Archive for the ‘Oakland serpentinite’ Category

A look at serpentinization

13 June 2016

Joaquin Miller Park contains excellent examples of serpentinite. This large boulder, placed by the park entrance, is a textbook example of how this rock type forms. (Click the photo for a 1000-pixel version.)

click for 1000 pixel version

Serpentine rock starts out as peridotite (“per-RID-a-tight”), a very important rock type that is rarely seen because it composes most of the Earth’s mantle, beneath the crust. Bear with me as I take you through the outskirts of plate-tectonic petrology.

The Earth’s mantle is hot and under pressure — so hot and under so much pressure that if you ease up on the pressure, even a little bit, it starts to melt. That’s what happens in places where the crust is spreading apart. Just a fraction of the mantle rock melts, only a few percent, and the melt — magma — leaks upward, seeking to erupt as lava. Magma isn’t the same composition as the rock it leaves behind. It’s enriched in elements like silicon and aluminum, and depleted in others like magnesium and iron. Silicon is the biggie that governs all of magma chemistry, and geologists track it (like other elements) in terms of its oxide, SiO2 or silica.

The mantle is real low in silica, around 40%. The first melt that comes out of the mantle is about 50% silica and hardens into the rock called basalt. The crust of the ocean floor is almost entirely basalt. Beneath it is peridotite, the badass dregs that the magma left behind.

As plate tectonics keeps sweeping the dense ocean crust back down into the mantle, remelting keeps concentrating silica in the magmas, which become less and less dense until they end up in the continents. Granite, the continents’ workhorse rock, is over 70% silica. (Granite is made of the minerals quartz, which is 100% pure silica, and feldspar, which is about 50% silica.)

The upshot of all this is that peridotite, the dense left-behind dregs rich in magnesium and iron, rides along on the bottom of the ocean crust, and almost all of it stays deep in the Earth. Occasionally chunks of ocean crust end up on land, where they’re called ophiolites (“OH-fee-alights”). Oakland contains bits of the well-known Coast Range ophiolite.

Peridotite, badass as it is, is helpless against superheated water, which reacts with its minerals (olivine and pyroxene) to form a hydrated mineral, serpentine. Let me show you what happens in these photos from the Klamath Mountains, America’s largest exposure of peridotite. This is a split-open boulder sitting in a roadside turnout up there. You can see similar examples up at the Serpentine Prairie preserve, off Skyline Boulevard.

serpization-klamath

When all of this rock was still a few miles underground, superheated water entered the gray-green peridotite along cracks, and the alteration spread outward from the cracks. Serpentine takes up more space than the unaltered minerals, so the outcome is just like driving wedges into the rock. Serpentine is also softer. That’s how this peridotite outcrop ended up looking like it does — alteration, then erosion.

serpweathering-klamath

This stage of alteration is preserved because conditions cooled off before the serpentinization process could finish. Usually, peridotite is completely altered. After that, serpentinite tends to slip and slide and flow, erasing any hints of its original structure.

Look again at the big boulder at Joaquin Miller Park. It shows those same spikes on its upper rim, plus a spiderweb of alteration cracks in the center. I find it mesmerizing.

Oakland geology ramble 1: Leimert to Redwood

6 June 2016

For a while now I’ve been envisioning geological rambles around Oakland — walks (hikes, really) that aren’t loops, but traverses. They rely on public transit, because that’s mainly how I roll. You can walk them in either direction. My ultimate idea is to work out a network of rambles that will cover the whole town. You could combine them into epic outings. This is the first ramble. It’s a little more than 4 miles.

The west end of the route is on Park Boulevard at the Leimert Bridge. The 33 bus line will get you there from either the MacArthur or the downtown BART station. Starting elevation is ~375 feet. Here’s the street route (1000 pixels):

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And here’s the corresponding geologic map:

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Briefly, the route takes you past Franciscan sandstone of the Piedmont block (Kfn), then crosses the Hayward fault into much older mixed rocks of the Coast Range ophiolite (basalt (Jb), serpentinite (sp), Leona volcanics (Jsv)) and a bit of Late Jurassic mudstone of the Knoxville Formation (KJk). (Search this site for more about all those rocks.) Remember to leave the stone alone.

Oakmore Hill looks pretty intimidating as you cross the Leimert Bridge. Part of that is because of Dimond Canyon below. The bridge is about 125 feet above Sausal Creek.

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Buy your fuel and water in the charming little Oakmore commercial district. Then make your way to Braemar Street along the top of the hill. Take any route you like. The intersection of Arcadia and Melvin, directly above the E in “Oakmore,” is a good shady spot to regroup and refresh.

Along the way you’ll see exposures of the sandstone.

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Braemar Street is nice and level. Truck along right to the end and enter the footpath like you do it all the time. As you descend the steps, look across the fault-line valley to the bare slopes of Joaquin Miller Park. That’s where you’re headed.

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Closer to hand, you’ll see that the rock has changed. This appears to be the Jurassic basalt, unit Jb.

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On the way to the freeway overcrossing at Lincoln Avenue, look at the lay of the land.

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The active trace of the Hayward fault isn’t precisely mapped here, but it runs from about the lower middle edge on the left side to the horizon directly behind the large tree (note the LDS Temple spire on the right edge). The next time the ground breaks, you’ll see it very clearly here.

Cross the freeway and take Woodminster Lane to Woodside Glen Court, where the road ends at a backdoor entrance to Joaquin Miller Park at about 700 feet elevation. Things get pretty steep here, and they’ll stay steep.

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The exposure appears to be either Leona volcanics or Franciscan sandstone; the important thing is that the bedrock changes abruptly as you enter the woods into the area mapped as serpentinite.

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Specifically, this is blueschist, the electrifying high-grade metamorphic rock that’s intimately mixed with greenish serpentine rock throughout this map unit. Enjoy the trail, which is the little-traveled west end of the Sinawik Trail, as you puff your way up to about 950 feet at Lookout Point. Stop a bit and check out the high-grade boulders there. (You’ll want to stop anyway.)

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This is where I show the route forking. It’s easier to go right, either on the trail or on Sanborn Road, going downhill to Joaquin Miller Road and across it to Butters Drive. I took the high route, up what I call Visionary Ridge, because I was returning two pieces of basalt to the locality where I got them. I thought better of that plan as I passed the park’s native plant nursery, where I added them to the little border at the bottom of this photo.

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The hillsides here are pure serpentinite and worth a close look. The high route continues along the ridge crest, around 1100 feet, to Joaquin Miller Road, where you cross and take Robinson Drive to where it meets Butters Drive at about 1025 feet. The high route will save you a loss and gain of 200 feet, but you’ll miss Butters Drive.

Butters Drive starts in some of Oakland’s most spectacular serpentine/blueschist ground, and it’s landscaped too. (See more photos from a 2015 visit here.)

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Continue past the hairpin turn into the headwaters of Peralta Creek in the Butters Canyon private preserve. Here the rock along the road is mapped as Leona volcanics.

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The intriguing thing is that right across the creek the rock is Knoxville Formation, a unit that’s generally shale and hence easily eroded. I think this contact is exploited by the creek to dig the canyon so locally deep. You can get a good look at the Knoxville right above the intersection with Robinson Drive, where the high and low routes meet again.

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Now the route plunges about 300 feet down Crestmont Drive and through Oakland’s largest area of serpentinite. Take in the prodigious exposure at Crestmont and Kimberlin Heights drives.

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The west edge of the serpentinite zone is a thrust fault, which means the rock here is quite pulverized. This part of the hike has several interesting exposures that I’ll let you discover on your own. The very easternmost end of Crestmont Drive goes through Leona volcanics, which you’ll see in boulders.

When you reach Redwood Road, truck on downhill to Campus Drive at about 650 feet elevation, where the 54 bus comes by regularly. It’ll take you to the Fruitvale BART station or connect you to major lines on MacArthur, Foothill or International boulevards.

Knowland Park knockers II: Rocks other than chert

2 November 2015

The distinctive landscape of Knowland Park owes much to its large exposure of Franciscan melange, in which lumps of various rock types stick out of the ground like raisins in pudding (or whatever culinary simile you prefer). A few weeks back I featured the chert knockers, because there are so many, and this week’s subject is the ones that aren’t chert.

Here’s the geologic map showing the Franciscan area, labeled KJfm (for “Cretaceous/Jurassic Franciscan melange”). The places featured in this post are numbered 1 through 8 from north to south.

knowlandKJfm-knockermap

Knocker 1 isn’t really a knocker, but an exposure in the fire road, of greenish serpentinite.

Knowl-JKfm-knocker1

I include it because there are relatively few in this piece of melange. Other melange areas, like those in San Francisco or Marin County, may be mostly serpentinite, but not here.

Knocker 2 is at the edge of a cul-de-sac overlooking the gorge of Arroyo Viejo. It’s a lovely greenstone.

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A closeup shows the greenish rock, which is a metamorphosed lava, along with its iron-rich weathering rind and the carbonate veins that are evidence of its deep-sea origin (more here).

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Knocker 3 is exposed along the road just above here, a nice graywacke, or dirty sandstone.

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The Piedmont block, Oakland’s other body of Franciscan melange, is largely graywacke.

Knocker 4 is the big one, which caught my eye the first time I set foot in the park.

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Its bluish color stands up to close inspection. This is a classic high-grade block, a body of rock that was carried deep into the Earth and returned to the surface quickly enough that the high-pressure blueschist minerals it turned into were preserved.

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You have to look closely at these rocks to see past the lichens that tend to cover every exposed surface. Geologists carry hammers to ensure fresh exposures, but rocks in the park should not be hammered.

Knocker 5 is just up the hill. I haven’t given it a good look yet, but my initial impression is that it’s lava.

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Knocker 6, across a small gully from knocker 4, is populated by a clump of trees. I think there’s a reason for that because the rock fractures nicely enough for the roots to reach deep.

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I interpret it as metamorphosed lava, from its greenish color, extremely fine grained (aphanitic) character and massive fabric.

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The next two knockers are outside the park — the Franciscan doesn’t honor property lines, and the Chabot Park neighborhood once looked just like Knowland Park.

Knocker 7 is on posted land at the end of the public part of Kerrigan Drive. I think it’s serpentinite . . .

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. . . because that’s what’s underfoot here.

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Knocker 8 is exposed along lower Lochard Drive and is too large to photograph easily. Looming over the road, it looks like basalt lava.

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But a fresh exposure shows some cryptic internal features, plus extensive deposition of iron oxides from weathering below ground.

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I’ve visited this site twice and am still not sure what to call it.

There are more knockers to be found in Knowland Park and south of the park. I plan to keep up my search to the south end of the Franciscan, along Chabot Reservoir.

Once again, I hope you’ll take part in the blog survey between now and November 20. It has prizes.

Rocks of the Skyline High neighborhood

6 July 2015

Walking the hills around Skyline High School makes for a nice workout, and there are some rocks, including serpentinite, the Joaquin Miller Formation and the Oakland Conglomerate. Here’s the Google Maps topography with the photo locations.

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And the corresponding geology.

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The neighborhood centered on Balmoral Drive has lovely views, but few visible rocks. This is looking west to the Serpentine Prairie and beyond.

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But at the very end of Tartan Way we behold some mixed shale and mudstone of the Joaquin Miller Formation (Kjm).

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JMF-tartanway

The area is mostly underlain by the Oakland Conglomerate (Ko), but the only glimpse of it you’ll see is on the high school grounds behind a fence.

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I think that the rocks here were examined during the construction of the homes in the late 1960s(?), because the strike-and-dip symbols are all located off the road, presumably in the excavations.

Now we’ll cross Skyline and explore the loop formed by Fernhoff and Bacon Roads. I particularly wanted to see this because it exposes the last bit of Oakland’s serpentinite I hadn’t yet visited. And at the entrance to Fernhoff Court, here it is.

serpFernhoffCt

The westernmost appendix of Bacon Road also dips into it.

serpBacon

But just uphill on Bacon, there’s some nice bits of Joaquin Miller again.

JMF-Bacon

The best place to see the Oakland Conglomerate is on Skyline Boulevard, specifically along the footpath in the median. Some places it’s well-bedded sandstone . . .

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. . . and other places expose the classic deep-brown conglomerate with its well-rounded river cobbles.

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There’s more along the roadside, but that’s probably not safe to visit. People drive fast here.

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Thank goodness for the path in the median! It’s a nice amenity.