165 million B.C. Mt Diablo began as volcanic rock beneath the surface of the Pacific Ocean was scraped into a mass between the Pacific tectonic plate and the overlying sedimentary layers of the North American plate. As ice ages affected sea levels, sedimentation continued in shallow coastal seas. About four million years ago, the older, harder volcanic material from the sea floor forced its way up from between the two plates heaving the weaker sedimentary layers up an angle. Over time, younger rock above eroded and by 2 million B.C. the older rock we recognize as Diablo’s peaks was exposed as low-lying hills.
Mount Diablo's mineral resources were recognized by the indigeonous peoples, followed by the Mexican settlers and eventually attracted investors and businesses from the late 1800's to the late 1900's. Many of these minerals were actively mined but these activities ceased due to declining profitabilty. These resources include quicksilver (cinnabar), copper, gold, silver, travertine, coal, sand and aggregate.
Mercury: Mercury has been mined on the northeast flank of Mt. Diablo off and on since its discovery in 1863. Prior to that, Indians used the colored mineral for ceremonial purposes. The mercury (also referred to as quicksilver) occurs in the form of cinnabar (red mercury sulfide) and metacinnabar (a black mercury sulfide). The host rock for ore is silica-carbonate rock, itself formed from the hydrothermal alteration of serpentine, lying in the boundary fault zone that separates Franciscan from Great Valley rocks. The silica-carbonate rock is made up of varying quantities of silica (chalcedony and opal) together with magnesian carbonates and stained rusty red by alteration of iron sulfide minerals. The rock is commonly spongy in appearance. Topographically, silica-carbonate rock forms resistant outcrops. It is believed that the mercury minerals were deposited from hydrothermal solutions which formed mostly in fractures in the silica-carbonate rock.
Serpentinite: Serpentinite is derived from the basal portion of the original ocean crust and uppermost part of the mantle, but has been metamorphosed by hydration from ocean water circulating through fractures in the ocean crust. Serpentinite, incidentally, is California’s state rock. On Mt. Diablo, serpentinite occurs in several localities. The largest is the prominent east-west band that runs through Murchio Gap extending west along Long Ridge, separating the ophiolite on the north from the Franciscan rocks exposed in the central core of the mountain to the south. This band is characterized by a noticeable change in vegetation due to the high magnesium content of the serpentinite. Exposures of the serpentinite are typically pale green to greenish-gray, locally black, weathering to grayish-orange. The body of serpentinite (pyroxenite) exposed along the Burma Road Trail on Long Ridge is coarsely crystalline, sparkling in the sunlight as you walk along the trail. There are several pods of silica carbonate rock (altered serpentinite) found in association with the mercury mines on the northeast flank of the mountain and other scattered locations along the serpentinite band.
Travertine: Travertine, a finely crystalline massive calcium carbonate deposit frequently associated with hot springs, was quarried along the northside of Mt. Diablo (Lime Ridge) for many years by the Cowell Cement Company.
Copper and Precious Metals: About 40,000 pounds of copperwas produced from the mines in the diabase in the 1860's, but there is no activity now. Minor amounts of goldand silverassociated with the copper were also produced. It was rumored that the best area to discover gold or silver was in the Back Canyon area (unfortunately inside the park boundary).
Coal and White Sand:North of Mt. Diablo and outside the park in the Black Diamond Mines area, lignite coal beds in the Domengine Formation were the largest known and most extensively mined coal deposits in California. From the 1860's to the beginning of this century, the Mount Diablo coal field supplied coal to the rapidly expanding urban and industrial centers of the San Francisco Bay area.
Mount Diablo's greatest value now lies with its recreational use as an open space and natural habitat.
In order to claim a find you will need to send me an email with the following information:
1. Name of this cache
2. Geo-handles of the cachers in your party
3. Pick up a rock near the posted coordinates that matches the one of the minerals described above. Using the descriptions above, identify the the type of rock and the possible minerals in that rock. Provide the coordinates where you found it, and if possible, post a picture of that piece of the rock.
4. Look around you, why do you believe that this area is was named Prospector's Gap