(Please understand the following information was gleaned by a rockhound, not a geologist, I’m trying to condense millions of years of geology into a few paragraphs.)
“Where were we when we saw seashell fossils in the stone building blocks of a museum?,” my husband asked. In our rockhounding family this could have been just about anyplace in the western USA but we further thought we remembered it was on top of Mt. Diablo in Contra Costa County.
While you could conceivably make this a one day drive from Mendocino County and battle traffic instead consider spending a night near there so you can drive up, down, enjoying its unique geology and looking at rocks.
First stop should be at the Summit Museum to take advantage of the Mt. Diablo Interpretative Association’s excellent displays and materials. From here on a clear day you can see Mt. Lassen 160 miles ton the north and Sentinel Dome in Yosemite to the south. Surrounding you is a 20,000 acre state park and 90,000 acres of protected lands in preserves, open spaces and parks.
At 3,849 feet this is a relatively young mountain made of very old rocks. It’s complex geology that can be divided into three groups. First comes the Mt. Diablo Ophiolite plate tectonic form providing a major part of the material for metamorphic rock. Not to dive too deep into technical jargon eons ago continental plates and oceanic plates collided in a subduction zone. In the Jurassic-Cretaceous era the old ocean crust started laying down materials to mask this mountain 265 million years ago (myo). Driving around visitors can find pillow basalts and diabase formed in dikes between the pillow basalts. Serpentinite, the California State Rock, is exposed on the mountain where there is noticeable change in vegetation patterns due to high magnesium.
The second group of rocks were formed during the Mesozoic era with geologic plate subduction that mixed a wide variety of rock materials.
This formation reflected 140 million years of uninterrupted east dipping subduction. Chert, graywacke, sand stone and shale were scraped off the subjecting plates, mixed together, and began forming new rocks. Franciscan complex rock is dark red and resistant to erosion and forms geologic features on the mountain and is 108 to 90 myo.
The third group of rocks were the Great Valley(Jurassic & Cretaceous) and Younger Sedimentary (Cenozoic). There were thick sedimentary layers laid down under water turning into sandstone and shale. During this time span in the Eocene era (35 to 55 myo) coal beds and glass sands were deposited in what is now Black Diamond Mines Regional Park on the north side of the mountain. Turritella fossils of marine snails, and caves, and open tunnels are accessible from a park road.
In the Miocene era (24 to 5 myo) subduction ended and faults began rearranging rock strata. Today you find steeply tilted formerly horizontal layers of rock forming Fossil Ridge. Building materials quarried here for the Summit Museum show clam and oyster shells on the exterior walls. Shell Ridge Open Space in Walnut Creek, just down from the mountain, have well exposed fossil marine shell beds.
It wasn’t until the Pliocene era (5 myo to present day) that Mt. Diablo grew to a geographic feature. The 4 myo tuff beds widespread around the mountain was laid on a relatively flat landscape, then steeply folded by earth forces into Mt. Diablo. There is no evidence the mountain area was above sea level in the Eocene era, but now the mountain keeps rising 2 millimeters a year. And something to look forward to? The US Geologic Survey in September 1999 projected a 4% probability of a 6.7 or larger earthquake on the earthquake fault under the mountain.
Important rocks and minerals mined around the mountain? Mercury, diabase, graywacke, white sands, blue schists, travertine and copper are found.
From 1863 forward mercury, also called quicksilver, was mined on the northeast flank of the mountain. Demand for mercury expanded during WWII and mining continued until 1958 and $1,500,000 worth was extracted. Diabase was mined to be used for crushed rock and rip-rap. Starting in 1860 40,000 pounds of copper was collected. White sands were used in glass making from 1920-1949. Coal was mined for industrial centers in the San Francisco Bay Area with 4,000,000 tons valued at up to $20,000,000.
So all you rockhounds out there, and anyone just interested in one big mountain with a lot to see, go check out Mt. Diablo — it’s worth the drive.
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