Sunday, November 19, 2017

An Iconic Bit of the Calaveras Fault in Hollister is Gone (But it will be back)

Corner of Locust and Central Avenues in 2016
I go on field studies trips year after year, and my camera is always around my neck, to the amusement of my students. They sometimes wonder why I would take pictures of things I already have dozens of pictures already. I sometimes wonder the same thing when I am trying to track down a specific shot from my archives of tens of thousands of pictures. But for a teaching geologist, there is a very good reason:

Things change.

Sometimes it is sudden, like a flood in Yosemite or on the Tuolumne River that actually alters the look of a landscape. It's nice to be able to catalog before-and-after views of a place. But in others, it is because of the incremental geologic changes. That's especially true with a couple of faults in Central California, the San Andreas and the Calaveras.
Corner of Locust and Central Avenues in 2013
Yesterday I updated the spot on Highway 25 near Pinnacles National Park where the San Andreas fault crosses the road (and I deeply appreciate the widespread response, especially on Twitter). The spot visibly changes every year. For decades, geologists have also been tracking the creeping of the Calaveras Fault in downtown Hollister. Generations of geology class field trips have walked several city blocks, tracking the fault as it offsets streets, sidewalks, curbs...and houses! I hope that all who do so remember to stay on sidewalks and not become nuisances to the residents. They have enough to contend with when you think about it.
The corner of Locust and Central Avenues in 2001

One curb has been iconic; it's been an illustration in any number of textbooks and PowerPoint presentations. The corner of Locust and Central Avenues is offset by the Calaveras Fault adjacent to the crosswalk, so it can be observed easily without bothering residents. It's one of the most vivid examples of right lateral offset imaginable, and if one knows the age of the sidewalk, the changes can be used to calculate the yearly rate of movement on the fault. The break is not a perfect measure because the deformation is spread out for several yards on both sides of the break (note how the sidewalk is curved in the pictures above).

So you can imagine my surprise on Saturday to find that the iconic curb disappeared sometime last year. It was for a good reason, as the city put in a wheelchair ramp, but it was still a shock. I was disappointed for a moment for my students until I realized that for the first time in 15 years, we have a brand new baseline of fault movement. Because as surely as the curb was offset before, it will continue into the future. We'll be watching for the first of the tensional cracks in the concrete, eventually to be followed by total rupture and offset curbs.
Corner of Locust and Central in 2017
Geology never stops.

Saturday, November 18, 2017

Why did the Road Cross the San Andreas Fault? 15 Years of Geologic Change (an Update)

I've been leading geology field studies trips to lots of places in the American West for 29 years and started to take digital pictures in 2001. I sometimes struggle to find new things to photograph when I visit a place for the 29th time, but in some cases it is not a problem. There are geologic changes that happen on a yearly basis, and with fifteen years of photos, the changes become obvious. This is a continuing update from a post in 2013, and I'll probably continue updating for the foreseeable future.
Highway 25 in the California Coast Ranges connects the town of Hollister with the access road to Pinnacles National Park (formerly Pinnacles National Monument). Along the way the highway crosses the San Andreas fault in a section where the fault creeps an inch or so each year (36°35'54.27"N, 121°11'40.19"W). Most years we've stopped to have a look at the effect the movement has on the pavement. In 2002 and 2004, the damage was obvious.
By 2008 someone had patched the road, and no fault motion was evident.
Little damage was evident in 2009 either. But by 2010 cracks had begun to appear as the fault stressed the pavement.
The fact that the fault creeps in this region is a good thing. It means that stress is not building along the fault surface, but instead is being released gradually. The sections of the fault to the north and south of the creeping section are locked by friction, and are building up the ominous stress that will eventually produce quakes with magnitudes in the range of 7.5 to 8.0. The quakes are coming and we need to be as prepared as possible.
By 2012, the road had been completely repaved, and  yet the shearing was already evident.
It became even more pronounced by 2013 and in 2014. Just by chance, the person working as a scale was the same individual as in 2004.

In 2015 the fractures were moderately larger. They'll need to start thinking of road repairs before long.
In 2016 Laura once again provided scale, as she did in 2014, and 2004.
Here in 2017, long-time trip volunteer Mary provides scale. The cracks in the road are just a bit larger, but they didn't look appreciably different than the previous year except for a twist (pun intended)...
The paint on the road was obviously offset, a lot more than last year. Even more interesting is the despite the offset, the paint was deformed (twisted), not split.
The offset paint strip reminds me of illustrations of elastic rebound theory, the idea that stress builds up on a fault line over time. In that model, the land on either side of the fault is distorted over time until the frictional resistance is overcome and the rock snaps back to its original shape. That won't be happening with the paint, but if they don't repair the road (as they often do; see above), it will probably show a clear break by next year.

The other "twist" is that a magnitude 4.6 earthquake took place last week less than two miles from the road crossing. It probably did little to change things on the road exposure, but serves as a reminder of the need to not be complacent about the existence of this fault in our state.

These little changes that happen at a rate visible in human lifetimes add up to huge changes when multiplied by thousands or millions of years. The nearby eroded volcano of Pinnacles National Park has been displaced 195 miles (315 kilometers) in the last 20 million years or so by movement along the San Andreas.
Until next year!

Thursday, November 16, 2017

Aerial Geology: A High Altitude Tour of North America's Spectacular Volcanoes, Canyons, Glaciers, Lakes, Craters, and Peaks. A Book Review...

A most interesting book landed on my desk recently. Timber Press asked me if I was interested in reviewing Mary Caperton Morton's book Aerial Geology, A High Altitude Tour of North America's Spectacular Volcanoes, Canyons, Glaciers, Lakes, Craters, and Peaks. I am most pleased to do so, but a bit of explanation is necessary to explain why.

If you have been following my blog from early on (the very ancient days of January 2008), you will know that I am thrilled by plane rides, and the unique perspective that aerial photographs add to one's understanding of geology. I didn't get to fly much as a child and teen, but I lived in southern California, and I was surrounded by high mountains: the San Gabriel and San Bernardino Mountains were only a short drive away, the Sierra Nevada and the Mojave Desert just a little farther. My favorite weekends were those spent climbing mountains that gave me a view. If I was stuck at home, I climbed to the very top of a deodar tree in my backyard that gave me a look at the surrounding neighborhood (I thought as a child that it must have been 50 feet high, it was more like 20...). When I took my first geology class in 1975, I was introduced to the perfect book that would feed my mania for high places: Geology Illustrated by John Shelton, published in 1966. Ostensibly it was a basic physical geology text, but it was illustrated with a stunning set of aerial photographs taken by the author. His book, despite being out of print for decades, is considered one of the 100 most influential books in science for the last century by American Scientist. A great many of the photos were of places I was familiar with, and so I treasured the book. My copy is barely holding together, but I still refer to it once in a while.

So why am I talking about one book when I am supposedly reviewing another? It's because some things have changed in fifty years. In 1966, we had barely been to space, much less seen the Earth in high quality photographs. The iconic Blue Planet photograph, taken by the astronauts of Apollo 17 was still six years in the future. A furious debate was taking place in the geological community about the strange concepts of continental drift and plate tectonics. It was only just beginning to be acknowledged by some geologists, but full acceptance was still a half decade away. Our knowledge base on all manner of geological processes has expanded astronomically. Our access to scientific knowledge has left the dusty halls of libraries and landed in our pockets through the internet and smart phones (this statement is not to be taken as a slam on libraries, which still provide so much good in our communities). And perhaps above all, our use and abuse of the resources of our planet has threatened to upset the delicate web that keeps our civilization intact: global warming, soil depletion, groundwater use, and so many other issues are affecting our daily lives. Just ask the people of Houston, Puerto Rico, California's Wine Country, and so many others who have been affected by intense geological and climatological processes.
Mississippi River Delta, a NASA image from Aerial Geology

There is still a vast dearth of scientific literacy in our society, especially when it comes to geologic processes. The internet can do much good, but it also is a source of bad information and stupid conspiracies. Wouldn't it be nice if there was an accessible and attractive book that could lay out the basics of geological processes accompanied by spectacular views that take in advances in satellite technology as well as beautiful old-school aerial photographs?

Yes it would, and that's why I can highly recommend Aerial Geology. Mary Morton has selected 100 significant sites and locales in North America that illustrate geological processes in action. Many of the sites chosen are familiar and expected locales, but there are also quite a few that are less known. Each site has a well-written vignette that outlines the basic geology and occasionally related environmental problems. The pages for the Mississippi Delta (above), for instance, describe the very serious problem of subsidence: the delta is disappearing into the Gulf of Mexico at the rate of a football field per hour.
NASA image of Craters of the Moon in Idaho, from Aerial Geology
People who fly often may find their interested in the world below rejuvenated by reading this book. A helpful feature is a notation on each vignette that explains what commercial flights may reveal the feature in question (even when I'm paying attention, I can get lost at 35,000 feet and not know what spot I'm looking at).

Aerial Geology is well-written and beautifully illustrated, and is a worthy successor to Shelton's Geology Illustrated. I enjoyed learning about some new places, and finding out new things and new angles on many familiar places as well. The author is a frequent contributor to Earth Magazine, and has an excellent blog on geological topics as well. The book is available at the usual places, including the publishers website and Amazon (and it is surprisingly inexpensive if you are looking for a cool Christmas gift). Check it out!

Disclosure: I was provided with a review copy of the book.

Sunday, November 12, 2017

A Landscape as Bizarre as They Come: The Volcanic Tableland of the Eastern Sierra Nevada

The Volcanic Tableland, with the White Mountains beyond.
There is a bizarre landscape on the far side of the Sierra Nevada between Bishop and Mammoth Lakes. It's not one of stark beauty exactly, it's barren and covered by little more than sagebrush. It's got few roads or trails, primarily because very little of this landscape is of much use to anybody. From above, the surface is riddled with scarps and grabens from numerous faults. This is a broken-up land. It's not...normal.
Source: US Geological Survey
The surface of this landscape isn't "right" either. There are no dark rich soils here. The underlying rock is pink or white, and so is the weathered soil and debris that covers it. Although the surface has an area of several hundred square miles or more, the underlying rock is remarkably uniform. It is a volcanic rock called rhyolite tuff. And with that name, the explanation for this landscape is revealed: it is the remnant of an ancient disaster beyond imagining.
767,000 years ago, an explosion took about 125 cubic miles of pasty magma from the crust and blew it into the atmosphere. The huge void collapsed inwards, forming an oval-shaped depression 20 miles long, 10 miles wide, and a mile or more in depth. The resulting ash spread far and wide, blanketing the western United States. Measureable deposits can be found in Kansas and Nebraska. But most of the ash came straight down. Some of it refilled the caldera, but much of the remainder buried the regional landscape hundreds of feet deep in hot ash. All life would have been extinguished for miles in every direction.

It is difficult to understand the magnitude of such events. From a human perspective, we have nothing to compare it to. An eruption at Tambora in Indonesia in 1815 produced less than a tenth of the ash as Long Valley, and that was enough to cause global cooling with related summer snowfall, crop failures, and famine across the northern hemisphere. The effects of an eruption the size of Long Valley on modern civilization would be appalling. I've heard it said that modern agricultural production has a month-long lead on consumption demand (No, I can't cite a source. It's a factoid I'm sure I heard or read somewhere). Try to imagine a disruption of agricultural production lasting several years. Governmental and societal structures would collapse, and the death toll would be unimaginable. Humans would no doubt survive, but it would be a dystopian landscape as bad as any sci-fi action movie, and maybe worse.

The only good thing that I can think of to say on this possibility is that studies of calderas like Long Valley or Yellowstone suggest that the eruptions will be predictable on a scale of decades or centuries. There would be time to prepare the eruption, or, however unlikely, geo-engineer the caldera to lessen the intensity and effect of the cataclysm.

What happens when a singular event completely reshapes a landscape? The eruption of Long Valley completely disrupted the drainage patterns of the eastern Sierra Nevada and Owens Valley. The land had to start over. Where there had once been river valleys and canyons, there was now a gaping pit miles wide and long. The evidence suggests that for 600,000 years the caldera depression contained a huge lake similar in plan if not in scale to Crater Lake. Crater Lake has no outlet, with the lake level determined by evaporation and seepage. The Long Valley Lake would have been similar, as no evidence exists for an outlet, at least until around 150,000 years ago.
Source: U.S. Geological Survey

For 600,000 years sediment washed into the basin, slowly filling it. Finally, along the south rim of the caldera near the present day site of Crowley Lake, the basin spilled over. In just 150,000-160,000 years, the Owens River carved a 400-500 foot deep gorge down the surface of the Volcanic Tableland, laying bare the full extent and history of the climactic eruption of the caldera. The rate averages out to about a foot every 400 years, but the rate was probably higher at the beginning. The small creek that flows through the gorge today is a mere shadow of its former self. The Los Angeles Department of Water and Power has co-opted most of the water, but a court order several years ago mandated that a minimum flow must be maintained.
Because the LADWP has utilized the water from the Mono Lake Basin to the north, they have constructed penstocks and pipelines that allow them to produce energy as they transport water down the long slope of the Tablelands. That means access roads, and it is thus easy to visit the gorge, and it is fascinating.
The mounds seen occasionally along the rim are the eroded remnants of fumaroles, where steam would have emanated from the interior of the ashflow, leaving mineral deposits that toughened the rock. The tuff at the rim is relatively soft, but as one walks deeper into the gorge, the rock becomes harder (it tuffons up?). When the hot ash landed, it was hot enough to remelt, forming welded tuff, or ignimbrite. Pieces of pumice caught up in the eruption became flattened and smeared in places.
As the rock cooled, it contracted to form columnar joints. Unlike Devils Postpile, a few miles away on the other side of the crest of the Sierra Nevada, these columns are not vertical. Most columnar jointing is not. The intense fracturing of the rock into these columns aided the Owens River in the carving of the gorge, as the crumbling rock could be quarried by the rushing water much more readily than solid rock.

The columns average six sides, but columns with 4, 5, or 7 sides are occasionally seen.
The Volcanic Tableland and the Owens Gorge are otherworldly, but they provide a hint of how quickly landscapes can adjust to new geological conditions. It took 600,000 years to fill a basin 10 by 20 miles with at least 2,000 feet of sediment, and 150,000 years to carve a 400 foot deep gorge. This is fast by geologic standards, but humans, had they been around this region at the time, would not have noticed much change in course of their lifetimes. Just like we aren't noticing the changes now...

Wednesday, November 8, 2017

What? Why Would I Ever Want to Give Up my Parking Spot in Yosemite Valley?

 "Why?", indeed. I have to admit that I get just a little tiny bit elitist at times, at least in my approach to visiting my favorite national park, Yosemite (program note: my "favorite national park" is likely to change on a nearly daily basis, depending on whichever one I happen to have visited most recently). I tend to emphasize the spots far from the madding crowd, and wow, does Yosemite get crowds. Last year included a huge spike in visitation, to five million people (the park has only exceeded four million twice). Lines at the entrance stations, lines at the parking areas, gridlock on the valley roads. If one arrives at the wrong time of year and the wrong time of day, one may be quite stuck and not see very much.
So what to say to the hapless traveler who waited for an hour or two at the entrance station, and circled the parking lots waiting for someone to leave, got into a shouting match with the person who tried to grab their spot, and finally has a chance to see things? Obviously, I am saying "go see more"! It's not enough to just see the rock cliffs from the bottom. Go to Glacier Point and get an entirely different perspective on things. I have a feeling that someone like that will not be willing to give up their coveted spot, and their time for visiting may be short anyway. And Glacier Point has a pretty good chance of being socked in as far as parking, too. The main lesson? Don't visit during the height of summer.
Still, there is nothing quite like seeing Yosemite Valley from above, especially from a cliff that is a sheer 3,200 foot drop straight down. It's dizzying, and exhilarating, and vertigo-inducing (that's about the same as dizzying, but only a thesaurus can describe a view like this). If you ever have the slightest chance of getting up there, go for it without hesitation (and while up there, don't miss Washburn Point or Taft Point).

My memories of Glacier Point go back quite a ways. I was here with my family around 1966 or 1967 when we watched the Firefall from the valley floor. This used to happen as the workers for the Curry Company pushed a bonfire over the brink.The embers glowed bright as they fell for several thousand feet, and it became a nightly tradition. A day or two later we were on the rim at Glacier Point where I could watch them push the fire off the cliff. It was interesting, but over the years the spectacle caused traffic congestion in the valley, and was not in keeping with the natural environment. The practice was ended in 1968. In an interesting twist, a fire fall happened naturally in the valley in 2008. A marvelous picture was captured by Edith Howe-Byrne, a photographer who spends part of the year in the valley. Check it out here.
Glacier Point is perched on a cliff-top at the upper end of Yosemite Valley across from Half Dome. The view includes Yosemite Falls and the valley floor to the north (above), the glacial U-shaped valley of Tenaya Creek at the base of Half Dome (below), and the alpine country to the east.
The Overhanging Rock at Glacier Point has been an iconic picture spot for a century or more. I can hardly estimate how many people have stood on, danced on, done handstands on, or pretended to be falling off of. It's outside the fencing, and thus technically off-limits, but without a ranger present people can't resist the tempting of fate. To be sure, I've stood on many a precarious rock in my day, but I have a serious problem with people who do so in such a crowded place. There's a certain peer-pressure (peer-comfort?) in seeing someone else standing in such a place leading to the thought, "if they can do it, I can do it too". Eventually you get someone who unexpectedly gets vertigo or faints. I don't know how many have fallen, but there have been a few. I cringed the other day as first one person went out, and then others climbed the fence and got their pictures too.
I much prefer letting the rock be part of the scenery, a nice bit of foreground for shots of the distant cliffs. I admit to taking great pains to make sure there are no signs of walls or fencing or people in my shots from the point. I like to imagine I'm the only person perched on this solitary cliff so far and distant from the hubbub below (and all around me).
The day had been unexpectedly cloudy, and the rocks across the valley lost a bit of definition in the gloom, but shortly before we decided to leave, the sun broke through and the granite began to glow. The contrast of shadow and rock was striking.
This might be one of my favorite shots of Half Dome, with both the foreground and background bathed in shadow.

Glacier Point is accessed by Glacier Point Road, which is about a 22 mile drive from the valley floor (allow an hour to get there). The road is paved, but narrow and windy, and filled with the most dangerous animal in the park, the scared flatland driver. There is a large parking lot, pit toilets, and a modest curio/snack shop open seasonally. The walk to the view point is perhaps a quarter mile, maybe less, and is fully paved, and partly accessible for wheelchairs (mainly the upper viewpoint). A number of trails start from the point as well.

Sunday, November 5, 2017

Looking Down on Yosemite: A Return to Taft Point

I don't know how it happened, but somehow 12 years slipped by since I last made the hike out to Taft Point above the floor of Yosemite Valley. It's not that tough of a hike, only 1.1 miles with just a bit of climbing on the way back. But what a payoff at the end. There is nothing quite like standing on the brink of the ultimate abyss.
The trail begins at a parking lot about two miles up the road from Glacier Point. Parking is limited and fills quickly, so be sure to arrive early in the day, or you'll be walking a lot farther from your parking spot. Or you can do what I did and wait until about the last day of the season before a major snowstorm, and come at the end of the day when the sun is quickly sinking below the horizon.

The beginning of the trail in the deep Red Fir forest gives few hints of the grandeur that lies beyond. It almost immediately passes an unusual outcrop of almost pure quartz (remember y'all, it's a national park; no collecting!). The trail plunges into the forest and crosses Sentinel Creek, which a short distance downstream falls over the brink of Sentinel Falls, more than 1,000 feet high.
The trail breaks into the open, and one can sense the edge of nothingness that lies beyond. And then there's a distraction! There are huge open cracks near the cliff edge that split the rock for hundreds of feet. These are called the Taft Fissures, and they are a bit of evidence for the geology that led to the formation Yosemite Valley itself. The fissures follow distinct cracks in the granitic rock that makes up the cliffs in this area.

One of the Taft Fissures. The "layers" are exfoliation fractures, a process related to jointing.
Granite forms miles below the surface as molten magma within the crust cools slowly, forming a coarsely crystalline rock. The rocks are under tremendous pressure from the overlying rock, but over time the rocks are uplifted and erosion strips away the "overburden". As the rocks approach the surface, they expand in volume, but being solid, they crack to form joints. On the scale of Yosemite Valley, the joints were points of weakness that could be exploited by flowing glaciers to carve and cut away at the cliffs. Many of the sheer cliffs of Yosemite are joint surfaces. Unfortunately, many rockfalls begin with joints that give way along the vertical cliffs.
At Taft Fissures, the joints allowed water and ice to get into the interior of the rocks, causing the minerals on the surface of the rock to crumble to quartz gravel and clay dust. As the cracks widened, boulders would occasionally fall in and become wedged inside, as seen in the picture above.
Just beyond the fissures, the world ends...the cliff edge is the beginning of a sheer 3,000 foot drop to the valley floor below. The perspective is both stunning and dizzying. It's hard to believe, but the viewpoint makes El Capitan look almost...small. It's scattered in with so many other steep cliffs.
The fast-setting sun illuminated the cliff of El Capitan and the scar of the September rockfall from the cliff at Horsetail Fall. The first view in the picture above is not actually Taft Point. The point is on the upper left. I would have to head up there to get the entire view.

There are four stunning viewpoints within a short walk of Glacier Point Road. Washburn Point, seen in the last post, emphasizes the wilderness lands upstream of Yosemite Valley (and a side view of Half Dome). Glacier Point provides a look straight down into the upper end of the valley, extending from Half Dome and Tenaya Creek Canyon to the area around Yosemite Falls. Sentinel Dome provides a 360 degree panorama of the entire region, but without the dizzying look straight down sheer cliffs. It is accessed from the same trailhead as Taft Point.

Taft Point picks up the view of the valley floor at Yosemite Falls and provides a panorama of the western parts of the Yosemite Valley, taking in the Three Brothers, El Capitan, and the Cathedral Rocks which soar above Bridalveil Falls (which are hidden on the far side). The sweeping view also takes in the western slope of the Sierra Nevada, leading down to the Mother Lode foothills and the Great Valley beyond.
Although the rockfall at Horsetail Falls in September has garnered all the news these last few weeks, the view from Taft Point takes in a perspective on the largest historic slide in the valley, which happened in 1987. An estimated 600,000 cubic meters of rock came down the cliff of Middle Brother (on the right side of the picture above). Luckily, no one was hurt as the Park Service had shut down the road when loud sounds of fracturing rock were reported coming from the cliffs above. The September rockfall, in contrast, was a bit over 10,000 cubic meters.

The cliff of El Capitan and the sheer cliffs of the Cathedral Rocks form a constriction in the western part of Yosemite Valley. Sometimes called the "Gateway", the rocks stand out because they are relatively unjointed, and thus were more resistant to the cutting and quarrying action of the glacial ice. The barrier even extends underground. Beneath the cliffs, the sediments are only about 300 feet deep. Farther up the valley, the sediments are around 2,000 feet deep. The glaciers at times had to flow up and over the sill of granite.
Taft Point is an amazing place. If you have a fear of heights, this will be a good place to either cure it forever, or amplify the condition to the point that you will be a whimpering mass of a nervous wreck. I don't mind looking over the edges of cliffs myself, although it will sometimes be on my belly. One never knows when one might have a fainting spell, after all. But I cannot watch others standing on cliff edges, especially if they are my students. I tell them to do their antics on their own time.

The sun hit the horizon and the sky exploded into flaming orange and pink. I quickly headed up the trail and back to the car so Mrs. Geotripper and I could have a nice dinner back down on the valley floor. It didn't matter that it was dark. The moon was up and we could still see the cliffs above, including a constellation of lights from at least fourteen climbing parties on El Capitan.