In the Heart of the Devil: The Damning of Del Puerto Canyon

This beautiful canyon is under serious threat

California geology is complicated. Unlike any other state, it is affected by the interactions of all three kinds of plate boundaries: divergent (the crust pulling apart), convergent (the crust compressing together), and transform (the crust sliding laterally). All of these forces have formed a complex landscape with incredible scenery unlike any other place in the world. 

The Coast Ranges province is one of those unique regions. Extending some 400 miles from the Oregon border to the Transverse Ranges near Point Conception, it is one of the least familiar parts of California (aside from the Pacific Highway 1 corridor through Big Sur and the Marin Headlands/Point Reyes region). The province is defined by a series of individual mountain ranges that trend roughly parallel to the coast, but the variation in rock types and structure is astounding. Within the province there are active volcanic fields, older inactive volcanoes, vast tracts of tilted sedimentary rocks, exposures of twisted and folded rocks formed deep within subduction zone complexes, and even a displaced section of Sierra Nevada granitic crust. Numerous active faults slice through the province, including many of California's most dangerous: the San Andreas, the Hayward, the Calaveras, and many others.

We just explored the heart of the devil: the so-called Diablo Range. It is one of the largest individual ranges in the province, running for around 150 miles from Mt. Diablo and the Carquinez Strait on the north to the Coalinga area in the south. The region is largely undeveloped, and few paved roads cross range. We followed one of those few roads, the one that traverses Del Puerto Canyon. It's a one-of-a-kind experience, the equivalent of driving into and through the crust of the Earth and into the mantle below. It is the path to the nether-world that has often been called the home of the devil.

Del Puerto Canyon lies just west of the Central Valley town of Patterson. A paved road, state route 130, connects Patterson with the Santa Clara Valley, but anyone thinking it would make a shortcut between the two localities is in for a rude awakening: curvy, narrow, with steep drop-offs, it is not a road for the faint-of-heart. It also has some extraordinary scenery and some absolutely fascinating geology. 

The lower canyon exposes 25,000 feet of late Mesozoic and early Paleogene marine sediments deposited in the forearc basin of the Cordilleran subduction zone that stretched the length of California. The middle stretches reveal the oceanic crust on which the sediments were deposited, the Coast Range ophiolite (the second-most complete section found in California). The uppermost canyon is the strangest environment of all, consisting of rocks that were once part of the Earth's mantle. The rocks are interesting, and so are the plants that survive on the ultramafic soils.

One of our students discovered an ammonite fossil on this trip!

The canyon also has a place in the history of California paleontology. The 25,000 feet of oceanic sediments provide an extensive record of fossil species, including the clams, snails, ammonites and shark teeth that are expected in such environments. Mesozoic marine reptiles have also been found in the region, including plesiosaurs, ichthyosaurs, and a new species of mosasaur, Plotosaurus bennisoni. The canyon was also the site of the discovery of California's first dinosaur, a species of duckbilled dinosaur called Saurolophus. It was discovered by 16-year-old Al Bennison of Gustine in 1936. We found a single fossil this trip, an ammonite. Someday, it'll be a dinosaur, right?

To me, the most interesting rocks are found in the upper canyon. The mantle of the Earth is a 1,800-mile-thick layer that starts at a depth of 15 or 20 miles beneath the continental crust. It is generally composed of a rock called peridotite or dunite, made up of the mineral olivine with varying amounts of pyroxene and various ores of chrome, mercury, magnesium, and copper. Peridotite is chemically unstable in surface conditions and alters mostly to serpentine. Many of the rocks we observed showed some degree of alteration. The rock below that looks like alligator skin (below) is composed of fractured chunks of pyroxene (the reddish-brown) and serpentine (the green fracture filling).

In a few spots one can find some relatively unaltered peridotite (below).

We also found some samples of chromite ore. Chrome contributes to the production of stainless steel and has applications in forming armor. During peacetime, there are cheaper sources of chrome overseas, but during wars the supplies may be cut off. During the world wars, chromite was mined in the upper canyon and transported by rail down the canyon to Patterson to be processed.

The little black grains are chromite

The upper canyon was also a source of cinnabar, a mercury sulfide mineral. Despite its toxic nature, mercury was a critical component in the processing of gold ores during the Gold Rush. Miners could often make a good wage mining the ores, but they would do so at great risk to their health. Luckily the temptation to gather mercury ore was not possible as the mine properties are fenced off.

The last part of our journey is the saddest. The lower canyon, the first five-and-a-half miles, is under threat. A local irrigation district is intent on building a reservoir that will flood much of the canyon under hundreds of feet of water. It will serve no purpose other than to store excess water from the California Water Project (in the rare years when such excess is available). It would then be used in subsequent drought years. In other words, it would be an evaporation pond, a waste of water. There would be no recreational facilities. Numerous archaeological sites would be flooded, and precious prairie and riparian ecosystems would be destroyed.

Del Puerto, ("the Gate") Credit: Elias Funez, Save Del Puerto Canyon

There are geological concerns. The dam itself would be constructed a quarter mile from a potentially active fault system (there was an earthquake swarm in the canyon earlier this year). There are seven gigantic earthflows and slumps, one a mile long, that would be partially inundated and potentially reactivated. And it's true I'm not an engineering geologist, but I question the stability of the shale, siltstone and sandstone that the dam abutments would be anchored in. I wasn't reassured by the environmental impact report.

In any case, there is community opposition to this misguided plan. It will do nothing to benefit the local community even while it threatens nearby cities. If you would like to learn more, and support efforts to stop this boondoggle project, please contact the organization Save Del Puerto Canyon. It would be such a shame to destroy yet another beautiful place in service to economic benefits for the very few.

 

A Netherworld Incompatible with the Existence of Life: The Mantle Exposed at Del Puerto

Our journey into the interior of the Earth has reached a remarkable boundary, the base of the oceanic crust. Beyond the crust, underneath the Mohorovičić discontinuity, lies the mantle, a layer that extends halfway to the center of the Earth. We have been driving up Del Puerto Canyon, a scenic route that crosses the Diablo Range in the central part of California's Coast Ranges. The rocks at the headwaters of the canyon have formed a landscape unlike any other. It's a landscape of the underworld, the netherworld.

Serpentine in the upper canyon. Serpentine is highly altered mantle material.

The netherworld is a place in the mythology and religion many cultures, a dark and hot place deep within the Earth that is the habitation of death. It's no place that a human would ever want to be. And yet, here we were, on a sunny December afternoon. It's not necessarily a place of death, but at the same time the rocks were never really able to support life. They formed in an environment where life was impossible, and life at the surface is ill-adapted to survive in such rocks.

The Earth's mantle is composed of a variety of ultramafic rocks, rocks rich in iron and magnesium and a suite of elements not commonly found in the overlying crust: mercury, chromite, nickel, platinum and cobalt. The rocks found in the upper canyon include varieties of dunite and peridotite, which are olivine-rich rocks. In most places such rocks are quickly altered into serpentine, California's state rock, but in upper Del Puerto Canyon, you can find largely unaltered ultramafic rock.

The plant cover is strange, and quite unlike any found on "normal" crustal rocks. Grass is almost nonexistent, and the ubiquitous California oak trees are nowhere to be found. There are too many toxic substances in the soil, and not enough of needed nutrients. The few scattered plants that can survive on such soils include the California endemic Gray Pine and some species of manzanita. A great many wildflowers have adapted to these soils, and the slopes can be quite colorful in wet years.

The rocks have yielded economical amounts of mercury, chromite and mercury. Our stop at the head of the canyon was at an old chromite mine. Despite the obvious use to make cars sparkle and shine, chrome has more important uses in making stainless steel and armor. During peaceful times, chrome is cheaper from overseas sources, but in wartime, the ores in the canyon become valuable. They were last mined during World War II.

Mercury was sought for use during the Gold Rush as a way to separate gold from the ore. There are extensive deposits in the upper canyon and just beyond the pass at the head of the canyon. Mercury mining was lucrative, but deadly. Miners who absorbed mercury into their nervous system quickly developed symptoms and often died. Mercury continues to poison water and sediments in the Central Valley and San Francisco Bay.

Altered ultramafic rocks from the upper canyon

The upper canyon has numerous kinds of rocks to observe. The two pictures above show ultramafic rocks that were in the process of altering to serpentine. The core sections are composed of enstatite pyroxene (which I believe is also known as bronzite when slightly altered like this).

Chromite ore from the upper part of Del Puerto Canyon

It's not too difficult to find small metallic grains of chromite. It takes a bit more searching to find some relatively unaltered peridotite or dunite. It's a beautiful rock, with the rich green of the olivine, which is also known as the gemstone peridot. That's right, there are outcrops of gemstones in the upper canyon! If you are ever up that way, take a close look at the rocks near mile marker 18.

Peridotite from the upper reaches of Del Puerto Canyon

So, our journey into the interior of the Earth is pretty much done. The rocks of the core are just too deep to visit, even with unobtainium (movie reference! The Core, and Avatar). It's a pleasant way to spend an afternoon, exploring the netherworld in the bright light of day!

If you want to pay a visit to Del Puerto Canyon, it can best be accessed from the town of Patterson in the Central Valley southwest of Modesto. A road (Mines Road) reaches the area from Livermore, and a slow winding road travels over Mt. Hamilton and Lick Observatory from San Jose in the Bay Area. There is a campground and day use area in the upper canyon (Frank Raines Park and Minniear Day Use), but most of the rest of the canyon is private land. Stay on the road!

Driving Through the Most Dangerous Plate Boundary in the World: At the Portal of Hell in the Diablo Range

At the portal of hell? For the miners who once worked these tunnels, it was...

I miss the tabloids sometimes. Sure, there's the Onion, which gets mistaken for real news sometimes. But there was nothing like standing in the grocery store lines perusing headline after headline of "real" news, like Elvis sightings, UFO reports, and "Loch Ness Monster Meets Yeti" stories. On extended field trips when group morale was slipping a bit, I could pick up a copy of one of those tabloids and read stories around the campfire, and it worked like magic to bring a group out of the doldrums.

Source: http://www.snopes.com/religion/wellhell.asp

One story has stuck in my mind over the years, the one about the Russians (or Alaskans, or Norwegians) who were digging the world's deepest well on the Kola Peninsula in Russia (or on the North Slope, or in Scandinavia) and when they reached a depth of 7 miles (or 9 or 12), the drill started spinning wildly, and they measured temperatures of 2,000 degrees, but scariest of all, they lowered a microphone (into the 2,000 degrees...) and heard the unmistakeable sounds of screaming souls in Hell. In some versions, gases rose and took the form of the devil, and...well, you get the general picture. As can be seen from the link at the start of the paragraph, the story spread so widely that it merited a Snopes article. What does it tell you that some people (including one or two of my students) took this seriously?

In any case, we are back on our journey through the most dangerous plate boundary in the world, and when we finished our last episode we had passed through the Franciscan Complex, the rock unit that formed in the accretionary wedge of California's Mesozoic subduction zone. We are now making our way down the eastern margin of the Diablo Range through the gorge of Del Puerto Canyon. The association of "Diablo", and "Puerto" ("Devil" and "Portal") is what brought to mind the "screams of the damned" in the old tabloids.

The geologists were trying to dig a hole into the deep crust. They wanted perhaps to break through the continental crust and into the underlying mantle, a layer of peridotite and other ultramafic rocks that make up about 80% or so of the Earth's volume. They failed, but what an effort they made! The thing is, if one really wants to reach the mantle, there is an easier way: find the places where the mantle has risen to the Earth's surface. And that's what happened in Del Puerto Canyon.

Peridotite is composed largely of the mineral olivine (the gemstone peridot). The Earth's mantle is made of gemstones!

As we travel down this canyon we are going to do the literal equivalent of traveling from the Earth's mantle upwards through four or five miles of the oceanic crust, and then "climb" through 25,000 feet of oceanic sediments. All while coasting down a pleasant country avenue in one of the most scenic of the Coast Range's canyons. Along this journey we will also finally leave behind the accretionary wedge of the Franciscan subduction zone, and enter into the intriging forearc basin.

The ultramafic rocks of the upper canyon host a number of unusual minerals and ores. Among them are mercury and chromite. The mercury mined here was used in the process of separating gold from the tailings in the Mother Lode on the other side of the Great Valley. As I understand it, one could make very good wages mining the mercury, but you didn't last long at the job. The mines literally killed the miners. For them, the tunnels really were the portals of Hell. The mercury vapor in the air got into their nervous system, first destroying their minds, and then their bodies.

Chromite from upper Del Puerto Canyon

The chromite that was mined in the upper canyon was not nearly so dangerous. Chromite is of course the source of chrome, which most people associate with shiny car accessories. The most important use of the metal is much more pervasive: it is a major component of stainless steel. The United States usually imports cheap ores from overseas, but during periods of war domestic sources were exploited, and that's what happened in the upper part of Del Puerto Canyon. The road we are traveling was once a railway that serviced the mines.

The orange streaks in the ultramafic rocks of Del Puerto Canyon are slickensides, scratch marks left by faulting

There is a large quarry face in the upper canyon where the rocks can be readily observed. The unweathered rock is dark in color, but weathers readily into an iron oxide rich patina. The region came to be known as the Red Mountain Mining District. The ultramafic rocks are very poor in important in plant macronutrients, and also contain some toxic elements as well. The vegetation on the slopes reflects the difficulty of surviving in this poisonous chemical environment. A number of the species found here are endemic to California and the Coast Ranges. And few invasive species ever gain a roothold on these slopes.

Next, we'll see if we can find the oceanic crust, the rock sequence known as the Coast Range Ophiolite. Stay tuned...

The Other California: A Journey to the Center of the Earth (kind of...)

How many of you tried to dig a tunnel to China in the backyard when you were a kid? Given the soil conditions in the yard I grew up in, I'm probably lucky to be alive. I dug tunnels looking for buried treasures, gemstones, fossils and sometimes I was just curious what was down there. Geologists, I've found, are the kids who tried to find all those things, and never really grew up.

So how far do these overgrown kids get? It turns out that the deepest tunnels that humans have ever been able to dig reach depths of about 12,800 feet, a little over 2-1/2 miles. That might seem like a lot from our point of view, but the depth to the center of the Earth is around 4,000 miles. We've barely scratched the surface, yet the temperatures of the rock at these depths is well over 100 degrees, and the rocks are under so much pressure that explosions of rocks from the walls are a constant danger to the miners. Kids, there's got to be a better way to see what lies deep below. And there is, in the Other California, one of those places not found on the postcards. The adventure lies in the Klamath Mountains, and the most dangerous thing you have to face is slipping on a slick river rock, because geological processes have brought the rocks many miles up from the depths. You need only explore the rivers flowing off the mountains to see what the deep interior of the earth looks like.

The Klamath Mountains are a collection of bits and pieces of the earth's crust that have been carried great distances from their point of origin and slammed (at geologic speeds of inches per year) into the western edge of the North American continent. A huge variety of igneous and metamorphic rocks are found around the province, and some of the most interesting are those that once resided deep in the Earth's mantle, a layer that extends from just below the crust, from maybe 15 or 20 miles beneath our feet, to a depth of about 1,800 miles. Here are a couple of bits of the Earth's deep hidden places that I found on a short trip to the Eastern Klamath Terrane in the vicinity of Gazelle.

The oceanic crust is usually described as being made of basalt, but a few miles down in the crust the basaltic magma cools slowly to form a coarse-grained basaltic rock called gabbro. Sometimes, as can be seen above, the crystals that form are huge, with black hornblende and white feldspar crystals several inches long. Igneous rocks with such large crystals are called pegmatites.

Going even "deeper" into the interior, we pass the Mohorovicic Discontinuity, the dividing line between the crust and mantle. The upper part of the mantle is composed of olivine-rich rocks like dunite or peridotite. Olivine is best known to most people as the green gemstone peridot. That's right, much of the Earth's interior is made up of gems! The rock in the picture above is dunite, in part slightly altered to serpentine.

In many parts of the Klamath Mountains, the mantle rocks are completely altered to serpentine, the state rock of California. These ultramafic rocks are fairly rich in a number of unusual metal ores, including platinum, nickel, magnesium and mercury. One of the most important ores is chromite, which is the only significant source we have for chromium, the metal that puts the "stainless" in stainless steel. We import most of the chromium that we need from foreign sources, but in wartime (especially the two World Wars), the ores were mined domestically, and a number of operations were present in the Klamaths. The black semi-metallic crystals in the picture above are chromite, with green serpentine across the top.

In our next post we are going to "climb" into the underside of a volcano...

Journey to the Center of the Earth (well, the mantle at least)

Continuing our trip up Del Puerto Canyon (and into the earth's mantle; see Photo of the Day - Feeling Crushed?), we reached a huge pod of mantle peridotite and dunite in the upper canyon that had escaped the most extensive alteration during the long journey to the earth's surface (in most places the rock is serpentinized). A few unusual minerals are found in the area, including mercury (cinnabar) and chromite (shown above). The chromite crystals are dark and metallic-looking.

The United States imports almost all the chromium that it uses, but in wartime, imports are disrupted, so the mines in the upper canyon were utilized off and on during the two world wars. The mercury was considered far more valuable, given its role in the production of gold during California's Gold Rush of the 1850's. I've heard that the mercury miners could earn a much higher wage mining mercury than they could mining gold, but they also tended to go mad, and die a lot sooner. It's not as if the gold miners had such a great life expectancy, though. We make jokes and complain sometimes about workplace safety regulations, but it is a great deal safer in the mines today than it was 140 years ago. And sometimes, workers even get to enjoy some kind of retirement plan!