Friday, March 27, 2015

Driving Through the Most Dangerous Plate Boundary in the World: A New Blog Series

Source: adapted from National Park Service and R. J. Lillie. 2005, Parks and Plates
Before I get accused of "cable-newsing/click-baiting" with my choice of a headline, I'll amend it to say "Driving through the most dangerous kind of plate boundary in the world".

Where in the world do we find the worst earthquakes, and many of the worst volcanic eruptions? Looking at maps of earthquake epicenters and volcanic eruptions, it doesn't take long to realize that there are specific zones where disasters and human misery occur. They follow oceanic trenches and their associated volcanic arcs (curving series of active volcanoes). Horrific events like the Sumatra earthquake of 2004, the 2011 Tohoku earthquake near Japan, and the 1991 eruption of Mt. Pinatubo in the Philippine Islands were the result of oceanic crust and upper mantle (the lithosphere) sliding beneath the adjacent continental or island landmasses. It is the sliding and grinding of the oceanic plate in these subduction zones that produces the huge earthquakes, and it is the complicated interaction of extreme heat and volatile materials in the mantle that leads to the formation of magmas and resulting volcanic eruptions.
Mt. Shasta and Little Glass Mountain, in the active part of California's subduction zone

Subduction zones are complicated places, and it can be difficult to study active systems. We may get highly accurate maps of the seafloor, and geophysical data may reveal the broad outlines of what lies beneath the bottom of the sea, but direct sampling and observation of the deep crust is mostly beyond our technology. So how to study and understand the dynamics of these zones of terror?

Maybe you have noticed that I sometimes say nice things about the geology of California. Once in a while, anyway. The state has such a rich variety of geologic and tectonic landscapes that one could spend a lifetime exploring them all (which for the record is what I am currently doing, although I am known to explore other places as well). As such, the state provides a nice outdoor laboratory for looking in the active parts of a convergent boundary, as well as a marvelous place to observe the deep interior of a fossil subduction zone complex.
California's Great Valley may seem to be a monotonous flat valley (clarification: it is a monotonous flat valley), but it hides a violent and complicated past.

Besides the trench, there are three structures within many subduction zone complexes: an accretionary wedge, a forearc basin, and a magmatic arc (see the diagram at the top of the post).

The accretionary wedge is a gathering place for the flotsam and jetsam of the seafloor and oceanic crust, as well as sediments from the continent. These trench deposits include a clay- rich sandstone called graywacke, dark colored shale, pillow basalt, deep-ocean chert, and the occasional volcano or coral reef. These rocks are carried deeper and deeper into the subduction zone, and are put under tremendous pressure. The rocks are churned up, faulted, and deformed into a chaotic mass called a mélange (from the French word for mixing).

A forearc basin forms in a relatively shallow sea between the crest of the accretionary wedge and the volcanic arc inland. Sediments, primarily sandstone, siltstone, and shale derived from the continent, accumulate to depths of tens of thousands of feet. This can happen in a shallow basin because the weight of the sediment pushes the crust downward, making room for more sediments. The foundation of the forearc basin is ocean crust rocks collectively called an ophiolite sequence.

A magmatic arc is a chain of volcanoes fed by magma generated when the subducting slab of oceanic crust reaches the semi-molten layer within the mantle called the asthenosphere (from the Latin "weak shell"). Water in the subducted slab serves as a catalyst to lower the melting points of the silica-rich minerals, causing the rock to melt and form plutons of magma that rise through the continental crust. If the magma reaches the surface and erupts, it may form andesite, dacite, or rhyolite lava. If it cools slowly deep in the crust, it will form a variety of granitic rock, such as actual granite, granodiorite, tonalite, quartz monzonite, or diorite.
Part of the Diablo Range, a subdivision of the Coast Ranges, from the summit of Mt. Hamilton, which houses the Lick Observatory complex.
California's complicated geological history includes a period of nearly 200 million years when the entire state was influenced by a subduction zone. Beginning about 29 million years ago, the subduction zone was progressively replaced by a transform boundary, a series of lateral faults known as the San Andreas fault system (yes, that San Andreas). The process is not yet complete, as the subduction zone still exists in the northern part of the state where it feeds the eruptions of Mt. Shasta and Lassen Peak. The remains of the ancient subduction complex now make up the Sierra Nevada, the Great Valley, and the Coast Ranges. One can conveniently explore this incredible complex in a car or on foot without the threat of magnitude 9 earthquakes, or catastrophic rhyolite caldera eruptions. Probably.

I hope you'll join me on this coming blog journey across California and through the guts of an ancient subduction zone. I got the seed of an idea for this series when I finally drove the winding road from San Jose to the Great Valley past Lick Observatory and down Del Puerto Canyon (I've been in Del Puerto many times, but never drove beyond the head of the canyon). I can't believe it took me this long to get around to it, but that's what happens sometimes.

This series is also meant to coincide with the long-awaited opening of our Great Valley Museum of Natural History, which opens to the public on April 4. Information is available on Facebook at  https://www.facebook.com/GreatValleyMuseum?pnref=story, and at http://www.mjc.edu/instruction/sme/gvm/. I hope to see you there!

Wednesday, March 25, 2015

So, Besides Fish, What's the Last Thing One Would Ever Expect to See in Death Valley National Park?


It's one thing to find that a number of fish species survive within the boundaries of Death Valley National Park, which preserves the hottest and driest desert in North America. But sure, springs will persist in many dry environments. But a permanent, year-round waterfall? Yes, there is one in Death Valley National Park.
When Death Valley was changed from a national monument to a national park in 1994, the boundaries were vastly expanded, and came to include a portion of the Darwin Plateau at the south end of the Inyo Mountains southeast of Owens Lake. Part of the reason this area was included was the presence of a permanent stream, and its importance as a wildlife area. And there was Darwin Falls as well.
The falls are a short walk up a canyon just upstream of the Panamint Springs Resort on Highway 190. Indeed the water at Darwin Falls isn't just for the wildlife. A pipeline runs the length of the canyon providing the precious liquid to the resort as well.
There is some excellent geology exposed in the canyon walls on the way to the falls. One example is this excellent exposure of a dike, an intrusion that cuts across the sedimentary layers. There are nice examples of jointing in the granitic rocks exposed a little farther upstream.
Then comes the unexpected thickets of vegetation and the sound of trickling water. The trees are so thick it is a little tricky getting a clear shot of the lower falls. The falls are about 20 feet high, pretty small by any standards, but we are talking about a desert environment here. I understand several other falls are present upstream for a total drop of 80 feet or so.
These pictures are from 2008. I've only made it to the falls the one time, but I'd like to get up there again, maybe when the sun hasn't practically set! Thanks to commenter "twoeightnine" for jogging my memory about this extraordinary place in Death Valley!

The

Tuesday, March 24, 2015

One of the Most Astounding Viewpoints in America: At the Outer Ring of Hell (but not really)

A place not to missed. That's what I have to say about Dante's View in Death Valley National Park. And if you ever get there, don't just stand in the parking lot. Short walks in several directions offer even better views of the incredible landscape surrounding the lowest place in North America. Death Valley may get hellishly hot sometimes, but it's not a view of hell but is instead a dramatic perspective of some of the most interesting geology in the American West.
The Black Mountains form the eastern margin of the deepest part of Death Valley, almost directly across from Telescope Peak, at 11,043 feet (3,366 meters) the highest point in the park. The viewpoint is a bit over a mile in elevation at 5,476 feet (1,669 meters). From the viewpoint one can take in almost the full length of Death Valley, a distance of more than 100 miles. Once also look east across the numerous mountain ridges of the Basin and Range geological province to Charleston Peak above Las Vegas, Nevada. A few summits of the Sierra Nevada peek out over the ridge of the Panamint Range across the valley.
Directly below one's feet is a steep one mile drop to Badwater on the valley floor. The slope is so steep that the parking lot and Badwater pond are not visible, but very small humans can be seen on the white trail leading out to the salt flats, and very small cars can be seen driving on the highway that circles the edge of the alluvial fan.
Here's a zoomed shot to see the people a bit better...
The Black Mountains are composed mostly of the oldest rocks in the region, a complex of gneiss and schist dating back 1.7 billion years. Similar rocks are found in the depths of the Grand Canyon in Arizona. They were once covered by several vertical miles of Paleozoic sedimentary rocks, but those rocks have slid westward to form the Panamint Mountains. The gap between became the graben of Death Valley itself.

The rocks making up the summit area are volcanic rocks, mostly rhyolite, which were erupted in violent eruptions around 6-7 million years ago. Such rocks have been found across the Basin and Range province, indicating extensional stretching and failure of the crust, which ultimately formed the grabens and horsts of the region.
I take my use of superlatives in describing viewpoints seriously. You can see my ten (eleven, actually) most precious "spots" in the world, and Dante's View is on that list. There may be other places in the world that allow one to see far, but I suspect few reveal a more stunning and fascinating landscape.
There is a myth sometimes repeated that one can see Mt. Whitney from Badwater, which is simply impossible because the Panamint Mountains rise 7,000-8,000 above the adjacent valley floor. One can see the tips of several Sierra Nevada peaks from the hill north of the parking lot at Dante's View, but I think the highest peak visible is Mt. Williamson at 14,389 feet (4,383 meters). It is possible to simultaneously see both the highest and lowest points of the conterminous United States, but you have to find your way to the summit ridge of the Panamint Mountains and Telescope Peak to do so.
It's truly a place and a view not to be missed.

Monday, March 23, 2015

Is There a Deserted Corner of Death Valley? Ubehebe Country

Is there an empty quarter of Death Valley National Park? An area so isolated that tourists are almost never found there? The short answer is: of course. Much of the park is near-primeval wilderness, roadless and untrammeled, and largely devoid of humans. That's as it should be. We need to have these kinds of places, a reminder of the natural world where humans haven't mucked everything up, places that have value measured in something other than dollars. 
Places like Badwater, Furnace Creek, and Dante's View are popular tourist destinations, and are certainly wonderful places to visit. But in my travels I like the places that feel like the end of the road, the entrance portal into a wild and even dangerous world. One of those kinds of places is Ubehebe Crater at the end of the paved road in northern Death Valley. 

Ubehebe Crater is part of a volcanic field that includes a number of basaltic cinder cones, and seven or so phreatic (groundwater) explosion pits called maars. The volcanoes are quite young, with some that erupted as recently as 800 or so years ago (though the dates are still being debated).  The biggest crater is 750 feet deep and half a mile across. The groundwater explosions were caused when magma approached the surface, heating the water and causing it to flash to steam. Dozens of explosions would have accompanied the formation of each crater.

The craters are a fascinating destination for Death Valley travelers, although few make the effort unless they are also going to Scotty's Castle. Walking to the bottom of the crater is one way to gain an appreciation for the power of exploding magma. So is a walk around the biggest crater. In the picture above, the cars in the parking lot on the opposite rim are barely visible.

But the reason I love stopping in at Ubehebe is the sense at being at the edge of the wilderness. The pavement ends, and only gravel roads continue, towards Racetrack Playa in one direction, and the distant town of Big Pine in the other. The lands seen from the edge of the crater rim feel like terra incognita. It's wild, lonely country.

The fourth day of our recent journey to Death Valley was devoted to an exploration of the north end of Death Valley, with stops at Bonnie Claire Playa and Ubehebe. The day was ending as we drove back down the valley towards Stovepipe Wells, but the sky was gorgeous.


Saturday, March 21, 2015

Work to Begin on the Ferguson Slide on Yosemite Highway 140


The problem with a lot of beautiful national parks in mountain landscapes is that they lie in mountain landscapes. The rugged terrain is subject to landslides, and my favorite nearby park, Yosemite, is no exception. The park has four entrances, but only one can be considered an "all weather" access point, as it follows the Merced River, and does not have to surmount any snow-covered passes. Highway 140 may be the lower route, but it is not without its problems. The Merced Canyon downstream of Yosemite Valley is rugged and steep, some 2,000 feet deep in places. Instead of granitic rock, it carves through the metamorphic rocks of the Mother Lode, including the sometimes unstable slate, phyllite and chert of the Calaveras Complex.

There was a prehistoric slide near Savage's Trading Post. The so-called Ferguson Slide had caused problems in the past, and Caltrans had been looking to stabilize it, but in 2006 it overwhelmed the mitigation efforts and gave way, covering 600 feet of Highway 140 with hundreds of thousands of tons of metamorphic rock. The highway was totally blocked, and there were serious fears that the slide could completely block the river valley and flood the small village upstream.

Local communities, especially Mariposa, were devastated by the Ferguson Slide. Tourism makes up a huge portion of their economy, and with the highway closed, no one was coming through town. The first temporary fix, two quickly built bridges that detoured around the slide, were impassable to buses. Mariposa continued to suffer economic losses, so by 2008 the bridges were realigned. At present there are are delays of up to 15 minutes waiting for traffic to pass the one-way road.

I missed the story at first in the newspaper last week ("newspaper": a form of information transfer that predates the internet), but work has begun on a "permanent" fix on the section of Highway 140 beneath the slide. The first part will involve the removal of more than 100,000 cubic yards of slide debris, followed by the construction of a 750-foot "rock shed" that will channel future debris slides over and across the highway. I've seen snow sheds before on many alpine passes, but this will be only the second rock shed in the country according to Caltrans. The project will cost $133 million and will be completed in fall 2019. The work is not expected to block traffic, as the "temporary" detour will remain open.

I understand that the rock shed approach will be one of the least environmentally intrusive approaches to mitigating the slide (assuming it's done right), but I'm struck by the fact that this one road repair will be only slightly less expensive than the complete renovation of Yosemite's infrastructure following the devastating floods of 1997 ($182 million in 1997 dollars). And it's been nearly twenty years since those renovations, and Congress has never been one to keep up with the care of our national parks. It would be nice if we could take care of our parks as well as we care for the access to those parks.

In any case, this is a good time to be visiting Yosemite via Highway 140. Despite the drought years, the spacing of the few rainstorms we've had has left a lot of greenery along the river corridor. I haven't been up there yet to check, but I'll bet the poppies will be blooming soon, and in force. I'll have a full report about April 11.

Thursday, March 19, 2015

The Incredible Dunes of Namibia: Geotripper becomes an Armchair Geologist for a day


Here at Geotripper I usually write about my travels, geological and otherwise. I feel inordinately lucky to have a job that allows me to travel a lot, introducing my students to the incredible world that lies beyond the borders of their town. But there are certainly a great many places I have never visited, and may never have the chance to.

I am also lucky to have a cadre of former students who also travel a lot, and some of them get to places I could not reach myself. For instance, one of my former students was on the logistical crew of an Everest expedition a year or two ago, and he brought back a few precious stones from high on the mountain. And then there are Noel and Art, a couple who are spending their time in Nepal and Africa. This week Art sent me a beautiful collection of photographs from their exploration of Namib-Naukluft National Park in Namibia. So today, I join you as an armchair geologist, enjoying the scenes of an incredible place in a far-flung location of our planet.
Deadvlei, a former oasis in Namib-Naukluft National Park. The dead trees are acacias.

Namib-Naukluft National Park encompasses just over 19,000 square miles, an area larger than Switzerland, or for you Californians, the area of about 18 Yosemite National Parks. It's the fourth largest national park in the world. It preserves the Naukluft Mountains and a vast region of sand dunes.
The main visitor destination in Namib-Naukluft National Park is Sossuslvei, a portion of the dune field that includes playas and an almost always dry river channel. In the Google Earth image above, on can see the light-colored playas (dry lakes), and the dry channel to the right. One of the world's tallest sand dune can be seen in the area. It's called (very imaginatively) Dune 7. It is 1,256 feet high (383 meters).
The second Google Earth provides a regional context (Sossusvlei is on the bottom of the image on the left side; it is the long narrow gray area). The north-south length of the map is around 150 miles. Namibia is on the west coast of South Africa, and it was part of South Africa until 1990, when the country became independent. The region has been a desert region for tens of million of years, which explains the wide distribution of the sand deposits. The sand is carried into the region along the coastline, and constant onshore winds blow the sand inland. The orange area on the left next to the coast is the vast north end of the dune field.

The Namibian desert is one of the driest in the world. It is dry because of a cold ocean current offshore that prevents the evaporation of water and causes the condensation of the atmospheric water offshore. Sometimes the fog will come onshore, providing a bit of moisture, but generally it is as dry as any place in the world except for some parts of the Atacama Desert in South America. Death Valley in California is practically humid in comparison.
The desert is described as one of the oldest in the world, dating back tens of millions of years. Contrast this with the Mojave Desert and Death Valley, which have existed as deserts for only 2-4 million years, with wet climates during the occasional ice ages.




The source of all the sand in the Namib is the coastline of the Atlantic Ocean. Rivers provide sand to  the coast and longshore currents and beach drift move the sand into the desert shoreline. The onshore breezes carry the sand inland.
There are several kinds of dunes, based on the amount of available sand, and the variability of wind directions over the course of the year. Many of the dunes of the Namib desert are seif (or longitudinal) dunes, long parallel ridges of sand that extend for many tens of miles. When the dunes get backed up against a mountain range, they will have a much more irregular outline that sometimes resembles a starfish. They are called star dunes. Such dunes can accumulate vast amounts of sand and grow to great heights.


Deadvlei is one of the most popular photography spots in the park. The barren dead acacia trees were once watered by an ephemeral stream that changed course. Some of the dead trees may be hundreds of years old.

Dunes move when sand is carried from the base of the windward side and dropped down the lee slope, called the slipface. The dunes will migrate until they reach mountain ridges that interrupt the winds.

Many of the park visitors elect to do a flyover of the park. The vastness and loneliness of this landscape is overwhelming. I would not want to be lost here.

One section of the park contains exposures of a much older example of sand dunes. The billion year old rocks display the same kind of cross-bedding (diagonal layers representing the buried slip-face of dunes) that the modern day desert dunes possess. It's a marvelous example of uniformitarianism, the idea that processes that we can observe happening today also happened in the past (because the laws governing nature have not changed with time).
Many thanks to Art and Noel for sharing these wonderful photographs! Please regard them as copyrighted, however. I think they won't mind if the shots are used in school projects and the like, but if you want to use them in any formal publications, please notify me and I will forward their contact information.

Wednesday, March 18, 2015

Another Cool California Roadcut: Dike and Sill on Highway 190 near Panamint Springs

Dike (vertical brown rock) and sill (horizontal brown rock) along Highway 190 near Panamint Springs
I've been working through my memories of beautifully instructive roadcuts in California. Some, like the Big Pumice Cut and the Charlie Brown Outcrop, are quite famous to geologists and generations of field studies students. Others have received less attention, and today I'm showing a particularly nice spot to learn about the difference between a dike and a sill.

A dike is an intrusion of molten rock that is tabular (thin, but deep and wide, as it fills fractures in the rock), and discordant (it cuts across rock layers). A sill is also tabular, but it is concordant, meaning it squeezes between layers rather than cutting across like a dike.

Highway 190 provides access to Death Valley National Park from the west at Lone Pine and Olancha. It travels around the south edge of Owens Lake, and then crosses the Darwin Plateau before beginning a steep descent into the Panamint Valley. From there it crosses the Panamint Mountains at Townes Pass and enters into Death Valley.
The Darwin Plateau is capped with multiple layers of basaltic rock covering a sequence of Paleozoic limestone layers. Midway down the stretch between the turnoff to Darwin and the resort of Panamint Springs there is a spectacular exposure of a dike and a sill in the same roadcut. The basaltic intrusion has been weathered to an orange-brown color. The Paleozoic-aged limestone is gray and has clearly delineated layering that makes it a straightforward exercise to distinguish the two types of intrusions that cut through it.
I wish I could say that this is a favorite stop on our way to Death Valley, but unfortunately we are usually running late by the time we reach it late in the afternoon on our way to Stovepipe Wells. It is a marvelous example of both kinds of intrusions.