Comments on: The Rocks of Leona Quarry

By: GeoMAn

GeoMAn — Mon, 24 Dec 2012 18:54:33 +0000

Geological Bull Puckey! Get a real geologist to explain this lithology. [That's not a very helpful comment. — Andrew]

By: Roland De Wolk

Roland De Wolk — Mon, 20 Feb 2012 22:56:25 +0000

roger that. thx!

By: Andrew

Andrew — Mon, 20 Feb 2012 22:49:54 +0000

If you’re talking about the Leona Quarry, call it a metavolcanic rock or a metatuff. Tell ‘em I told you.

By: Roland De Wolk

Roland De Wolk — Mon, 20 Feb 2012 21:47:59 +0000

meaning it is technically incorrect to call this handsome red rock at the quarry (and beneath my home) as rhyolite OR a volcanic basalt? what name can i accurately call the reddish rock?

By: Andrew

Andrew — Mon, 20 Feb 2012 21:21:10 +0000

Not at all. Those are very different rock types. Basalt (low silica) makes up the Moraga Formation lavas exposed at Sibley. Rhyolite (high silica) makes up the bouldering grounds in the Berkeley hill parks. The Leona Quarry rocks are of intermediate composition.

By: Roland De Wolk

Roland De Wolk — Mon, 20 Feb 2012 20:58:42 +0000

is it correct to call it rhyolite and a volcanic basalt?

By: Andrew

Andrew — Sat, 12 Feb 2011 19:28:55 +0000

Yes, I see I posted contradictory statements. The unit is mapped as the basal part of the Great Valley, but the rocks are volcanic and can just as easily be considered part of the ophiolite. Russ Graymer, in his notes to the Oakland geologic map (dated 2000), says that these appear to be arc rocks, of intermediate and silicic composition, laid down on top of the ophiolitic rocks (which would be basaltic or ultramafic). The fault between it and the younger Knoxville Formation is of indeterminate nature, so the stratigraphy is no help. If I recall correctly, it was Shervais in 2008 who put this unit in the Coast Range Ophiolite. It’s a judgment call among the experts.

Thanks for the chance to clarify.

By: Diodogo

Diodogo — Sat, 12 Feb 2011 07:07:52 +0000

The rocks of Leona Quarry belong to the Coast Range Ophiolite, not the Great Valley sequence. The Leona rocks are lower plate. They’re part of the Pacific (and/or Farallon?) global tectonic plate that was subducted, thrust, under the North American plate during Cretaceous and Paleogene time, roughly 146 to 23 million years ago. By exposure to the high temperatures of the Earth’s mantle, the Leona ocean-floor volcanic rocks were metamorphosed. (In contrast, the Great Valley sequence, as part of the upper plate, relatively speaking, stayed near the Earth’s surface and avoided metamorphism.) Also, the great amount of subduction-zone thrusting and shearing, on the order of 1,000 miles, produced the slickensides seen in (at least one of) the photos of the Leona rocks.

By: Renate Lellep Fernandez

Renate Lellep Fernandez — Thu, 30 Dec 2010 02:52:06 +0000

Is Indian Rock of North Berkeley of similar geological stuff in the “arc”? I remember it as a kid climbing around on the Rock the stuff looking just like salt and pepper granite. I don’t remember the many variations as pictured of quartz keratophyre.

By: Andrew

Andrew — Sun, 08 Nov 2009 00:29:35 +0000

I've been doing some reading about the Coast Range Ophiolite, of which the Leona Quarry rocks are a part. The most recent work I've seen considers this rock not a lava, but a highly compressed and altered tuff, that is, the ash and rubble erupted from volcanoes. So the little flat features in the last photo would be the remnants of lapilli: pebble-size particles, like bits of pumice or lava, that are now almost erased. It's thought to be about 150 million years old, unlike the far younger volcanic rocks of Sibley volcano.