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...

Spending a Day in the Earth's Mantle: Exploring Del Puerto Canyon in California's Coast Ranges

There aren't many places where you can drive into the Earth's mantle. It is the 1,900 mile thick layer that constitutes something like 80% of the volume of the planet, but on the continents it lies hidden beneath many miles of crust. We've never drilled into it, so getting to it isn't easy. Unless you live near California's Coast Ranges or Klamath Mountains. In these places, fragments of the mantle have been brought to the Earth's surface.  The rocks of the mantle are described as being ultramafic, and they are beautiful. They are literally composed of gemstones: lots of olivine (peridot), as well as bronzite pyroxene, diopside and other unusual minerals. They are valuable in other ways, as ultramafic rocks serve as ores for chrome, mercury, magnesium, platinum, nickel and other rare metals.

It was an interesting day. The geology club at our college decided on Del Puerto Canyon in the Coast Ranges near Patterson as the destination for their field trip this semester, so 19 intrepid explorers set out to explore the mantle. The journey starts as we drive down-section through 25,000 feet of ocean sediments of the Great Valley Sequence, the same sediments we live on back home. We can do this because they have been tilted to near vertical angles. That's that equivalent of driving to the top of the Himalayas, only going the opposite direction, into the Earth's interior.

About ten miles up the canyon we cross the Tesla-Ortigalita fault and enter the Coast Range Ophiolite, the volcanic and intrusive rocks that represent the oceanic crust on which the Great Valley Sequence sediments were laid down. We encounter dikes, pillow basalts and gabbro plutons as we drive farther up the canyon (and "into" the crust). Finally, just beyond Frank Raines OHV Park, we find the rocks of the mantle. Most of these rocks on the long journey to the surface are altered into serpentine, California's State Rock (there was a bit of a ruckus about our state rock last year in the media and legislature).

But not all of the rock is turned to serpentine. In the upper reaches of the canyon, a mass of rock several miles across was pushed to the surface without quite the degree of metamorphic change, and as such it preserves some of the original minerals and textures of the mantle rock. We found some very nice samples of the chromite ore (below), a heavy metallic mineral that occurs in small grains within the ultramafic rocks.

We could also see where alteration was starting to affect the rock, releasing magnesium into hot water solutions that later formed magnesite, which also was mined here during the World Wars. The "ma" in "mafic" refers to magnesium, and the "fi" refers to iron. Can you see any clues that there might be large amounts of iron in the rocks below? Or why the entire region is referred to as the Red Mountain Mining District?

The rocks also provide clues as to how these rocks traveled the many miles from the depths of the Earth to the surface. Marks on the rocks called slickensides are the scratches and striations where the rocks were ground into each other and sheared on the way up along fault zones.

I've been up Del Puerto Canyon a few dozen times over the years, but I rarely have time to explore. We usually come up with a laboratory class and our time in the canyon is severely constrained. On this day we took our time and wandered widely, and I discovered something new this trip. The peridotite we usually see at the chromite mine is very fine-grained and almost black. Except in a microscope, the beautiful olivine crystals are practically invisible. On this trip I wandered further up the canyon than normal, and rock hammer in hand I picked at the deeply weathered red rocks. One outcrop was unexpectedly tough. I pounded all the harder, and the rock split to reveal sparkling green crystals of olivine, larger and purer than any I've ever seen in Del Puerto. It was a beautiful rock, and all the more interesting when one considers the journey the rock has made from the depths of the planet.

As the day ended, we jumped back into our Miss Frizzle's Magic School buses (ok, white school vans) and headed back up to the surface of the Earth, and the Starbucks in Patterson.

A Trip to the Moon and a Trip to the Mantle (well, sort of...)

I'm interrupting my journey through the Pacific Northwest briefly with other things, mainly a trip to the moon and a trip into the Earth's mantle. I'm currently on the road, just for the fun of it this time, no students, just me and Mrs. Geotripper, and we are exploring the center line of the American West, a line that runs east through Nevada, Utah and Colorado.

Today's discovery was on what I consider the real "loneliest highway in America", Highway 6 from Tonopah to Ely, Nevada. Not a single gas station or business for 170 miles. Along the way is a really interesting volcanic area, the Lunar Crater Volcanic Field, an area of 100 square miles or so of basaltic cinder cones and lava flows dating from the last million years or so, with the most recent activity in the last 20,000 years. Astronauts trained in the area during the Apollo missions.

About 7 miles off the highway about 75 miles east of Tonopah, Lunar Crater is a big hole, about 3,800 feet across and 430 feet deep. It is not a cinder cone or anything like one. It is a pit caused by the explosion of groundwater when magma came close to the surface. These craters are called maars. Similar features are seen at Death Valley (Ubehebe Crater), and at Mammoth (Inyo Craters).

The other interesting item of the day was an olivine crystal that came to the earth's surface from the deep mantle, via a lava flow near Lunar Crater. It's the biggest I've ever found (note the penny for scale). Chunks of mantle that are carried in lava like this are called xenoliths.

More when I get a chance, internet access is sparse out here!

From hell's heart I stab at thee: Accretionary Wedge #28

"...to the last I grapple with thee; from hell's heart I stab at thee..."

The Accretionary Wedge challenge for the month is to show off a favorite "deskcrop", those beautiful and significant rocks and minerals that have won a coveted position on the desk of a geologist/teacher. The convergence of the wedge with Halloween added a layer of complexity by asking for spooky items. Well, I have a lot of deskcrops on my desk, so many that there is precious little room for books and office supplies and that sort of thing. It was hard to choose.

That "spooky" thing made it even harder, but what is Halloween about anyway? The day was co-opted by the Christian religion from earlier Celtic traditions, but the upshot of the cultural intermingling of meanings is that on All Saint's Day Eve there are spirits from the underworld set loose on Earth. So I looked for the deskcrop that best exemplifies the underworld, and I realized it could be nothing else but the walls of hell itself: olivine dunite xenoliths from the deep mantle, brought to the surface of the Earth by volcanic eruptions.

Olivine is a beautiful mineral, one of my favorites, with a glassy luster and bright green and yellow color which varies due to a substitution of iron and magnesium in the crystal structure. It is also known as the gemstone peridot, the August birthstone. The streets of Heaven are paved with gold, but the walls of hell are apparently lined with gemstones! The mineral is relatively unstable at the Earth's surface, and readily alters to serpentine when exposed to water in the upper crust.

As for the quote, it comes to us from Moby Dick by Herman Melville, although it was also a memorable quote from the movie Star Trek II: The Wrath of Khan (and I hate to admit that I first knew of it from the movie).

Thanks to Research at a Snail's Pace for hosting the Wedge this month!