Orientation

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, subduct, melt into the mantle, and rise again to form fresh crust at the seams. During the reign of the dinosaurs, the ancient Farallon Plate collided with the North American Plate and subducted beneath it. The Farallon Plate disappeared almost entirely under North America, and it dragged the adjacent Pacific Plate into contact with the California coast. The Pacific Plate slides northward past the North American plate along the boundary known as the San Andreas fault. The San Andreas is a transform boundary where the two plates grind past each, generating earthquakes. Although these quakes can be destructive, they are relatively small compared to those that occur at subduction zones. And although transform faults are not normally associated with volcanic activity, the tectonic forces that created volcanism at Sibley are connected to the earthquakes that we experience in the East Bay today.

As the Farallon plate was pulled under the continent, the seam that divides the Farallon and Pacific plates was also subducted. The subduction of this spreading center created a slab window, a gap under the continent that was filled with hot, upwelling mantle, leading to volcanoes! The slab window was initially formed in Southern California around 25 million years ago together with the San Andreas fault system, and both began moving northward along with the Pacific plate. The zone of volcanism reached the Bay Area around 10 million years ago, and the eruptions created Round Top. As the slab window continued northward, the East Bay volcanics went dormant, and new volcanoes arose in Sonoma County. While the volcano at Sibley is extinct, the processes that created it are still very much a work in progress!

Geologists and geology students at Cal have long used Sibley’s rich geology as a training ground for mapping rocks, and much of what we know about the rocks of Round Top volcanics comes from Berkeley scientists. Garniss Curtis was a professor of geology at Cal and a pioneer in the field of geochronology (the dating of rocks with radioactive isotopes). Curtis is best known for dating ancient hominid fossils in Africa, but the rocks of the East Bay Hills were the testing ground for his new methods. Curtis and his students found that Round Top erupted over the course of a few hundred thousand years between 9 and 10 million years ago, during the Miocene period. Stephen Edwards, a Cal grad student, mapped the lava flows and geology of Sibley when he was preparing for his qualifying exam. Edwards designed many of the interpretive signs you will see around the park, and you will find the signage full of valuable information.