The Leona “rhyolite” is one of Oakland’s most intriguing rock formations. We have other volcanic rocks here — the true lava flows at the Sibley Volcanic Regional Preserve — but the Leona is ten times older and has a very different story.
I put the word rhyolite in quotes because the rock isn’t technically rhyolite, although geologists used to think it was. Rhyolite is a type of lava, generally light-colored and very viscous, the kind of stuff you see in the Inyo Domes south of Mono Lake, or at Lassen Peak.
Instead, the Leona was originally a thick pile of mostly volcanic ash, part of a chain of volcanoes out in the deep Pacific Ocean. Volcanic ash is a glassy material. Later it was invaded by actual lava flows and hydrothermal features like the “black smokers” of the deep sea floor. These things cooked the ash beds into hard rocks as the glassy ash broke down (devitrified). The result looks somewhat like rhyolite, but it’s formally called quartz keratophyre on the geologic map. Cliff Hopson, a leading expert in this part of California geology, described it in 2008 (GSA Special Paper 438) as the top part of the Coast Range ophiolite, “mostly altered, devitrified volcaniclastic sedimentary rocks” of the “volcanopelagic remnant,” a mixture of ash and deep-sea ooze.
The hydrothermal activity, in particular, added sulfur into the mix in the form of the mineral pyrite. The mineral oxidizes upon exposure to air and rainwater, yielding sulfuric acid and iron oxides.
Over the years on this blog, I’ve documented acidic waters draining the Leona “rhyolite” almost everywhere it’s exposed. The most notorious place is the former sulfur mine at the top of McDonell Avenue, where “yellowboy” oxides stain the streambed below.
Would-be pyrite miners have poked their picks into the Leona all over the place. I recently located a long-abandoned adit — a horizontal tunnel — left behind by one of those guys.
You don’t want to go here. The adit is about 10 meters long, smells funny and is lined with a powdery deposit signifying a steady decay. The city ought to seal it, but it’s difficult to find and is well enough left alone, so far.
Nearby are numerous pits with the telltale red-brown linings that develop in the Leona after a few decades of exposure.
Bits of rock below the adit, with the pyrite leached out of them, are very lightweight. Elsewhere I’ve seen this stuff turn to pure clay.
Experience has shown me that the Leona seems to release acid drainage wherever an incision is made in it. Had I read the literature first, I’d’ve known this long ago. In a 1969 report for the U.S. Geological Survey (Map GQ-769), Dorothy Radbruch noted, “Fresh rock contains abundant pyrite in many places. . . . runoff from rhyolite hills [is] very acid and corrodes concrete sewer pipe.”