Perhaps the most salient fact is that, although the rocks of which it is composed are very old, Mt. Diablo only began rising recently in geological terms. The rocks are old, but the mountain itself is young. 

To better understand the complex geology of Mt. Diablo, it is useful to divide the mountain’s rocks into three main groups. Each group has a different history and is characterized by different types of rocks. 

           Group 1—Mt. Diablo Ophiolite  (Jurassic) 

            Group 2—Franciscan Complex (Jurassic and Cretaceous) 

            Group 3—Great Valley Group (Jurassic and Cretaceous) and Younger Sedimentary rocks (Cenozoic) 

Plate tectonics played a major part in the formation of the Mesozoic rocks of Mt. Diablo. We now recognize at least 11 separate major plates of oceanic crust and rigid upper mantle rocks around the globe. These plates “float” on a layer of semi-molten rock, all moving against and jostling each other, creating new land forms in the process. Continents ride atop these ocean plates, being rafted along as the plates move. New oceanic crust is being continually created by the eruption of submarine volcanic material forming along ocean-spreading ridges such as the Mid-Atlantic Ridge. To compensate for the newly created oceanic crust, older existing oceanic crust is driven beneath the continental crust at subduction zones, and recycled into the earth.

                                               
                                                  Geologic Map link 

This is a very generalized SW to NE cross-section across the summit of the mountain.
The Ophiolite is not exposed in this section. The section emphasizes the strong folding of
Mesozoic and Cenozoic sedimentary formations against the Franciscan core of the mountain.

Group 1 - Mt. Diablo Ophiolite (Jurassic-Cretaceous)

It is generally believed that near the close of the Jurassic a subduction zone developed along what is presently represented by the modern California coast. The oceanic crust caught between this subduction zone and an earlier shoreline in the ancient Sierra foothills was preserved as the Coast Range Ophiolite and later partially exposed.  Ophiolites are thought to form at oceanic spreading centers in the middle of the oceans, associated with oceanic island chains (arcs), or in narrow oceans such as the Gulf of California. 

Ophiolites generally form a uniform vertical rock sequence consisting, from bottom to top, of ultramafic peridotite from the top of the mantle, mafic intrusive gabbros and/or diabase that formed one or more miles below the sea floor, and mafic extrusive rocks, often in the form of pillow lava extruded beneath water. 

The rocks of this old ocean crust on Mt. Diablo have been named the Mt. Diablo Ophiolite and is considered a fragment of the Coast Range Ophiolite. The Mt. Diablo Ophiolite underlies the mountain north of a line drawn from Long Ridge through Murchio Gap, encompassing the Zion Peak rock quarry, Mitchell Rock, and Eagle Peak. Radiometric and fossil-age determinations date the ophiolite as having been formed approximately 165 million years ago during the Mid-Jurassic.  

Mt. Diablo Ophiolite Basalt: The basalt, which makes up the upper part of the Mt. Diablo Ophiolite, is mostly interbedded pillow basalt lava flows. As the lava erupts under water, the outer surface of the flow “freezes” in contact with the water. More lava breaks through and again the outer surface “freezes.” This process leads to the accumulation of “pillow” structures and the resultant rock is referred to as pillow basalt or pillow lava. The basalt has a microscopic crystalline texture with a black to greenish-brown color, weathering to a yellowish-brown to dark reddish-brown soil. Well-developed pillows can be seen on Mitchell Rock.

Pillows shown here in the Marin Headlands are similar to structures found in the basalt flows on Mt. Diablo

 Mt. Diablo Diabase: The pillow lavas are fed by a series of vertical fissures, or dikes, that allow the molten rock from below to reach the surface. The molten material in the dikes solidifies into a rock called diabase, which has the same chemical composition as basalt, but with a coarser texture. Diabase is exposed in quarries at Mt. Zion and on Eagle Peak.

Mt. Diablo Serpentinite: Serpentinite is a rock frequently found in association with an ophiolite. 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 forms by addition of water to minerals in peridotite, changing them from olivine and/or pyroxene to

the serpentine minerals antigorite, chrysotile and lizardite. 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. In addition to the highly sheared serpentinite, ultramafic rocks of harzburgite (a variety of peridotite) and pyroxenite are present in this band as well, but are less sheared than the serpentinite. The body of pyroxenite exposed along the Burma Road Trail on Long Ridge is coarsely crystalline, sparkling in the sunlight as you walk along the trail. Exposed blocks of massive harzburgite on the westerly end of Long Ridge frequently contain veins of fibrous chrysotile. 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.

 

Diagram of a subduction zone
showing the position where the
Franciscan Complex formed.

The Mt. Diablo Ophiolite is part of the Coast Range Ophiolite that underlies the Great Valley Sequence. The ophiolite was underthrust and uplifted by Franciscan wedges driven eastward by subduction.

(courtesy National Park Service)

 

 

Group 2 - Franciscan Complex (Mesozoic)

The central Mt. Diablo summit area and North Peak is underlain by an assemblage of Mesozoic rocks that have been a puzzle to California geologists for years. Our relatively new understanding of plate tectonics and subduction has finally provided an important clue to unraveling this mystery. This diverse complex of rock types is common up and down the coastal ranges of California and has been given the name Franciscan Complex. The processes of subduction can account for the mixing of such a wide variety of rock materials.

The Franciscan Complex records over 140 million years of uninterrupted east-dipping subduction, during which the Franciscan formed as an accretionary complex. As the oceanic plate subducted beneath the continent, part of the upper section of the ocean crust (pillow basalt) and the material riding on the plate (chert, graywacke, shale, small islands, and sea mounts) were scraped off the upper part of the subducting plate, mixed together, partially subducted and accreted on and under the continental crust.  

Mt. Diablo and North Peak are composed of faulted blocks of resistant basalt and chert with some graywacke and minor shale, and are expressed topographically as rugged and jagged rock masses. Wrapping around the two peaks in a rough “figure 8” shape are the more gentle treeless slopes of “mélange.” Such a diverse mixture of rocks, is called a "mélange" by geologists from the French for "mixture". The Franciscan mélange is essentially a chaotic mixture of an intensely sheared sandstone and shale “paste” in which are embedded blocks of basalt, chert, and graywacke along with rare exotic rocks. It is often difficult to distinguish between the mélange topography and local landslides. 

Franciscan rock accretion ceased with the ending of subduction in our area. Franciscan-like rocks are currently forming north of Cape Mendocino offshore or beneath the continent where the oceanic Juan de Fuca plate is still subducting beneath North America. Recent studies using modern dating techniques and temperature history studies suggest that the Franciscan Complex appears to have undergone metamorphism around 108 million years ago at a depth of approximately 12 miles. As a result, the Franciscan rocks are frequently referred to as metabasalt or metagraywacke reflecting a history of metamorphism by heat and pressure deep underground.

Franciscan basalt: The blocks of basalt exposed in the Franciscan on Mt. Diablo are altered oceanic pillow basalt. On the surface the rock weathers to a dark yellowish-brown to dark reddish-brown while fresh exposures are grayish-green to light olive drab. It is locally called “greenstone.” The green color comes mostly from chlorite, a green alteration mineral. The basalt blocks in the Franciscan are believed to be fragments scraped off of the upper part of subducting basaltic oceanic crust.


Franciscan chert: The chert bodies in the Franciscan form prominent dark red exposures and talus slopes. Made up of silica, they are resistant to erosion and form such features as Devil’s Pulpit and Turtle Rock. Typically red in color (green and white less common), the chert layers are typically interbedded with reddish-colored shale. These banded rocks are often referred to as “ribbon chert.” The red color is derived from iron oxides. The chert in the Franciscan was formed far out at sea. Silica skeletons of minute ocean animals called radiolaria settled to the ocean floor forming a silica ooze that ultimately solidified into chert. The chert continued to slowly accumulate on top of the ocean floor as the ocean crust drifted away from the spreading center on its long journey toward subduction. The Franciscan chert ranges in age from 190 myo (million years old) to 90 myo, representing 100 million years of accumulation.

Franciscan cherts are formed from the tiny (0.5 to 1.5 mm) silica shells of radiolaria. Many of these radiolaria are tropical species indicating that the sediments were deposited near the equator and were later transported northeastward by plate movements.

 

 

Franciscan graywacke: Graywacke is less common on Mt. Diablo than the greenstone and chert. It is typically fine - to medium-grained and massive (no stratification or bedding visible). It breaks along distinct joint planes, which helps distinguish it in outcrop from the more “shatter fracturing” of the greenstone. The graywacke consists mainly of angular quartz, plagioclase feldspar, chert fragments, and dark volcanic rock fragments. Calcite and quartz occur commonly in the criss-crossed white veins. The graywacke is younger in age than the greenstone (basalt) or chert, ranging from 90 to 108 million years in age. These rocks are thought to have formed in a subduction trench environment off the coast of North America (some researchers suggest Mexico, subsequently moving north). 

                                                Photo right of Graywacke showing joint planes along
                                                Mary Bowerman Interpretive trail near summit.

Franciscan shale: Approximately 10% of the Franciscan on Mt. Diablo is made up of shale, most of which has been altered to argillite as a result of the earlier period of metamorphism. Most of this clay-sized material was probably deposited in less turbulent current conditions in association with the graywacke deposition. 

 

Franciscan exotic rocks: The most common so-called exotic rock present on Mt. Diablo is a glaucophane schist, or “blueschist,” named for the noticeable blue color of the glaucophane. Blue schist is largely altered basalt and reflects a history of hi-pressure/low-temperature metamorphism, a condition found in subduction environments and rarely any other place. On the Summit Road as you drive toward the summit, just past the Rocky Point Picnic area, you will notice a dark blue-black boulder of blueschist about five feet across protruding from the bank on the left side of the road.

 

 

Group 3 - Great Valley (Jurassic - Cretaceous) and Younger Sedimentary Rocks (Cenozoic)

The name Great Valley Group refers to the thick sedimentary rocks of Upper Jurassic through Cretaceous age that were deposited between the ancestral Sierra Nevada to the east and the subduction zone to the west on top of the ophiolite basement that underlies California’s central valley.  The Great Valley sequence is composed mostly of deepwater marine shale, sandstone and some conglomerates accumulating to a thickness of 60,000 feet near the western margin of the present day Great Valley, and then, in our area, thinning toward Mt. Diablo. The Upper Jurassic   Knoxville Formation is 140 million years old and are the oldest beds of Great Valley in this area. Great Valley deposits on-lap the Mt. Diablo area and thinner deposits intermittently covered it during this time. 

The general interpretation of these rocks is that they were deposited in the submerged central valley as intermittent underwater "turbidity currents" and the deposits are called turbidites.

On the south and west sides of the mountain, the depositional contact between the Eocene and the Miocene rocks can be recognized by the abrupt change from clean, thick-bedded, light tan sandstone in the Domengine formation (Eocene) to poorly sorted, dark gray, pebbly sandstone of the marine Miocene rocks. There is a large gap in the geologic time record between these rock units, representing erosion or non-deposition. The interval of missing time and rock equivalents includes the upper Eocene, the entire Oligocene and the lower (or earliest) Miocene.

During middle Miocene time, the general drainage was directed from the east into an open ocean to the west, a pattern similar to the deposition of the earlier Eocene. By about 10 mya (million years ago), subduction had ended in central California and there was a major change in the pattern of deposition. A highland developed to the west and the Diablo Range south of Livermore began to rise. The Mt. Diablo area began to accumulate marine and later non-marine deposits from these sources. Now steeply tilted upward from an original horizontal orientation, the vertical beds form the prominent “hogbacks” on Fossil Ridge and Blackhawk Ridge. Building material quarried from Fossil Ridge was used to construct the summit museum building, and numerous clam and oyster shells can be seen in the exterior walls of that building.

DEVILS SLIDE
Hogback ridges plunging into Sycamore Canyon
as viewed from South Gate Road. The ridges are in the Briones Formation (Miocene).

These fossiliferous beds are called the Briones Formation. Following Briones deposition, the direction of sediment transport shifted again, bringing sands derived from the east, rich in volcanic material washed from the Sierra highlands. These volcanic sands have been named the Neroly formation. They form the grass-covered rounded hills immediately south of the underlying ridge-forming Briones strata on the south, and can be found on the west and north sides of the mountain as well. 

Andesitic Neroly sandstone alters easily, and in most places the sand grains are coated with a thin layer of bluish clay that is clearly exposed in an often visited site in Shell Ridge Open Space in Walnut Creek. Beds rich in fossil marine shells are well exposed at this site and also in Sycamore Canyon on the southern flank of Mt. Diablo. Around nine million years ago, during the late Miocene, the sea again receded from the Mt. Diablo area, marking a permanent change from marine deposition to non-marine stream and lake deposition. 

One of the nine million-year-old stream deposits on the south side of the mountain has captured and preserved an abundant and diverse collection of animal fossils. The Blackhawk Ranch Quarry has yielded numerous vertebrate fossils of horses, rhinos, camels, and smaller animals. A large mastodon skull, a Gomphotherium, has been removed from this site. All give evidence that late Miocene mammals abounded in the newly created forests and flood plains stretching away to low hills to the west and south. There are several volcanic tuff deposits in the late Miocene and Pliocene derived from the volcanic fields of Sonoma County. There was still no Mt. Diablo at the time.

Plio-Pleistocene to Recent Rocks (5.3 mya to present):

Non-marine deposits continued to collect in the area during Pliocene time (5.3-1.8). It was during Plio-Pleistocene time, by 4 mya and continuing to the present, that Mt. Diablo was formed as a topographic feature. From that time on, Mt. Diablo has been feeding erosion materials into surrounding valleys. Pliocene sources were predominantly from Great Valley rocks. Pleistocene sources were predominantly from Franciscan, indicating unroofing and erosion of deeper terranes. The 4.83 million-year-old Lawlor Tuff is a widespread marker bed around the mountain. The fact that it was laid down on a relatively flat landscape and is now steeply folded indicates that Mt. Diablo must have begun its growth after the tuff was deposited.

PART II:    FORMING THE MOUNTAIN - Mt. Diablo’s Tectonic History 

Although Mt. Diablo is old in terms of its rock history, it is very young as a topographic feature. The rising of Mt. Diablo to its present height is the result of a complex interplay of tectonic forces.  

The Franciscan Complex presumably underlies all of Contra Costa County. It was emplaced below the Coast Range Ophiolite by accretionary faulting during Cretaceous time, so the contact between the Franciscan and Coast Range Ophiolite (locally called the Mount Diablo Ophiolite) as well as the overlying Great Valley Sequence is everywhere a fault plane. This fault is known as the Coast Range fault. 

Recent field studies suggest that following the emplacement of the Franciscan Complex at depth in this area, the rocks underwent a period of metamorphism. This appears to have occurred at depths of approximately 12 miles. During late Cretaceous through early Eocene time, the overlying cover rocks were significantly thinned by extensional faulting along the Coast Range Fault plane and ductile thinning in the serpentinite component of the ophiolite. As a result of this slow structural thinning of the overburden, the Franciscan rocks of the future mountain rose vertically to a depth of around two miles. There appears to be no evidence that these rocks were above sea level during this period.  

The final two miles of uplift and exhumation of the Franciscan Rocks occurred during Pliocene to present time. It was during this last phase that the Franciscan rocks, and overlying Great Valley strata, were folded by compressional forces associated with what is believed to be a blind thrust fault beneath the mountain. These geological processes have created a complex uplifted compressional asymmetric fold that has been moving southwest on the blind thrust. The recognition of the possible existence of a blind thrust fault under Mt. Diablo has resulted in the issuance of an advisory by the USGS in September, 1999, predicting a four-percent probability of a 6.7 or larger earthquake occurring on the blind thrust fault underlying the mountain.  

Geologists believe the mountain is still rising at about 2 millimeters per year. Extensive erosion has exposed the Franciscan in the center of the fold to produce the majestic Mt. Diablo we see today.

 

PART III:     MT. DIABLO MINING HISTORY - Boom to bust

The most important minerals and rocks that have been mined or excavated on and around Mt. Diablo include mercury, diabase, graywacke, white sands, coal, blue schists, travertine, copper, and farther north and east, gas and oil. 

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.   Ryne Mine produced most of the cinnabar while metacinnabar was produced at the Mt. Diablo Mine.

A man named Welch discovered cinnabar at what is known as the Ryne Mine in 1863.  It operated for about 10 years before becoming uneconomic.  In 1933 it was discovered that black metacinnabar in the area known as the Mt. Diablo Mine also contained mercury and was more abundant than cinnabar.  The demand for mercury during the second world war resulted in an expansion of operation that was to continue until 1958 when mining operations again ceased.  It is estimated that about $1,500,000 worth of mercury was extracted from the mines.  Unfortunately a continuing legacy of the mining is the acid mine water emptying into and contaminating Marsh Creek. 

 

Building Stones and Rip-rap

The diabase quarries (left) on the northside of the mountain (Zion Peak) are currently being excavated for crushed rock and rip rap material.  There were several excavations in graywacke on the northside of the mountain for the same purposes,  but they are now abandoned.  Blue schist from the Franciscan rocks on Mt. Diablo yielded good dimension stone and was popular for building construction due to its color. 

Copper and Precious Metals

About 40,000 pounds of copper was produced from the mines in the diabase in the 1860's, but there is no activity now.  Minor amounts of gold and silver associated 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).

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.

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.    Finally closed as newer and cheaper energy sources were discovered, during its lifetime the mines produced approximately 4,000,000 tons of coal valued between $15 and $20 million. 

At the base of the Domengine Formation exposed in the Black Diamond Mines Regional Park, there is a thin section of white sands called the "Ione" sands, a description carried across the Central Valley from major white sand deposits in the Ione Formation on the east side of the Valley.  The sands appear to be continuous across the valley subsurface and of equivalent age.  The white sands, that were used for making glass, were mined from two deposits in the area from 1920 until 1949 when they ceased operation. The caverns are a fascinating place to visit on guided tours. 

Gas and Oil

The Domengine Formation also acts as a reservoir for natural gas and the Martinez Formation produces oil in the subsurface northeast of Mt. Diablo.

--------------------------------------------------------------------------------

 PART IV:       SUGGESTED READING LIST

Go to Suggested Reading List

Return to Geology Table of Contents

 
Last updated:  4-09
Website editor invites visitors to send additional information on the geology of Mt. Diablo to keep the site updated and comprehensive, but still useful for the average reader.  Please send to:  mdiamail@aol.com