Taking in the View from the Bay Area's Devilish "Not a Volcano"

Mount Diablo looms over the Bay Area, rising 3,849 feet above the coastal plains. The two prominent peaks are miles from any other high mountains, and thus the mountain offers an incomparable view of central California, from Mt. Lassen in the far north to Mt. Dana and other high peaks of Yosemite National Park to the south. Sitting above the Carquinez Straits and the delta of the Sacramento River, it also offers unparalleled views of the Bay Area including the Golden Gate Bridge and the skyline of San Francisco (on non-foggy days). One minor surprise: if you actually stand on the true summit of Mount Diablo, you won't see any of the things mentioned above. It's inside a building (see the picture below).

The monument in the visitor center describes the use of the mountaintop as the initial point for all of survey lines for Central California and Nevada. Any property lines using the township and range system of measurements in the region are based on measurements from the summit that started in the 1850s.

The most prevalent story of how the peak came to be called "Diablo" involves an effort by the Spanish to force members of the local Native Americans into servitude at the nearby missions. Some were hiding out in willow thickets that the Spanish called the Monte Diablo, the "devil's thicket", but the "monte" was later mistranslated as "montana".

Despite the mountain's distinctive shape, it is not volcano. Even though some of the rock making up the mountain is volcanic, it is basalt and related rocks that were once part of the ocean crust, and not something erupted from a terrestrial cone. The origin of the mountain can best be described as ocean floor crust and Franciscan trench deposits breaching the Earth's surface like (oh, geologist friends forgive me) some gigantic Godzilla character in a movie. Described more properly, the rocks from deep in the crust have been thrust upwards along active compressional faults.

A rather narrow and exciting road climbs to the summit of Mt. Diablo, the centerpiece of Mt. Diablo State Park. The summit building contains a visitor center and store, and includes an observation deck. Trails lead out in several directions from the summit, but one, the Mary Bowerman Nature Trail, circles the summit in about 0.7 miles.

North from the summit, the hills drop away rapidly to the Sacramento River and Carquinez Straits, where about 60% of all of California's fresh water flows. 32 of the 35 islands in the delta have subsided to below sea level. They are protected by poorly designed levees that are a century or more old. If a moderate earthquake strikes on one of the nearby faults, liquefaction will cause widespread failure of the levees, and the delta will be flooded with salt water. The California Water Project will be shut down for many months while the delta is slowly flushed out. The implications for water deliveries to the San Joaquin Valley and Southern California are frightening.

An odd sight is visible just to the north, the large open pit mine called the Mt. Zion quarry. The basaltic diabase is some of the oceanic crust mentioned earlier. It is a tough and stable rock, useful for breakwaters, road beds, and railroad lines. Note how the quarry lies on the ridge side opposite of the suburban developments of Clayton. Out of sight, out of mind...

North Peak is about a mile northeast of the main summit, and only about 300 feet shorter. Like the main summit, it is composed primarily of rugged Franciscan chert. Chert develops on the deep ocean floor as myriads of one-celled creatures with silica shells sank to the bottom and became more compacted (diagenetic alteration). The rock was carried on the Pacific Plate for thousands of miles before being scraped off in the subduction zone along the western edge of North America.

 The view to the east and southeast reveals my home. It's flat. They grow lots of stuff there.

Actually though, the oak-covered ridges in the foreground reveal something of interest. The Great Valley is a deep trough with upwards of 25,000 feet of sedimentary layers dating back to the Mesozoic Era, the age of the dinosaurs. We would know little about them except that along the eastern Coast Ranges, the sediments have been curled upwards and exposed by erosion. The rocks surrounding the core of Mount Diablo are in fact called the Great Valley Group, and they contain a rich fossil record, including plesiosaurs, ichthyosaurs, mosasaurs, and even an occasional dinosaur bone.

The easternmost section of the trail offers an excellent exposure of the Franciscan chert called the Devil's Pulpit. No one walking by was able to resist the temptation to climb it, including me!

The southern flank of Mount Diablo was burned last September. With the horrific drought, regrowth of vegetation has been limited, but there were lots of interesting flower species. The views included the Livermore Valley and the South Bay. A short distance later, I reached the end of the trail and we headed down the mountain. We made one more stop, at the very cool sounding Rock City.

After visiting with the resident raccoon, we set out to explore the bizarre rock exposures. We were looking at the Domengine Sandstone, a coastal estuary deposit that included extremely pure quartz sandstone and low grade coal called lignite. Earlier in the day we had explored one of the coal mines at the Black Diamond Mines Regional Preserve.

We made our way through a warren of criss-crossing trails and followed a sandstone ridge to Sentinel Rock. It is an eerie landscape, but it's also a lot of fun to explore.

Knowing that Sentinel Rock would be an irresistible but dangerous climb, the park administration installed cables and carved out steps.

Mt. Diablo State Park is an absolutely fascinating and beautiful island of wilderness in the midst of the Bay Area. It is a great place to visit, and a marvelous geological wonderland. If you are ever in the region, check it out!

Through the Looking "Glass" and into the Rabbit Hole: A Tour of the Black Diamond Mine

Coal mining...

Barren open pits, overburden stripped away, ruined rivers, flattened mountain tops. It doesn't have a very good reputation and probably doesn't deserve it. Yet even today it provides a significant part of our energy mix, despite the destructive effects it has on our atmosphere, land, water, and the lives of the miners themselves. A hundred years ago it provided much more of our energy.

California doesn't have much of a reputation for coal mining. During the middle and late Paleozoic, when widespread parts of the eastern United States and parts of Europe were covered by forests, swampls and coastal estuaries, California was pretty much underwater, and no major coal seams ever formed.

The situation was a little different about 50 million years ago. The Ancestral Sierra Nevada, the mountains that formed while granite was cooling deep in the Earth, had been worn away, and the modern Sierra Nevada had not yet begun rising. Large meandering rivers coursed across the landscape, bringing sediments into the coastal complex from sources as distant as central Nevada and Idaho. A shallow sea filled the forearc basin that paralleled the Pacific coast, and along its margins there were beaches and barrier islands, along with coastal estuaries and jungles of tropical vegetation. The river and coastal complex formed a sedimentary layer called the Domengine Formation (in the Sierra Nevada foothills, it is known as the Ione Formation and the Auriferous Gravels). It was here that coal formed in California. It was a low-grade form of coal called lignite. It had none of the quality or energy content of  bituminous or anthracite coal, but it was close to San Francisco, and it was the only coal for hundreds, even thousands of miles. Coal mining began in the Coast Ranges above Antioch in the 1850s, eventually producing four million tons before shutting down around a half century later. Several thousand miners and their families lived in five villages in the immediate vicinity, and dug miles and miles of tunnels into the hills (I've heard of upwards of 200 miles of passageways).

The towns faded away and many of the underground workings collapsed. In the 1920s, a new resource was being mined here: sand. Sand? What in the world for? For glass-making. It turns out that the sandstone of the Domengine Formation is very pure, almost 100% quartz. If you've ever stood on a sandy beach in California, you would know that the sand in the state is usually gray or brown in color because of the many other minerals that are present. It's only sands that have been transported along lengthy rivers and deposited in coastal regions where they would be washed back and forth for millennia that they reach that level of purity. Those were the conditions present as the Domengine was being laid down 50 million years ago. Glass mining took place through the 30s and 40s, and ultimately 8 miles of huge tunnels were excavated. That's a lot of glass bottles...

California paleogeography from interpretive signs at Black Diamond Mines, mapping by Ron Blakey of Northern Arizona University

So it was that last Saturday, the Geology Club at my college finally conducted their semester field trip, two weeks after the semester was done. We headed out to the Black Diamond Mines Regional Preserve, a unit of the East Bay Regional Park District to have a tour. Not a tour of the former towns...we were going underground!

We gathered outside the portal of the Hazel Atlas mine where we met Pat, the mining technician and engineer for the park. He came to work here in 1998 to prepare for the reopening the mines for tours after safety concerns shut them down in 1989 after the Loma Prieta earthquake.

We donned our hardhats, grabbed flashlights, and headed into the main portal. One thinks of mines as dark dank places, and I'll bet they were to the original coal miners, but with lighting and white colored walls of sand, the tunnel was actually fairly bright, at first not really different than walking down a corridor in a factory.

The main adit intersected one of the thinner coal seams in the Domengine Formation. The miners in the 1800s generally followed the seams until they thinned out to a foot or so. The thickest of the coal seams, the Black Diamond vein, averaged 40 inches thick

One can see in the picture above that the layers in the mine slope about 30 degrees. The miners accessed the coal seams from the lower end and excavated their way upward, allowing gravity to do some of the work for them.

Diagram from interpretive signs in the visitor center

The walls of the mine reveal other signs of the depositional environment of the sandstone. In the picture below are some crossbeds, formed in coastal dune complexes.

Farther in, the sand layers broke away to reveal symmetrical ripplemarks, which indicate oscillating waves in shallow water. Although I didn't get good shots, there were trackways of worms in some sand layers, and burrows from crabs or shrimp. Such disturbances of sediment by biologic activity is called bioturbation.

By this point we had gone several hundred feet into the mountainside, and the tunnel turned 90 degrees to run parallel to the surface.

We had reached the main part of the mine, where excavation was done by the room and pillar method, where large chunks of sandstone were left in place to support the ceiling of the mine, in many places 6o feet above our heads. It's not often that I am in underground chambers this big. I almost expected a Balrog to come around a corner ("You shall not pass", he said, wielding his rock hammer like a wizard...)

There were dark unlit pits too. The dark squares on the left in the picture below are rock bolts, drilled into the rock to hold unstable jointed rock in place. Pat reported that chunks of rock do occasionally split off the walls and fall. One of his jobs is to locate such rocks along the main tour tunnel and either stabilize them or pull them down. That actually sounds like fun, even if a bit dangerous.

We had reached the end of the regularly scheduled tour, and found that there is an escape route out of the mines in the event of a collapse. We continued on, following the escapeway, because there was some good geology still to be seen!

We were now following a stope back towards the surface.

We intersected one of the coal seams, and could see why the coal mining was so dangerous. The rock was much softer and incompetent. They occasionally have problems with the coal because it generates noxious gases. When the sand mine intersected with one of the old coal mining passageways, there would occasionally be a spike in carbon dioxide or other gases, and the coal tunnel would have to be isolated to keep the air fresh.

I was surprised by how good the air was. The temperature was about 56 degrees and there was usually a current of air blowing through. It was not at all like some mining tunnels I've been in. I don't think there is any worse feeling than warm stagnant air in a dark tunnel.

Near the exit, we had a view down into the Eureka slope, one of the coal mines active in the 1860s. The picture below doesn't provide the downward perspective. It sloped about 30 degrees or more and disappeared in the darkness below. Visiting a coal mine? Fine. Working in one every day of one's working life? No, thank you. I like the job I have now.

We approached the surface after close to a half mile of walking underground. We had barely begun to explore the intricate boxwork of passageways in the mine. The visitor center occupies a particularly large tunnel at the other opening to the mine (the "emergency exit").

We emerged back into the world of sunlight and blue sky.

Although there are occasional tailings piles of coal here and there, the region has recovered nicely from the mining days, and is a nature preserve today with miles of trails and picnic areas. More information about the park can be found at http://www.ebparks.org/parks/black_diamond. It's well worth a visit!