Thursday, April 17, 2014

One of the World's Most Precious Places, Under the Volcano

Yosemite Valley, hands down, is one of the most extraordinary places on our amazing planet. I have been going to Yosemite National Park three or four times a year for the last quarter century, and I never get tired of spending time there. The thousand square miles of national park that surround Yosemite Valley are incredible, but the valley itself is hypnotic. I would hope that everyone could visit the park at least once, but it becomes something special when you can see it throughout the seasons, in all the different moods of the place.
The mood in the park on Saturday was expectant. The snowmelt has been filling the rivers a little (the drought continues unabated), and the first hints of green are showing up in the meadows and oak woodlands on the valley floor. Snow still lingers in the high country. The Dogwoods are just hinting at the possibility of a bloom. Changes will be coming fast in the next few weeks (and the long dry spell of summer will begin soon; much sooner than we have hoped).
The valley is a showcase for learning about glacial features and glacial erosion, although several aspects of valley scenery are not anything like typical. But if the subject is hanging valleys, Yosemite has no peer.

Big glaciers carve deeper valleys than little glaciers. When the massive Merced River glacier joined the Tenaya Creek glacier (with its spillover of additional ice from the Tuolumne Meadows icecap), the combined force of the two ice rivers produced the deep trough of the main valley, 3,000 feet deep (even deeper if the sediments filling the valley floor are removed). The tributary glaciers in Yosemite and Bridalveil Creeks were much smaller and couldn't cut nearly as deep. Their valley floors were left hanging high above the main valley, and today high waterfalls spill over the edges. Bridalveil Fall (in the pictures above and below) is 620 feet high, nearly four times the height of Niagara Falls, but it's one of the smaller waterfalls in Yosemite Valley.
Yosemite Falls is usually described as the highest waterfall in Yosemite Valley, but that depends on which geographer one is arguing with. It has three sections, an upper fall with a drop of 1,425 feet, a cascading central section, and a final sheer drop of 300 feet. It may the fifth or the seventh highest water in the world, but if one is talking about essentially unbroken drops, it's not even the tallest waterfall in Yosemite Valley. That honor goes to Ribbon Fall, shown in the photo below. It drops 1,612 feet, nearly 200 feet more than the upper section of Yosemite Falls. I imagine some first-time visitors mistake it for Yosemite Falls as it is seen first during the drive into the valley. Most people never see it at all though, because it dries up by late spring in most years.
I started up the valley trail from Bridalveil Falls, and ended up with a new view I have not seen before. The trail winds along the base of the Cathedral Rocks, with a sheer precipice of thousands of feet. Being at the base of such high cliffs is awe-inspiring.
Half Dome gets all the attention, but North Dome is beautiful in its symmetry as well. It stands across from Half Dome on the other side of Tenaya Canyon. And it's a whole dome!
And then there is Yosemite Falls. It never fails to amaze me with its stunning drop of nearly half a mile, and it becomes even more amazing when one realizes that it is misplaced in a manner of speaking. Can you see the cleft in the shadows to the left of the waterfall? The cleft provides a route for the trail that climbs up to the top of the waterfall and nearby Yosemite Point.

The dark cleft used to be the path of Yosemite Creek! The falls used to be an inconsequential side canyon but the glacier coming south from the high country pushed up a moraine, a pile of glacial debris, and blocked the normal channel of the canyon. The stream's new pathway took it over the brink of the cliff.
I mentioned the term "under the volcano" in the title because when you stand in the bottom of Yosemite Valley, you are within the frozen magma chambers of a series of volcanoes that once existed here, just five miles or so above. There are eight or nine individual intrusions that make up the valley walls and different susceptibility to erosion has caused a series of reentrants and coves along with the bold battlements of cliffs like the Cathedral Rocks or El Capitan. Many glacial valleys have long monotonous walls that aren't nearly as appealing as a place like Yosemite.

Yosemite is indeed one of the world's most precious places, and I am forever appreciative of living nearby, and being able to share it with you. Enjoy!

Tuesday, April 15, 2014

Meanwhile, in the Skies Tonight...

Lunar eclipses are always interesting, and they can be shared by most of a planet, unlike solar eclipses that follow a narrow strip of land across the globe. I was a little frustrated tonight because high cloudiness affected the view of the unfolding eclipse, but I did what I could.
I was reminded of one of the greatest teaching moments I've ever had, among them having an earthquake take place while teaching about earthquakes. During one of those moments, I was watching the students intently taking a test on quakes when the room shook. Barely anyone even looked up and I told them to look at the swaying monitors on the ceiling and then told them they all failed for not getting under their desks. The ultimate surprise quiz.

But the truly best night was an earth science class a decade or so ago. I was introducing the course and commenting on the shape of the Earth and the students answered in various versions of round, circular, or spherical. I argued that we were on the back of the turtle. They laughed, but I then asked any of them to prove it wasn't.

Dead silence....

Someone said we have pictures from space. I responded that we had pictures of the Death Star and the Enterprise too.

More silence...

So I was able to take the whole class outdoors and point at the ongoing lunar eclipse and ask them what shape the Earth was casting across the surface of the Moon. None of them said turtle. They were impressed (for once!).
In any case, here are my somewhat clouded shots of tonight's eclipse. Other skywatchers
in areas with clear skies will no doubt have finer shots, but what can I say? It was certainly more fun than the taxes I was working on...



It's way too late to catch the other side, so here is totality, and I'm calling it a night...

Monday, April 14, 2014

The Yosemite No One Sees in Summer...the Merced River Canyon

The drought in California is horrific. It is quite probably the worst drought in centuries, but we received a slight respite in the form of showers and snow during the last part of March and early April. It was a drop in the bucket towards relieving the huge water deficit that has built up in the last few years, but it gave a shot of energy to the seedlings of grasses and wildflowers. They perhaps should have sprouted and grown months ago, and they will be dried out in a few short weeks, but this week, the Sierra Nevada foothills were alive with color.
We were on our way to Yosemite Valley for our geology field studies course last Saturday, and I joked (only in half-jest) that the students had to "earn" the right to learn the geology of Yosemite Valley by first exploring the canyon of the Merced River. We had to prepare ourselves by undertaking a journey through the rocks that predate the granitic rocks that form the walls of the iconic valley. Our route took us through the foothill towns of Snelling and Hornitos, and then we drove up Highway 140 through Mariposa, over Midpines Summit and down into the deep gorge of the Merced River (originally the "Rio de Nuestra SeƱora de la Merced (River of Our Lady of Mercy)".
Highway 140 winds back and forth through a series of metamorphic terranes, bands of deformed and twisted rocks that traveled across the Pacific Ocean to be mashed into the western edge of the North American Continent. The rocks include slate, schist, marble, phyllite, quartzite and greenstone. These were the ancient rocks (between 600 and 200 million years old) that were intruded by the granitic rocks that eventually were exposed and eroded to form the dramatic cliffs of Yosemite Valley.

The Sierra Nevada is a huge westward tilted block. The slope has allowed the rivers draining the mountain range to carve deep gorges, some deeper than the Grand Canyon (Kings Canyon to the south is about 8,000 feet deep!). The depth of the Merced is more like 2,000 to 3,000 feet deep downstream of Yosemite, and in any other setting in most any other state, this would be enough for the establishment of national parks and the like. Instead, it is "just" the preliminary canyon one must pass through before arriving at Yosemite.

The carving of such deep canyons can have serious geologic consequences for society. The slopes of such canyons are inherently unstable, and mass wasting (landsliding) is a constant hazard. The building of roads and railways adds to the instability by undercutting the already steep slopes. Such is the case with Highway 140. CalTrans has struggled for years to prevent the collapse of a slope near the village of El Portal. In 2006, the slope flicked away the mitigation efforts and collapsed onto 600 feet of highway. The Ferguson Slide, as it is called, closed the highway for months and caused economic hardship for the surrounding communities. Two temporary bridges were constructed to cross the river and circumvent the huge slump. Decisions have yet to be reached regarding the permanent disposition of the highway.
The Ferguson Slide. 600 feet of Highway 140 is buried under the rocks.

As we drove deeper into the Merced River Canyon, the slopes seemed to come alive in orange and gold. The California Poppies have reached the zenith of their blooming cycle. In contrast to my previous gloom and doom post regarding fires, rejuvenation does indeed occur, as many of these slopes have been cleared of brush and chaparral by repeated wildfires, allowing for dense concentrations of wildflowers.

We passed El Portal, and then the Yosemite View Lodge. The name is not totally dishonest; the boundary of the national park does indeed lie a few yards east of the hotel complex, so the view of the granite slope beyond is indeed a view of Yosemite. But it is not yet the iconic valley with Half Dome and El Capitan and Yosemite Falls. It is pretty, though, and the Merced River makes a dramatic dash through the maze of gigantic fallen boulders.
 We arrived at the park entrance station, drove a few more miles, and made our last stop before Yosemite Valley. It was another site of mass wasting, but this time it was a rockfall, and it included granitic rock instead of metamorphic material. The so-called Cookie Slide took place in 1982, and involved around 100,000 cubic yards of material that bounced and fell down the slope onto Highway 140. Once again, the road was blocked for weeks.
 The size of some of the fallen blocks is astonishing. It's hard to see how a block like the one in the picture below held together during the downhill descent.
The rocks are part of an intrusive series called the El Capitan granite, which was intruded about 103 million years ago. A contemporaneous intrusion of dioritic rock lead to some co-mingling of magmas, including the enclave of darker rock in the granite (below).
I've always thought of granite as a particularly beautiful rock. The sparkling mass of quartz (glassy gray color), orthoclase feldspar (white grains), and biotite mica is pleasing to the eye, and tells an odd story: Yellowstone National Park, off in Wyoming, sits on top of a huge magma chamber that may one day explode again (but not tomorrow). In Yosemite, we are sitting within a former magma chamber, under the volcano. As such we are exploring the inner depths of an extensive magma system, brought to light by long periods of uplift and erosion.

In a coming post, we'll see how Yosemite looked on Saturday.

Sunday, April 13, 2014

You Don't Know What You've Got 'Til It's Gone: They Burned Paradise...

My God...

Nothing beautiful today; nothing inspiring...

Twenty years ago I paid my first visit to the destruction wrought by the eruption of Mount St. Helens. I stood on a high viewpoint on Windy Ridge after a long drive through downed trees and destruction and looked at a vast area and found it hard to believe that one explosion had so completely devastated a region. The ash blast from St. Helens destroyed an area of 230 square miles (600 square kilometers).

402 square miles (1,040 square kilometers). Nearly twice the area of St. Helen's destruction. That's what the Rim Fire did to my home forest, Stanislaus, and Yosemite National Park. I saw it for the first time today, and it devastated me.

I've been to Yosemite perhaps 80 times in the quarter century that I have been teaching at Modesto Junior College. I grew to love the entire region surrounding the park, and some valleys and streams became as familiar to me as my own backyard.

And now it's changed forever, if forever is the time remaining in my life. All because some clueless moron lit an illegal fire to cook his lunch in the forest, despite a crippling drought, extremely dry conditions, and a restriction on all fires. Are people really this stupid?

I know the whole argument that arose after three-quarters of Yellowstone National Park burned up decades ago. Fire is natural, fire is good, fire leads to rejuvenation of the ecosystem, all those things that are no doubt true. But that doesn't relieve the awful feeling that so much that was good was destroyed all at once, and there really wasn't a damn thing we could do about it.

Global warming and climate change will no doubt lead to many changes in the forest above my home. The new forest will be quite different from the one that passed away into nothingness last fall. Probably more drought-tolerant shrubs and fewer trees. 

The areas we traversed on our drive home still contained standing trees, as if there were some hope that life survived in the trunk somehow. But no, the needles were brown because even though the trees were not burned entirely, their cambium layer within the trunk was boiled away and the trees are dead as surely as if they had burned to the ground.

I can only offer this little bit of hope.

There were pockets of green here and there. Bits of forest had been passed over by the fast-moving flames. The horrific drought continues, but late rains in March provided enough moisture for grasses and flowers to blossom and start recolonizing the burned landscape. It's not much, but it will have to do...

Friday, April 11, 2014

Out of the Valley of Death: What the Heck Happened at Ubehebe?

The geology at Death Valley National Park is naked and raw. Nothing is hidden beneath a soft matte of vegetation. It's in your face, all the processes of tectonism, volcanism, erosion, and deposition. Every era and most of the epochs of the geologic time scale are represented somewhere in the boundaries of the park. Not even Grand Canyon National Park can make that claim. On the previous day we had explored the oldest rocks in the American West, the contorted metamorphic rocks of the Black Mountains and the ancient sediments of the Pahrump Group.

Our first destination on our third day of exploration in the park was the Ubehebe Craters at the north end of the valley, a place representing the opposite extreme in time. The events that took place here can be measured in just hundreds or thousands of years. On the drive there we were treated to amazing views of fault scarps, sand dunes, and alluvial fans, which formed mostly in the last few tens of thousands of years.

To arrive at Ubehebe Crater is kind of an otherworldly experience. For miles, the road has been following the broad rocky surfaces of the many alluvial fans that flank the Grapevine Mountains. The valley floor between the Cottonwoods and the Grapevines narrows and soon the alluvial fans merge in the center of the valley and in a few places the road climbs in earnest. But soon the Cottonwood Mountains come to an end, with the final ridge tapering down to the valley floor. We've reached some kind of a structural nexus. The formerly deep trough of the northern part of Death Valley seems to pinch out, and another valley merges with it from the south. The Ubehebe Craters have formed along the intersection of the faults that formed the valleys. The crustal weakness of the fault zones allowed the magma to rise from deep in the Earth almost to the surface.

It's clear when standing on the rim of Ubehebe Crater that something big happened here. This hole is a half mile across (~1 km) and around 700 feet (215 meters) deep. Whatever happened here did so more than once. There are perhaps a dozen overlapping craters, and each of them is rimmed with dozens of layers that represent numerous explosions. What in the heck happened?

The red, orange and yellow layers below the rim provide some clues. They are partly composed of alluvial fan gravels, but mostly they are fine-grained lake sediments. And they used to be saturated with groundwater. The darker rocks along the rim are composed of bits and pieces of basalt and volcanic ash. The volcanic rocks mantle several square miles around the craters (below).

The best way to see the details of Ubehebe Crater is to walk the rim (and to walk down to the bottom, but that will be a story for a different time).  The gullies that have been carved into the sediments are shallow, indicating the recency of the events here.

Still the trail is steep in a few spots. It's about one and a half miles of up and down pathways leading around the rim. From high points, one can see the overlapping cones and the layers of basaltic material.

From the highest part of the rim, one can see the strong contrast between the orange and yellow sediments in the wall of Ubehebe. They mark the location of a fault line related to the Cottonwood Mountains just to the south.

It's quite a view from the top!

The only actual lava at the complex can be seen inside the crater of Little Hebe, in the picture below. Little Hebe is the remains of spatter cone that was later blown apart by the kind of explosions that formed the rest of the craters in the vicinity.

So here at Ubehebe are the ingredients for an explosive event. Saturated lake sediments, a fault line providing a conduit for magma to approach the surface, and the tendency of water under pressure to explode when it gets too hot. The Ubehebe Craters are world-class examples of maars, the craters that result from groundwater (phreatic water) flashing to steam in an explosive manner.

It must have been awe-inspiring to witness the explosions, and considering that human artifacts have been discovered underneath some of the ash deposits, there is a very good chance that people saw the event take place. Precise dating of the explosions has proven challenging, but few researchers think that the eruptions were more than 6,000 years ago, and some recent findings provide hints that the eruptions could have been only a few centuries ago...and the conditions responsible for the eruption are still present. What a spectacle it would be if it happens again!

There was one thing extraordinary about our visit last February...the air was calm! The narrow constriction at the head of the northern stretch of Death Valley causes winds to be funneled across the rim of the craters, and it's usually...um..."breezy" up there. As in barely able to stand up straight. But this day was one of the most pleasant I have ever experienced.

The display at the parking lot at Ubehebe provides a nice overview of the overlapping craters. It is easy to combine a tour of Scotty's Castle (interesting but not very geological) and Ubehebe Craters. They are only a few miles apart. Ubehebe is also the starting point for the rough road leading to Racetrack Playa and the sliding stones. Another rough road heads north across Eureka Valley to Big Pine in Owens Valley. It is another road that I intend to explore one of these days!

We got in the vans and headed south with the intention of walking into an upside-down mountain. But that will in the next post!

Monday, April 7, 2014

Out of the Valley of Death: Geology at Fifty-five


One of the first things I tell my students (and occasionally even with some success) is "don't sleep while traveling in the vans". Death Valley National Park is the largest national park in the lower 48 states, and no matter how much time one has, it's hard to take it all in. When you only have four days, it's pretty well impossible, but there is still much to see in transit between stops. On our third day out we were set to explore the northern end of Death Valley, which in a park that is mostly desert wilderness, feels even more isolated and lonely (despite the presence of Scotty's Castle up one of the side canyons).
The day started with a stunning sunrise as seen from one of the most isolated RV parks in the American west, Stovepipe Wells. The campground is literally a parking lot, but it's a parking lot with one of the most incredible views possible. The resort is situated on the distal end of the huge alluvial fan that emerges from Mosaic Canyon on Tucki Mountain, which we'll check out in a future post. The elevation is sea level, but it somehow feels higher, given the spectacular and far-ranging view.
Two great desert mountain ranges form the boundaries of the northern reaches of Death Valley, the Cottonwoods on the left and the Grapevine Mountains on the right in the picture above. The mountains tower over the valley floor, reaching nearly 9,000 feet above a valley floor that is barely above sea level. The view extends thirty miles or more.
A drive north towards the end of the valley reveals a series of classic desert features, starting with the Mesquite Flat sand dunes, sometimes known as Death Valley dunes. Sand dunes are picturesque enough by themselves, but in Death Valley they have a dramatic backdrop of high barren mountains.
We raced by at 55 miles per hour ("Honest, officer!"), but cameras these days are versatile, capturing the image as if we were standing still. And this is the sort of incredible sight one could miss if one is snoring away.
The dunes have formed here because prevailing winds sweep down the northern reaches of Death Valley (and a number of destroyed tents over the years can attest to the power of these winds). The vast bulk of Tucki Mountain at the north end of the Panamint Mountains stands in the path of these winds, causing them to break up and form eddies. The sand accumulated in the region as the powerful winds lose energy and drop their load of sand and dust. They are sometimes referred to as star dunes, or modified sub-barchan dunes. Even if you've never been to Death Valley, you've probably seen these dunes anyway; they formed the backdrop for the droids lost on the planet Tatooine in the original Star Wars movie.

In the picture above, one can see the valley floor beyond the dunes is interrupted by a terrace of some sort. This is the scarp for the Furnace Creek fault zone which is one of the important structural features of the Death Valley graben.
A closer look provides a view of light-colored sedimentary rocks exposed in the face of the scarp. The fine-grained mud and silt layers are part of the Furnace Creek formation, which was deposited into a fault trough similar to present-day Death Valley, but oriented in a more northwest-southeast direction. Crustal stretching has effected the crust in the region more than once. The erosion of the Furnace Creek formation in this dry environment produces badlands topography, which we will explore in greater detail in another post.
The fault system interrupts the surface of the alluvial fan, shifting it in an right lateral direction (the rocks across the fault are displaced to the observer's right). These faults roughly parallel the San Andreas fault, which lies far to the west. The fault is presumably still active, but has not produced a major earthquake in modern times.
Another incredible sight visible from the road is the series of alluvial fans that extend from the edge of the mountains down to the valley floor. They build up as the rare but violent flash floods and mudflows carry boulders and debris across the valley floor. They have a somewhat convex slope, becoming steeper near mountain front. Death Valley is famous for the variety and number of fans it has.
The fans reveal variations in color. The darker surfaces on the fans result from desert varnish, a mixture of manganese oxides and clay that coat the exposed surfaces of the rock. It accumulates over time, and the origin is debated. Bacteria are likely involved in the process.

As we drove further north, the valley floor narrowed, and we soon reached an area where the alluvial fans from the two mountain ranges merged in the center of the valley. We were approaching the end of the Death Valley graben. In the distance we could see dark-colored rocks coating the surface of the alluvial fans. We had reached the volcanoes of Death Valley.

In the next post: the Ubehebes!