Archive for August, 2009

Geologists at work

31 August 2009

cpt truck

A local gas station has been undergoing remodeling, but one day it had geo-specialists pay a visit. On the left is a small drilling rig, and on the right is the typical panel truck used for cone penetrometry testing (CPT). It’s propped up on stout legs to make a stable platform, and as I peeked underneath I could see the penetrometer shaft sticking into the ground like an ovipositor.

CPT consists of pushing a steel shaft with a standard cone-shaped tip straight into the ground. Sensors on the truck measure how much power it takes to do this in the tip measure the pressure, which varies as the tip penetrates different types of soil. Sensors in the tip also measure the electrical conductivity of the soil, pore-water pressure, and other things. In deep ground like this part of Oakland, a CPT tip can be pushed hundreds of meters down, but here they’re probably going down no more than 10 or 20 meters, just deep enough to see if the underground fuel storage tanks will be stable there. In chronically wet ground, empty tanks have been known to rise out of the ground during, say, earthquake shaking. Learn a little more on my About.com site.

This is hardhat work, but it’s not very dirty. Geotechnicians can get steady and varied work doing CPT with no more than a high-school education. The crew of this truck included a woman, too.

By the way, I have entered the “Blog Your Way to Antarctica” contest, which runs through September. Please see my entry here, and if you like the idea, give me a vote—the earlier the better. I know it would mean a break in this blog, but I’d make it up to you.

Elks Peak

25 August 2009

elks peak

The Elks Order has a picturesque plot in Mountain View Cemetery on a south-side hilltop. This elk-topped crypt is surrounded with rugged boulders of serpentinite, probably of local origin. The stone is well chosen, being inhospitable to plants and moss, and adds a bit of miniature grandeur.

Cemetery Creek canyon Moraga Canyon

18 August 2009

cemetery creek canyon

The valley of Glen Echo Creek, “Mountain View Valley,” is much larger than the current creek has the power to excavate. This is especially apparent at its head end, above Mountain View Cemetery where the creek is also known as Cemetery Creek. In this view (click it for a larger version), the valley is a steep-sided, tree-filled dogleg, almost a gorge, with homes perched high above a landfilled floor and Moraga Avenue running through it. I think of it as Cemetery Creek Canyon. The Google map below gives a feeling of how odd the canyon is, almost a box canyon with no sizeable watershed to feed it.

UPDATE: By the way, a story in the paper referred to this as Moraga Canyon, so I should probably call it that too, even though Moraga itself is way over the mountain. Once upon a time, Moraga Road did go to Moraga.

I wonder if, like Dimond Canyon, there is an explanation that involves a beheaded watershed from movement on the Hayward fault. The rock at the valley head, above the narrowest spot at Maxwellton Road, is Franciscan mélange, which is not an especially rigid rock. A bit of compression across the fault might squeeze it up, like clay in the hand. I don’t have a way to test this hypothesis, with rock exposures so scarce there, but the idea tickles me every time I look here.

The Tunnel Road quarry

13 August 2009

As you go up Tunnel Road past the turnoff to Route 13, on the left is a long expanse of bedrock. Right now it seems to be at its best.

tunnel road quarry

Click the photo for an 800×600 version. This is mapped as a little outlier of the same rock exposed in the big Leona Quarry, a keratophyre of Late Jurassic age (that would be about 150 million years old) also known as Leona Rhyolite. It’s pretty hard to study, but easy to just admire. Watch the traffic, though. A bit further up is this thick fault zone:

vein

It has signs of more than one episode of motion at some long-ago time: cracks recracked, layers sheared. If you aren’t required to decipher it as part of some devilish class exercise for a Cal geology course, it’s fun to contemplate. Hammers are OK, too, but won’t do you much good. This rock is heavily altered, but it apparently was originally volcanic and shallow subvolcanic material, of a composition richer in silica than seafloor basalt—perhaps part of an island arc that was carried up against North America back in the late Mesozoic.

This whole hillside was quarried at one time before it became part of a major highway route. It yielded rugged yellowish rock useful for filling things in and not much else. The stone of much of the WPA hardscaping around town may have come from here.

Stonewall Road View

8 August 2009

stonewall view

If you go up Stonewall Road, pretty soon you’re high above the Claremont Resort and the rest of Oakland. The contrast in elevation across the Hayward fault is very great here; it may be the steepest scarp on the whole fault (although Revere Road, at the other end of Oakland, is a contender). Everything in this view is across the fault, except possibly the house below on the right. Click the photo for an 800-pixel version.

When a big earthquake strikes this stretch of the fault, shaking will be very intense, with seismic energy coming from north, south and below. Trees will snap off at their trunks. Boulders will come barrelling down from above. Every car and burglar alarm on the street will sound, during the mainshock and during aftershocks for weeks afterward. Some homes will fall down the hill. Water and sewer lines will break and begin leaking out of the ground. Natural springs will arise at the same time. And smoke from dozens of nearby fires will begin to fill the air, and the sea breeze will push flames toward the hills.

Tanglewood Path

5 August 2009

hayward fault

Tanglewood Path crosses the Hayward fault just on the Oakland city line (at the chainlink fence); you can see the rightward offset of the path just above a set of steps. (Remember that wherever you look across the Hayward fault, the other side is moving to the right.) I’m standing at the first bend in Stonewall Road, looking west into Berkeley. This area is already unstable being on a steep slope, and the disruption of the path is as much due to landsliding and soil creep as it is to the fault. But the slope of the hillside naturally tends to push everything to the left, whereas the bend in the path goes the opposite way.


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