An icon lost: The Hayward fault’s Rose/Prospect curb

27 June 2016

Certain places are prized by geologists, especially teachers, for their educational value. Out-of-towners make pilgrimages to them. Sure we all enjoy the Grand Canyon, but real geologists have Siccar Point, Darwin’s outcrop, the Carlin unconformity and other obscure sites on their life lists.

One of those places was right nearby in Hayward, until very recently. At the corner of Rose and Prospect Streets is a corner curb that happened to be built precisely across the Hayward fault, where the steady progress of aseismic creep slowly wrenched it apart.

The best time series of this corner was compiled by Sue Ellen Hirschfeld, a now-retired geology teacher at UC East Bay. It goes back to 1971, and even then there was a sizable offset. Probably the curb was first emplaced in the late 1950s.

It’s a popular site for folks in the know, and there’s at least one Flickr group with lots of photos. The neighbors are probably sick of us, though. I’ve visited it many times, and I sometimes took pictures. This photo is from 2006.

RosePros-mar2006

We’re looking east across the fault. This side is moving northward a few millimeters per year. I came back the next year and took this shot of the “echelon cracks” in the street, with the iconic curb in the corner.

RosePros-sep2007

In 2012 I brought a few enthusiasts to see it; they asked for anonymity but I can show you where they stood.

RosePros-may2012party

A closeup at the time shows that it had a total offset of about 7-1/2 inches, or 20 centimeters, since it was built. The painted arrow at the left shows the offset in the six years since 2006.

RosePros-may2012

Last week I joined a party of visitors there, and to my dismay the corner has been dismantled. It looks like the plan is to put in a cutout for people with disabilities, which is a good thing and undoubtedly overdue. Still.

RosePros-jun2016

Anyway, I’m here to put the word out: Rose and Prospect is defunct. It is no more. Come back in 20 years. In the meantime, downtown Hayward is full of other examples of bent curbs.

Haywardfault-downtown

There’s always the Old City Hall, too, which was built directly on the fault and has long been abandoned. The first time I visited there, maybe 25 years ago, I was looking at the street adjacent to it. As I watched, a little tongue of water emerged in the center of the street and started trickling downhill. Assuming that an old iron water main had just cracked, I found a phone booth and alerted the city. Cleaning up after a creeping fault never ends.

Faceted spurs along the Hayward fault

20 June 2016

A lot of geology involves glimpsing the ideal behind the real. As you look around Oakland, the Hayward fault isn’t easy to see without a bit of training. For this post, let me start you from the ideal. The process of faulting has very specific effects on the land that you can learn to look for, then see.

Motion on the Hayward fault is mostly sideways, but a small proportion of its motion is compression across the fault. Compression has been pushing up the east side of the fault for at least the last million years, building the Berkeley/Oakland Hills. Where streams cut their valleys across such a fault, the ideal result is something like this example from the Manti-La Sal National Forest, in Utah. The image is from the Open Topography site and is derived from lidar (laser “radar”) data.

faceted-spurs

The flat, triangular faces of hillside are called faceted spurs. As you look at this, imagine the motions and processes that create the landforms. The high part is being raised; the streams are cutting downward; the low part is sinking while the streams dump their sediment onto it.

That’s about as geometrically perfect as faceted spurs get. In Oakland, they’re much more subtle. The rest of the images in this post are large; click each one to see it full size. Here’s a 1000-pixel view looking north from the top of Mountain View Cemetery.

spurs-pano

The numbers are as follows: 1 is the north side of Claremont Canyon, 2 is the south side, 3 is Grizzly Peak, 4 is Hiller Highlands, and 5 is the nameless ridge (Powerline Ridge, I guess I’d call it) south of route 24 as you approach the Caldecott Tunnel. Except for Grizzly Peak, those are faceted spurs.

Here they are labeled in the 1915 topographic map, made before the Caldecott tunnel construction changed the hills. The asterisk is where I was standing.

spurs-1915topo

I’ve also marked them in this view from above in Google Maps, turned so the fault runs straight across the image.

spurs-google

And finally, here’s the same view in Google Earth, including the lidar data along the fault. The beauty of lidar data is that you can digitally subtract buildings, trees and so on to show the pure shape of the ground.

spurs-GE-with-lidar

Do these help you see? I hope so. There are other faceted spurs in Montclair and around Sheffield Village, at the far east end of Oakland.

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.