Will California eventually split away from mainland America? (along the San Andreas fault)
What do you mean by eventually? If California continues moving northwest at its current rate (about 2 inches per year) it will be part of Alaska about a million years from now. Only the western half of the state moves northwest, though. The eastern half stays put. Also, Alaska is part of the North American mainland, although it is not part of the contiguous United States. Which were you referring to?
Will California become an island
...Probably neither, since the question stated split away from mainland America? (along the San Andreas fault). This makes it sound like, "Will the portion of California west of the San Andreas fault eventually sail away westward, and, in so doing, become an island?"
The San Andreas fault is what they call a "right-lateral strike-slip" fault. If you stand on one side of the fault, and if you are very patient, you will eventually see features on the opposite side move to the right.
A few years back I took the highway that runs west from Bakersfield. It runs through Buttonwillow and through the small oil town of McKittrick. Just west of there it climbs over the Temblor Range and down into the Carrizo Plain. I knew the fault runs across the highway there, and I kept looking for a sign saying San Andreas Fault, but there was no sign. There was a stretch of road (200 yards or so, I am thinking) where the pavement was newer. I wondered if that was where the fault was, and maybe they have to keep re-doing the asphalt there to fix cracks.
hitssquad was right, i meant "Will the portion of California west of the San Andreas fault eventually sail away westward, and, in so doing, become an island?"
what exactly is happening to western California in relation to the north american mainland?
It probably depends on how liberally you want to interpret "eventually." Because of the fact that the Temblor Range ("temblor" meaning "earthquake" by the way) is there, I suspect that at least in that location the fault is under some compression, and has pushed up the range immediately to the east of the fault. That would mean the two plates are not trying to separate from one another there. But in 10 million years, will that still be the case?
For the sheer heck of it, I will mention that there is a gigantic Frito Lay factory amidst the cotton fields west of Bakersfield. It has its own railroad spur, with tanker cars that may hold tropical oils.
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