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Soundings: Sierra rising

CREDIT: NPS

The Sierra Nevada today tops out at over 14,000 feet. Tomorrow it may be higher.

Demolition derby: Cars careen over the track, drivers gunning their engines, slamming on the brakes, swerving back and forth, throwing it into reverse.

Ramming, butting, scraping against each other, drivers attempt to render opposing cars junk.



Sideswipe: The scraping, sliding collision folds doors, twists fenders, mangles frames. While some metal merely gives way, other portions remain intact enough to transmit the force to other parts of the car.

Tens of millions of years ago, motions in the deep, dark rock put North America and the Pacific on course for a head-on collision.

Metal far from any point of contact twists and bends.




Geologic demolition derby: North America is a raft of rock, floating on denser rock below. The North American raft is pushed and pulled by “currents” – slow-motion flow – in the deep, dark rock below.

The floor of the Pacific is also a raft of rock; it, too, responds to currents in the denser, darker rock on which it floats.

Tens of millions of years ago, motions in the deep, dark rock put North America and the Pacific on course for a head-on collision.

Ocean floor and continental rock today meet head-on in the Pacific Northwest – extreme northern California, Oregon, Washington and BC.

Here in central California and points south, the collision has become a sideswipe.

The San Andreas Fault in San Francisco marks the line where North American and Pacific rock-rafts meet. The scraping sideswipe pulverizes the rock of both rafts.

Descending a ladder, into a trench dug across the fault, you can dig out that pulverized rock – fault gouge – with your fingers.

But not all of the rock is pulverized. Some rock remains solid enough to transmit the stress inland.

The Hayward Fault, running through the East Bay, marks another major line of motion.

Stresses are transmitted further inland yet, all the way across California.

In a region on the east side of the Sierra – eastern California, western Nevada – one finds rock pushed and pulled in response to the Pacific-North American sideswipe.

This region of eastern California and western Nevada, running roughly from Death Valley to Honey Lake in extreme California Sierra, is the Walker Lane.

The Walker Lane responds to more than just the San Andreas sideswipe.

Currents in the deep, dark rock beneath Nevada are pulling the state apart. Stretched beyond its breaking point, rock on the surface has broken into huge blocks, miles across, which bob up and down, slide and twist and tilt.

The parts of blocks that jut upward become mountains; the parts that sink form valleys.

Tilted blocks, one after the other, create the Basin and Range (valley and mountain) Province of the American West, comprising most of Nevada and parts of neighboring states.

The eastern-most rock-block of the Basin and Range forms the Wasatch Range of Utah. The western-most block, the Sierra Nevada.

The Walker Lane accommodates not just the sideswipe of the Pacific against North America … it also accommodates the stretching of the Basin and Range.

Rock is only so strong. Even as a block rises into the air it breaks, so that different parts may rise (or fall) on their own.

The evening of May 23, near the town of Susanville, part of the Sierran block shifted.

The Walker Lane is not as well instrumented as the much more densely populated San Andreas Fault Zone. (Which is not to say the quake caused no damage … it did.)

USGS geologist Dave Schwartz told me that new instruments have been deployed to the region of last week’s quake.

Aftershocks should delineate the position of the fault that moved … and, perhaps, whether the Sierra rose a bit higher into the sky.

Al Stahler’s science programs can be heard on KVMR (89.5 FM). He teaches classes to students of all ages, and may be reached at stahler@kvmr.org.


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