Voice of the Volcano
Stories in Stone at Sibley Preserve
by Gordy Slack on April 01, 2005
Ten million years ago, when three-toed horses and long-necked camels roamed our region,
Voice of the Volcano
Stories in Stone at Sibley Preserve
by Gordy Slack on April 01, 2005
Ten million years ago, when three-toed horses and long-necked camels roamed our region, volcanic eruptions tore through an alluvial fan, forming what would become Oakland's Round Top. This artist's conception shows what one of the Round Top eruptions might have looked like, with the still-volcanic Sierra in the distance.
Illustration by Carl Buell.
About 10 million years ago, something happened here in the Bay Area that must have made a big impression on the local mastodons, three-toed horses, and long-necked camels grazing on the alluvial plain that stretched east toward the young Sierra Nevada. That’s when buried magma found its way to the surface and changed the neighborhood in some fundamental ways. It was the birth of a volcano, or four, actually: a big one and three little brothers.
These days, one of those little brothers, a hill called Round Top, is the centerpiece of Sibley Volcanic Regional Preserve in the Oakland hills, just south of Highway 24 and the Caldecott Tunnel. But just where was “here” 10 million years ago? In geological time hereness is relative. According to USGS geologist Russ Graymer, the triple junction of the Pacific, North American, and Gorda tectonic plates—now off the coast of Mendocino—was then somewhat south of today’s Bay Area, working its way slowly northward, as it had been for millions of years. The tremendous forces generated by the colliding plates created a zone of subsurface molten material that finally found its way to daylight. Since then, the Pacific tectonic plate, moving northward, has dragged along the resulting geologic features, like volcanoes, that are near the edge of the North American Plate. The border between the Pacific and North American plates, usually marked as the San Andreas Fault, is actually a broad zone with numerous subsidiary faults: the Hayward, the Wildcat, the Moraga Thrust, the Calaveras, and others. The farther west the volcanoes are (closer to the Pacific Plate side), the faster they move north.
- The footprints of the volcano can be read in outcrops like this, which shows basaltic lava on the right and brick-red ash on the left. These layers were once flat but have since been turned neatly vertical by tectonic forces. Photo by Scott Braley.
So our landscape gets cut up, sometimes jarring us a bit in the process. In terms of latitude, the volcanoes were born southeast of modern-day San Jose, not in Oakland. As they were being torn asunder by the Wildcat Fault, the larger, more westward vent blew its own top off and then moved farther north and now resides under Lawrence Berkeley Labs, while Round Top resides “here,” where I stand today, in Sibley Volcanic Regional Preserve.
To the east of the then-active Round Top lay a vast alluvial plain where an inland ocean, known as the San Pablo Sea, had recently dried up. To the west, a range of hills rose where San Francisco Bay and the Peninsula are now. Rivers ran east down the range’s slopes, depositing millions of years’ worth of sediment in great alluvial fans. It was onto one of these fans that the volcanoes first spilled their lava, laying thick layers of molten iron-rich basalt over river sediments.
Those first moments of contact between the lava’s molten hot flow and the relatively mellow sediments were recorded in what geologists call contact zones, many of which are visible at Sibley today, baked brick-red by the action of steam and lava. Geologists find contact zones exciting, but the geologically uninclined might miss them altogether. Most of us haven’t been taught to listen to rocks very well or to interpret the stories they have to tell us about other times, and other kinds of time.
Before my recent visits to Sibley, the last time I thought so much about rocks was at Uluru, a kind of Grand Canyon in reverse that juts out of the pancake-flat central Australian desert. Uluru plays a key role in Australian Aboriginal creation stories, which unfold in a kind of alternate universe known as Dreamtime. In Dreamtime, the giant original creatures play out dramas that result in the landscapes we see today. Dreamtime is hard to grasp from a Western vantage point; but I did gather that it is longer (eternal), more significant (sacred), and more objective (real) than the ephemeral, subjective time of human experience. We are living in it, but unless we develop the sacred skills of perception, we are unable to sense it.
As I stood atop Uluru, it struck me that the notion of geologic time is not a bad secular counterpart for Dreamtime. Dramas engaging tectonic forces play out in geologic time and explain the formation of our everyday landscapes. Geologic time contains the short span 