders provide access to some of the cavern openings.

The trail provides a look at details of the rhyolite tuff as well. In shaded corners, plants have gained a roothold...

 ...including this tree that I can barely believe was alive.

 If you suffer from acrophobia, I will definitely not recommend the trail to the Alcove House (below).

It is 140 feet above the canyon, and is reached by a series of ladders. If you don't mind the exposure, the payoff is pretty amazing. The Alcove House was a dwelling with 22 rooms and a kiva, which has been reconstructed. The alcove provides a marvelous view up the canyon (check out the first picture in this post).

Bandelier was ravaged by two disastrous fires in 2000 and 2011. The most recent fire burned 60% of the park. Although the visitor center area did not burn, the area upstream did, and the slopes have lost the vegetation that would prevent flashfloods from developing. The floods have wreaked havoc on  the park, with trails and bridges being repeatedly washed out, while parking and picnic areas have been inundated. Private cars are currently being parked just outside the park and shuttles provide access. The park has a campground on the plateau above the canyon (including group sites).

I've written about Bandelier a few times before when I was working on "Time Beyond Imagining". Check out this post, and this one.

Monday Geo-picture: Bandelier National Monument

Monday geo-pictures (for busy geo-bloggers; what a great idea, Georneys)...here is another scene from the southwest, this time at Bandelier National Monument. The monument preserves ancient pueblo cliff dwellings and villages which are fascinating in and of themselves, but the scene is made even more interesting because of the geological setting. Bandelier sits on the flank of the gigantic Jemez (Valles) caldera, which covered the region with red-hot ash around 1.15 million years ago. The pueblo people found the ash cliffs easier to excavate than sandstone, and numerous holes pockmark the cliffs representing the rear rooms of the dwellings.

The kiva in this photo required a 140 foot climb to reach (mostly on ladders). It takes some imagination to understand the effort undertaken to live in such an inaccessible place.

The Yellowstone Media Storm: We're all gonna die! Oh, well sure, but...

Is Yellowstone gonna blow? Sure. Will everybody die? Sure, absolutely. But there is pretty much no connection between the first question and the second. Yellowstone caldera will in fact erupt again some day; it's a forty mile wide caldera with a huge magma chamber miles deep in the crust. That's the reason there are geysers there, and all those hot springs. And everyone will die, eventually. That's kind of a rule about living. But worrying about whether I'm gonna die from an eruption at Yellowstone is so far down my list of concerns that I am more worried about being gnawed to death by a pack of angry prairie dogs. It could happen, but it is highly unlikely.
I'm writing on the subject because a commenter asked for my take on the news this week about the activity around the Yellowstone caldera (for the record, "supervolcano" isn't a geological term). My best advice, before I finish with my own two cent's worth, is to check out the Eruptions blog here and here. Erik Klemetti does some excellent analysis of the story and how quickly things can be overblown by a media that thrives almost exclusively on spectacle, and has little use for reasoned discussions of the actual risks.

As I considered the question of an 'informed' response, I had to wonder if I'm not guilty of the same sort of exaggeration and sensationalism of cable news and other media. In my pursuit of educating others, I am not above discussing lurid tales of death and geological mayhem. After all, when I talk about volcanoes in class, I lead with Krakatoa, Tambora and Pompei, and I discuss the potential effects of a gigantic rhyolite caldera eruption such as those that took place at Yellowstone three times in the last two million years (not to mention broadly equivalent eruptions at Valles Caldera and Long Valley Caldera). I have a clear conscience on the matter, because I follow up with a discussion of the actual risk and probability of such events happening to our time; an eruption like the Yellowstone Caldera happens somewhere in the world every few hundred thousand years. Smaller, but certainly dangerous eruptions like those of Tambora (1815), Krakatoa (1883) and Katmai (1912) happen somewhere in the world on a scale of decades. And when these events take place there are clear signals that something is about to happen.

So, the Yellowstone Caldera floor rose around a foot during the last decade (that's the caldera in the picture above; I was standing on one rim, the mountains in the far distance form the other rim). There was an intrusion, a sill, that was filling and inflating with magma. There is nothing unusual or particularly alarming about this, as Yellowstone is an active magma chamber, and such events are normal. Although it has been 70,000 years since the last eruption in the park (and more than 600,000 years since the so-called 'supervolcano' eruption), it could happen again, but any activity would be accompanied by many other phenomena that would provide plenty of information about the scale of the eruption and the hazard to people in the region. My take? Learn about Yellowstone; it is a fascinating place, worthy of our attention and worthy of protection as a crown jewel in our national park system. But worry about the end of civilization as we know it in a vast explosion of fire and volcanic ash? No, there are other problems that we need to think about.

The best quote on the matter? From Max Read at Gawker: "But who do you trust, some kind of "professor," or your overactive imagination?" To which I might add: "But what about professors with overactive imaginations?"

Time Beyond Imagining - Where did the Ancient Enemy Go?

Bandelier National Monument on the flanks of the Jemez (Valles) Caldera is a fascinating place to observe Pleistocene geology, being the site of a massive rhyolite eruption only 1.15 million years ago (back to geologic time instead of human time; so much for yesterday's post on deep time...). Subsequent erosion carved canyons into the slopes of the volcanic edifice that later provided arable environments when humans arrived in the region within the last 10,000 years. Their construction of these dwellings in Frijoles Canyon provides a piece to an interesting puzzle.

Part of the mystique of the Colorado Plateau has to do with the disappearance of the Anasazi People ("Ancient Enemy" in the Navajo language. I know that name is out of vogue, but bear with me on this). A number of reasons have been advanced to explain why essentially the entire region was depopulated about the end of the 1200's. They point to a disastrous 25-year drought, soil and resource depletion, religious influences, invasions and warfare. For years it was standard procedure to promote this deep mystery of "where did all the people go, and why?" I haven't read the book in years, but even Louis L'Amour got in on it with Haunted Mesa, where I guess they disappeared into some different dimension or something (I didn't think it was one of his better efforts...).

Not being an anthropologist, I am perfectly satisfied with the idea of the stress of a drought as the root cause of the migrations out of the region. Making a living on the plateau has always been a dicey proposition, and it still is today. Big cities like Phoenix, Salt Lake City, Albuquerque and Las Vegas flank the plateau country, but few towns thrive in the midst: Flagstaff, Moab, Farmington, Cortez, and a few others, totaling a few hundred thousand people at most. Water is a limiting factor, and even today with all the technology we have for pulling water up and out of rivers and from underground, most towns on the plateau barely have enough. Water for use by the 4 million yearly tourists at Grand Canyon is captured at a spring on Bright Angel Creek deep in the canyon, and pumped up 5,000 feet to both rims.

There is, in fact, a tremendous amount of water available on the Colorado Plateau. The Colorado River offers something like 10-15 million acre-feet each year, even in the dry ones. But where is the river? Across most of the plateau, the Colorado flows through deep canyons, far from any kind of arable lands. Today, to make use of the water, we've had to construct giant monstrosities of dams and reservoirs to capture the water, and a huge network of canals and pipelines to distribute the water to where we have chosen for it to go. That technology wasn't available 700 years ago.

If you were a community of people living on the edge, depending on the yearly rains that simply weren't coming, the choice boils down to migrating elsewhere or starving. A prolonged drought would have serious consequences: crops would fail, hunger would be widespread, soil erosion would commence, and in an such an environment, one could understand that a fight for survival might mean a fight with others for limited resources. Religious beliefs would certainly play a part, since the gods would be presumed to be angry, and they would need to be placated to bring back the rain.

As to the "where" part of the question, that is somewhat less of a mystery. They moved to where dependable water supplies could be found. For some communities, these were the mesas that nowadays form the Hopi Reservation. Some of the towns there have been occupied continuously since the 1100's. Others headed south and east into the Rio Grande River country where two dozen different puebloan groups continue to live today. The Acoma and Taos pueblos have been continuously occupied for around 1,000 years, and the village at Bandelier was constructed about 1150 AD.

The term "Anasazi", mentioned before as meaning "ancient enemies" by the Navajo, has no precise meaning. The haunts of the people now called "Ancestral Puebloans" on the Colorado Plateau had been abandoned by the time the Navajo people arrived in the region.

Does the story of the Ancestral Pueblo people on the Colorado Plateau have any relevance to our society today? Ya think? The southwestern part of the United States is in the midst of a historically unprecedented drought that is probably influenced in large part by global warming. There are serious concerns that Lake Powell and Lake Mead could conceivably go dry in a decade. The effects are being felt at Bandelier as well, although ecologists don't see the effects as all bad. Just the same, millions of people depend on the Colorado River. If the modeling is correct, we have some serious choices to make concerning water use in this arid region.

The first picture of the day shows Frijoles Canyon in Bandelier with the rim of the Valles Caldera on the horizon. The ruins in the main canyon include the unique cliff houses (second picture) carved directly out of the softer parts of the rhyolite (the linear pattern of holes is where timbers were inserted to hold up floors and roofs). Harder chunks of rhyolite were used to make the bricks that form the buildings in the village called Tyuonyi on the main nature trail at the monument (third picture).

School begins tomorrow! If blogs become a bit more irregular, you'll know why...

Time Almost Not Beyond Imagining: Recent Volcanism on the Colorado Plateau and Deep Time

1.15 million years ago, a massive explosion shook the New Mexico landscape, and some sixty cubic miles of ash shot into the atmosphere in a catastrophic rhyolite caldera eruption. The collapse of the magma chamber produced the Valles (Jemez) Caldera. Some of the ash traveled as far east as Iowa, but most of it was dumped on the flanks of the volcano in layers hundreds of feet thick. It was so hot upon landing that the material fused back together into a solid light-colored rock called welded tuff. Smaller eruptions followed, the most recent around 50,000 years ago. Erosion has been eating away at the margins of the caldera, forming a number of deep canyons (see this map of the caldera). One of these canyons is called Frijoles, the "Canyon of the Beans".

Tucked away in Frijoles Canyon one of the more interesting national monuments in the park system: Bandelier. It preserves the remains of an Ancestral Pueblo village that was built nearby and sometimes right into the welded tuff generated by the Jemez Caldera eruption. It is a fascinating place to explore. Although humans had passed through the region for portions of the previous 10,000 years, the village was built around 1150 AD and abandoned about 1600 AD.

A couple of divergent thoughts crossed my mind when I was considering a post on Bandelier National Monument. They had to do with deep time, and the wanderings of people. Wanderings takes up the next post. Here, I wanted to provide a bit of perspective about geologic time.

A thousand years seems a long time to the casual tourist wandering through these very ancient dwellings. We can feel a connection to these people sometimes as we look at the drawings they made on the rock, or the personal improvements in the living spaces in the rock rooms of the village built into the cliff. And yet they seem far removed by time. The year 1600 seems impossibly distant to our daily lives.

Still, there were (and actually still are) people in my life who were personally acquainted with Civil War veterans. And those veterans would have known people whose lives reached into the 1700's. Two or three degrees of separation...it isn't that far back is it? We can perhaps visualize that kind of time if we work at it.

But 1.15 million years? Even though we have actually covered 2 billion years in this series on the Colorado Plateau, I sometimes find even a million years difficult to comprehend. One million. One thousand thousand. The entire history of Bandelier's village, from construction to abandonment to tourist destination could be repeated 1,000 times to reach the time when the volcano exploded. And beyond that: the history would have to be repeated 65,000 times to reach the time when the dinosaurs disappeared, and more than 200,000 to reach the time when dinosaurs first appeared. And they appeared relatively late in Earth's history.

Deep time is one of the four great discoveries that mark the biggest advances of my science, that of geology: the others were the principles of uniformitarianism, evolution, and plate tectonics. The first two are largely credited to one man, James Hutton, and evolution is credited to Alfred Wallace and Charles Darwin, although others were involved. Plate tectonics was the discovery of dozens, even hundreds of researchers, although Alfred Wegener is given credit for kick-starting the idea though sheer stubbornness (I guess this should give the Alfreds of the world some hope!).

Bandelier National Monument is a fascinating and scenic place to wander. The top photo is of a kiva set in a high alcove in the upper canyon. Getting to it requires an acrophobic climb up some high ladders and narrow trails. The rocks of the canyon walls are full of holes caused by cavernous weathering of the relatively soft rock. Along the canyon floor, the Puebloans scraped and scooped out the rock to make their living spaces larger and more comfortable. I will have some more photos of the ruins in my next post. Camping and other facilities (mostly built by the CCC in the 1930's) are available.

Time Almost Beyond Imagining: Who Do the Magic that Hoodoo?

Sorry for the title. Sometimes such things are irresistable, and geology terms sometimes seem to lend themselves to parody: subduction, orogeny, bedrock, schist and gneiss. Just the same, today's post is about one of the stranger places in the Colorado Plateau country: Kasha-Katuwe Tent Rocks National Monument on the flanks of the Valles Caldera, site of the previous post on the latest part of the geological story of the Colorado Plateau. It's got a lot of unique scenery packed in a small area.

The Valles Caldera is one of the nation's active supervolcanoes, and over a period of more than six million years, the eruptions of the center have blanketed the region with rhyolite tuff and ash. If you look at a physiographic map of the caldera you can see that erosion has sliced into the flanks of the volcanic complex, exposing evidence of older eruptions. The scenery at Kasha-Katuwe is the result of this geologically recent erosion of the soft volcanic rocks.

Some of the layers on the flanks of the caldera contain river and mudflow deposits with boulders and cobbles of all sizes. The larger boulders protect the softer underlying rocks from erosion until the boulders stand at the top of tall pillars (see the third picture). These are hoodoos. Eventually the boulders topple, and erosion attacks the underlying rock, producing the strange conical spires that give the monument it's name (the English part anyway; Kasha-Katuwe refers to "white cliffs" in the Keresan tongue). The erosion of the soft rock by flash floods in this arid environment have also led to the formation narrow steep-walled slot canyons (the fourth photo).

The monument is a bit tricky to find; the park brochure and location map can be found here. The New Mexico Bureau of Geology and Mineral Resources has a nice review of the geology of the region. If you ever pass through the Sante Fe-Albuquerque region on a field trip, check it out!

Time Almost Not Beyond Imagining: Recent Volcanism on the Colorado Plateau Pt. 3

Top picture of the Valle Grande by Garry Hayes. Bottom image of the Valles Caldera courtesy of the New Mexico Bureau of Geology and Mineral Resources.

I will probably get a bit of geographical argument about today's geological feature, as it might not really be considered part of the Colorado Plateau, and more a part of the Rio Grande Rift, but it is on the plateau side of the rift, and on my field trips to the plateau, it is an important stop. So there you go. I've been moving towards wrapping up my long running series of posts on geological history of the Colorado Plateau, and we are into the last few million years of plateau history. At 6 million years, the final parts of the Colorado River system were established and the main carving of the Grand Canyon had begun. Volcanism began in the Flagstaff region, with numerous cinder cones, lava domes, and a stratovolcano forming the San Francisco Peaks Volcanic Field. Further to the east, the crust was stretching and breaking the crust to form the deep trough of the Rio Grande Rift. Like the peaks near Flagstaff, the faulting of the rift provided an avenue for magma to reach the surface. Unlike the Flagstaff area, the volcanism manifested itself in a totally different and violent way. This is the Valles (or Jemez) Caldera of New Mexico.

Valle Grande in the midst of the caldera is a beautiful serene high altitude prairie surrounded by and containing numerous forested lava domes reaching elevations as high as 11,000 feet. The grasslands fill a crater-like depression 12 miles in diameter. The peaceful scene belies a catastrophic origin.

As America's first national park, Yellowstone gets a lot of press, historically for all the geysers and bison and stuff, but more recently because of documentaries about "supervolcanoes" on various science-related cable channels. I'm sure the film-makers would never stoop to broadcasting sensational stories about impending eruptions that would destroy life across a broad swath of the continent, but somehow people have gotten that impression. Just the same, Yellowstone in modern times has developed a reputation as a spectacular example of an active rhyolite caldera. What is not so widely recognized is that the continental United States has at least three active caldera systems: Yellowstone, Long Valley in California (Mammoth Lakes), and the Valles Caldera in New Mexico. And a major eruption from any one of them would be a true catastrophe on par with a asteroid impact. The good news is that such eruptions are rare, with hundreds of thousands of years of volcanic quiescence between major events.

Eruptions have been taking place in the vicinity of Valles Caldera for six million years. A major caldera formed in an eruption about 1.5 million years ago (the Toledo caldera), and then the eruption of the Valles Caldera destroyed the older caldera and formed the present depression about 1.15 million years ago. It is hard to imagine the scale of such events. The last eruption in the lower 48 states, that of Mt. St. Helens in 1980, produced about a quarter of a cubic mile of ash and lava. It killed four dozen people and devastated 150 square miles. One of the most famous eruptions in modern history, at Krakatoa in 1883, was the equivalent of 20 St. Helens eruptions, producing 5 cubic miles of ash. The eruption of the Valles Caldera produced just more than 60 cubic miles: the equivalent of 250 St. Helens. With so much material blown out from the magma chamber, the ground above could not remain stable, and it collapsed into the void. Ash from the eruption covered a multi state region; near the volcano it was so hot that as it landed it hardened into a solid rock, welded tuff.

In the millennia that followed, magma refilled the chamber and the center of the caldera rose several thousand feet. Smaller eruptions produced a number of lava domes, the most recent around 50-60,000 years ago, but the volcano is not dead. Magma still simmers deep in the crust under the caldera. Valles is now considered an ideal of example of a resurgent caldera.

During World War II, physicists first caused a nuclear chain reaction in Chicago but prudence dictated designing the atomic bomb in an isolated location. They chose the flank of Valles Caldera, and the Los Alamos National Laboratory was built on a potentially active volcano. Research continues there today, although the level of secrecy is somewhat less than it was; you can drive through town without being challenged by armed guards, although you may get irretrievably lost if you make a wrong turn on the confusing streets.

The caldera was a private ranch, but in recent years was given to the federal government and is now managed as a national preserve, which is kind of like a national park, but is supposed to be economically self-sustaining. Two other geologically significant parks are found on the flanks of the caldera: Bandelier National Monument, and Kasha-Katuwe Tent Rocks National Monument. They will be covered in subsequent posts.