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Geology FAQ

Mount Diablo Geology Frequently Asked Questions

Is Mount Diablo a volcano?

No. Although basaltic lava rock can be found on the mountain, the lava was formed far at sea, upwelling from deep in the earth through fissures in the ocean crust.

How high is the mountain?

3,849 feet. The summit is actually inside the museum. 

What kind of rock makes up the summit?

The hard resistant rock on the summit is mostly greenstone (a slightly altered form of basalt, a common igneous rock that makes up much of the upper part of the ocean crust) and hard reddish chert with minor amounts of graywacke sandstone and shale. The exposed rock that you can stand on inside the summit museum is greenstone. 

Why does it stand up higher than the surrounding area? 

The rocks have been folded and lifted by compressional stresses in the earth’s crust. The greenstone and chert on the two main peaks are very resistant to erosion compared to many of the rocks in the surrounding areas and thus stand higher.

Is Mt. Diablo still rising?

The stresses that folded and raised the mountain are still at work and the mountain continues to slowly rise (about 0.1 inch a year) while the forces of weathering and erosion try to keep pace. 

How old is the mountain? 

The oldest rocks on the mountain are the greenstones in the mountain’s core formed about 190 million years old. But as a topographic feature, the mountain is relatively young, forming only during the past one million years or so.  

Is there any gold or silver here?

Minor amounts of gold and silver associated with small copper deposits on the north side of the mountain were prospected, but production was not economical. 

Are the mercury mines still in operation? 

No. Mining operations stopped in the early 1970s after approximately $1,500,000 worth of mercury had been extracted. 

What are they quarrying on the north side? 

Diabase. Diabase is a dark igneous rock of fine crystalline texture. It is used primarily for building stones and crushed rock for roadbeds. 

Are there dinosaur bones in any of the rocks? Did they live here? 

No to both. The Mount Diablo region was under the ocean during the Jurassic and Cretaceous. The closest dinosaur bones to us were found near Pacheco Pass in central California. 

What about the bones at the Blackhawk Quarry? 

The quarry on the south side of the mountain is one of the richest mammal bone beds in the United States, second only to La Brea in Los Angeles. Bones from 9 million-year-old horses, camels, rhinos, and mastodons have been found here—but these are much younger than dinosaurs, which died out over 60 million years ago. The bones (only fragments are found, no complete skeletons) were preserved in stream deposits.

How old are the fossils in the building blocks of the Summit Building? 

The clams, oysters, and other fossil shells in the building stones of the summit building came from the “Briones Formation” of Miocene age near Fossil Ridge and are about 12 million years old. 

How are the caves formed at Rock City?

Although often called Wind Caves, they are only indirectly due to the wind. During the rainy season, water seeps into the rocks. The water is a very weak acid and slowly dissolves the cement that had been holding the sand grains together. The hot dry days of summer would draw the water to the surface of the rock where it evaporated and left behind the cement it had dissolved from deeper in the rock. After centuries of wet winters and dry summers, the inside of the rock had only a little cement, while the outside had a hard crust of sand grains tightly cemented. As wind and rain continued to beat on the faces of the rocks, holes developed in the crust. This exposed the loosely cemented grains underneath, which erode much faster than the surface, so that holes enlarged into little caves behind the hard crust. Very little of the hard crust remains on the surface of these rocks today.

This text was reproduced from "Geology of Mount Diablo - A Training Manual" by Roi Peers

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