Sibley Volcanic natural history

Geology of Sibley Volcanic Park

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Sibley Volcanic natural history

Oakland, California 94611, United States

Created By: Fin, Hoof, Wheel

Tour Information

This field trip will be to Robert Sibley Volcanic Regional Preserve, one of the original three parks established in the East Bay Regional Parks District, along with Tilden and Temescal (link). The park was originally called Round Top, a reference to the ancient volcano that forms the highest peak in the area, and which is the focus of this trip. The rocks here tell a dramatic history of the East Bay Hills, where megafauna roamed a landscape with shifting river beds and occasional lava flows. On your visit to Sibley, you will be able to peer inside an extinct volcano to fathom several millions years of local history in a way that is not possible elsewhere in California.


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What You'll See on the Tour

You are probably familiar with the idea of tectonic plates, the enormous chunks of Earth’s crust that float around on a viscous mantle and that constitute the continents and ocean floors. Over geologic time, tectonic plates drift, crash, ... Read more
Try to get your bearings here. Find Round Top looming above you, the old volcano now cloaked in eucalyptus trees. Now figure out which way is East, and look out over the hills in that direction. Imagine you are on a Miocene lanscape. To t... Read more
If you have walked up to this point, time to catch your breath! Use the sign to orient yourself northward for a close look at the Berkeley ridgeline. Try to find Grizzly Peak and Bald Peak (the latter may be referred to as Vollmer Peak on t... Read more
Now walk around the left side of the water tower, and carefully observe the rock face above you. Note the varying textures of the rock, such as the spheroidal structures and the crumbly, gravely folded areas. In front of the interpretive si... Read more
Take a load off on this nice bench! Check out the vegetation in front of you, both in the foreground and background. The tall trees that serve as the backdrop are mostly blue gum (Eucalyptus globulus), an Australian species that was wide... Read more
The massive pit in front of you is a former gravel quarry that was excavated by during the construction of the Caldecott tunnel in the 1930s. While the hole itself is not a natural feature, it exposes the inner workings of Round Top volcano... Read more
Not all of the rocks in Sibley are volcanic in nature. The exposure here shows layers of cobblestone, sandstone, and siltstone that would have been deposited by rivers running down an alluvial plain towards the paleo-bay to the east. We kno... Read more
This is a beautiful, quiet stop. Pause in front of the stone labyrinth and face the interpretive sign. Look up and to your left and notice that the the rock is the same as the basalt that you saw in the quarry pit. Now look up and to your ... Read more
The basalt here is broken up into consistent linear features that at first may appear to be depositional layers because they are currently horizontal. However, these are not bedding layers from successive lava flows or from river or lake de... Read more
The sequence of rocks at this stop tells a dramatic story. The dark basalt at the bottom of the sequence is overlain by light-colored tuff, the layers of ash that settled on the hot lava following an eruption. Like the Orinda rocks we saw e... Read more
This is an excellent spot to appreciate the variety of textures that volcanic rock can exhibit. If you are comfortable climbing up to access the rock, you can go and examine it up close. The many small holes in the rock are the vesicles ... Read more
At first glance, this large rock in front of the sign appears similar to other chunks of basalt that we have seen, and in some ways it is. But check out the crumbly, rough texture on parts of the rock face. These surfaces are evidence of au... Read more
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This site provides another great example where tuffs were baked by lava. In this case, note that the ash/tuff layer is sandwiched between two basalt flows, and it has been baked red on both sides.  Also, observe the extreme tilting of the ... Read more
Take a look at the brick-red cinder knobs indicated by the sign. Cinders, more formally known as scoria, are blobs of basaltic lava that get ejected during a volcanic eruption and harden in the air. They have a fine vesicular texture caused... Read more
Opposite you is the beautiful Siesta Valley, bordered by the low hills in front of you. The valley is the central axis of the Siesta Syncline, and all the rocks in Sibley tilt down toward the valley bottom. The layers that underlie the far ... Read more

 

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