top of page

The Rocks of Mount Diablo - Their Type and History

Geology Guide Part 1

The Rocks of Mount Diablo - Their Type and History

Wind Cave | Roi Peers

Rising 3,849 feet, Mount Diablo forms a prominent feature in the East Bay landscape. Our understanding of the geological history of the rocks and structure of Mount Diablo has undergone major changes during the past 30 years, and even now geologists are still trying to unravel the complicated history of the mountain. This complex history is not unique to the mountain, but to our region as a whole, since Mount Diablo has been caught up in the processes that have shaped the Coast Ranges for millions of years.

Perhaps the most salient fact is that, although the rocks of which it is composed are very old, Mount 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 Mount 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:  Mount 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 Mount 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.

Group 1 - Mount 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 Mount Diablo have been named the Mount Diablo Ophiolite and is considered a fragment of the Coast Range Ophiolite. The Mount 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.

Mount Diablo Ophiolite Basalt: The basalt, which makes up the upper part of the Mount 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.

Mount 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.

Mount 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 Mount 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.

Group 2 - Franciscan Complex (Mesozoic)

The central Mount 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.  

Mount 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 Mount 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 Mount 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).

Franciscan shale: Approximately 10% of the Franciscan on Mount 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 Mount 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 Mount 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. 

To summarize the Cenozoic in this area, it is perhaps easiest to think of the central valley of California as a low elongate basin, flooded intermittently by an encroaching shallow sea, and slowly being filled by sedimentary material eroded from the surrounding exposed land masses, primarily the “Sierra.” During the latter part of the Tertiary, newly formed highlands to the south (Diablo Range) and in the area of the present day San Francisco Bay also acted as source areas.  

The Mount Diablo area seemed to represent a persistent “high,” underwater, but less deep than surrounding areas and periodically exposed to erosion. Many of the formations seem to shoal out on the flanks of this area and when submerged, the strata thin over this “high.” The area, however, was not a “mountain” as we see it today, but rather a land of low relief intermittently submerged well into the Pliocene.

Paleocene Rocks (55.5 - 65 million years ago)

There are few Paleocene deposits present in our area indicating that the region was probably above sea-level and undergoing erosion following the close of the Cretaceous. The only nearby rocks of this age are restricted to the north side of the mountain outside of the park.

Eocene Rocks (33.7 - 55.5 million years ago)

During the Eocene, the climate warmed, resulting in heavy “ancestral Sierra” weathering that yielded large quantities of sands that washed into and across the Central Valley providing material to the Eocene deposits of Mount Diablo. A shallow marine basin, a sandy shoreline, a swampy backwater area—all existed in this area at different times or at the same time in different places. On the north side of the mountain, the Eocene is present in the Black Diamond Mines Regional Park. These strata contain coal beds and glass sands and have been described as a near-shore lagoonal swamp or tidal flat estuarine environment.

On Mount Diablo, Eocene deposits form the ridges of tan-colored sandy rock formations that wrap around the south and west side of the mountain. These sedimentary rocks have been given the name Domengine Formation and are well exposed at Castle Rock, Rock City, Knobcone Point, and Cave Point. These sands on the south side of the mountain are characteristic of deep offshore slope deposits. They represent deep erosion of the ancestral Sierra highland spreading over the Mount Diablo area as the ocean deepened westward. Some Domengine beds represent shallower near-shore deposits that contain beds rich in Turritella fossils (marine snails). These massive sandstone beds weather easily forming features such as caves and open tunnels. Rock City, easily accessible on the South Gate Road, is a good place to view these unusual features.

These massive sandstone  that form the Wind Cave beds weather easily forming features such as wind caves and open tunnels. Unusual "cannonball concretions" can also be found in these sandstone beds. Rock City is a good place to view these unusual features easily accessible on South Gate Road.

Oligocene Rocks (23.8 - 33.7 million years ago)

The only Oligocene rocks in the area is the Kirker Tuff on the north side of the mountain outside the park boundary.

Miocene Rocks (23.7 - 5.3 million years ago)

On the south and west sides of the mountain, the depositional contact between the Eoceneand 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 Mount 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.

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 Mount 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 Mount Diablo at the time.

Plio-Pleistocene to recent rocks (5.3 million years ago 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 Mount Diablo was formed as a topographic feature. From that time on, Mount 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.

bottom of page