BZZZTTT!!! The San Andreas is a strike-slip fault, which only involves horizontal motion, as the two continental plates are sliding in opposite directions. Cecil confused it with a thrust fault, like the one off the west coast of South America, whereby one plate slides underneath the other.
So a major break on the San Andreas will NOT cause any coastline to sink underwater. All that will happen is move Los Angeles a bit closer to San Francisco, which in itself is cause for alarm.
I dunno. I think that Cecil was saying that there might be some up-and-down as a side effect. Even thought the plates are moving antiparallel to each other, I would be surprised if it weren’t possible for some of the motion to get transferred into vertical movement.
For example, I can imagine that the earthquake that a strike-slip fault would cause could make some layers of earth collapse, causing a lowering of the area and possibly flooding.
Additional geological faux pas: California will not be subducted beneath the Aleutians. Continental crust is too bouyant for subduction. Any attempt to do so results in something akin to the Himalayas.
How dare you?! Don’t you understand that this means the end of civilization as we know it? The Master never makes a mistake. You either misread him or Ed changed his words.
From this Stanford site: “San Francisco Bay is there at least partly because the block between the San Andreas on the West and the Hayward fault on the East has been downdropped a bit.”
RM, don’t be obtuse. The rest of that sentence says
So, what are you objecting to, his use of “only”? I think it’s clear from the description that any vertical movement is incidental. Certainly nothing on the order of either an uplift (mountains) or subduction (sinking under the neighboring plate).
In the strictest sense the San Andreas as a" lateral transdexterous strike-slip fault" or something like that. But the San Andreas is not the only fault in California. There are dozens of other faults (Garlock [a lateral transinterous strike-slip fault], Elsinore, Hayward, Coronado Bank, etc.) all crunching or sliding against each other. California even has a rift zone located under the Salton Sea heading south to the Gulf of California and on out to the Pacific (that why Baja CA is a peninsular–its shearing and sliding away from mainland Mexico.
However, this tectonic collision does not act alone. Other areas of the North America plate have an effect on the San Andreas. The Basin and Range (Nevada & Utah) which is spreading and thinning (the crust is only ten mile thick in some areas) to the east has induced a kink along the SA fault in southern CA. A physical manifestation of this is an east–west mountain range known as the “Transverse” range (the San Gabriel Mountains would be part of this range). Clearly, this is demonstrative of an uplift along the San Andreas. The same goes for a “down lift”; in other words., a part of the earth that bows down like a valley (known as a syncline). An example of this would be the coastal area of Ventura county. Off the coast of Oxnard in the Santa Barbara Channel lies one of the deepest sedimentary fills in the world @ 36,000 feet+. This is an area where inland sediments have filled in this dip. To see the opposite end of these dips (the anticycline), drive west along Highway 101, past Ventura. You will note to your right, a set of low hills several hundred feet high that peter out along the coast by the time you hit Carpenteria. If you look closely, you’ll note a several cuts on the profile of these mountains. That is the result of periodic uplifts which have occurred in the past 200,000 years and ocean eroding cuts into the mountains as the rise occurs. For perspective, the volcanic Mt Shasta at ~14,000 feet is about the same age.
It’s a question of an academic convention verses reality. Southern California is a case of reality: Two plates sliding past one another, but NA plate is getting “old” and suffering some middle-age spread in various areas (The Rio Grande valley in NM, along with Basin and Range and the Imperial Valley in CA). Of the three, the latter two make their presence known day to day to those in California.
Besides, part of the San Andrea is underwater, courtesy of the man-made Salton Sea which, I might point out, lies below sea-level.
So. . . Unca Cecil was right. You just have to dig a little deeper.
I think you mean “oblate,” and I’m working on that.
The OP makes it clear what his objection to Cecil was–and my cite certainly supports Cecil. That’s the whole issue, in this thread, as the title is “Cecil made a mistake!!”
The OP says “So a major break on the San Andreas will NOT cause any coastline to sink underwater,” whereas it appears that breaks on the San Andreas have done so in the past.
So how far east of Yuma do I need to go to avoid becoming part of the Gulf of California (assuming no rise in sea level)? And when should I pack my bags?
Well, you do get a bit intense about geological discussions, but I’ve not witnessed you burning incense and chanting.
But rereading the column vs. what KGS says, I agree he overstated his point and that what Cecil said is compatible with the motion would mostly be sideways, with some possible vertical movement.
Appreciate that. Cecil has an unusually high SNR, considering his propensity for abrasive responses. No sense changing what ain’t broke, just see what happened last time.
And let’s just say I’m bulging a little at the equator, OK? We can leave religion out of this, thank God.
Um, isn’t the western part of California actually part of the Pacific Plate? If so, it is not Continental crust, but basaltic oceanic crust, which is more dense and may subduct at the Aleutian Trench.
Yes, the crust west of the San Andreas fault is grafted to the Pacific plate, but that doesn’t make it basaltic. In fact, it is the usual granites, etc. seen in “continental” crust.
But won’t it follow the rest of the plate as it subducts?
I am asking, not telling. I teach high school Earth Science (among other things) and would like to get this straight. We are covering Plate Tectonics right now.
IIRC, oceanic (basaltic) crust is about 10 g/cc, and continental (granitic) crust is about 7.5 g/cc.
The oceanic crust is fractionated less from the mantle than the granitic crust. The logic is that since the continental crust is less dense, it will not go down a subduction trench very far before popping back up.
This is how continental margins accumulated continental crust during the early epochs of Earth’s geologic history. It was a bit like a conveyor belt of new granitic rock being added to the continental margins.
