Iron is 7.8
Iron is 7.8
Other than that, I don’t know what to say.
I’ve got it right in front of me:
Continental Crust is 2.7 g/cm^3
Oceanic Crust is 3.0 g/cm^3
Anyways, thanks for the info, Sidewinder. It came just in time. I just showed my classes a video that was apparently incorrect.
What was incorrect in the video?
The video suggested that California west of the San Andreas Fault was basaltic oceanic crust which just happened to be thick enough to rise above the surface of the ocean.
Sidewinder says its granitic continental crust that happens to be stuck to the oceanic plate. Makes more sense to me.
It suggested it? How’s that?
OK, well, it showed the entire Pacific Plate, including that part of California, as one color, and the North American Plate as another color. Then the animation showed the entire Pacific Plate, including that part of California, moving NW towards Alaska. Then the narrator said something to the effect of “The basaltic Pacific Plate will subduct at the Aleutian Trench.”
There was never any mention of the fact that California is attached to, not part of this oceanic plate. Other than that it was a good video.
I returned the video so I don’t have the title.
Ah. The devil is in the details, eh?
That is a problem with science popularization in general. If all the details are included, it ends up too complex for general dissemination. But one has to be careful about inferences drawn from generalizations.
Do we tell kids that compasses point north? Sure, but actually they don’t at all. They point in the direction of the local magnetic field–which usually happens to approximately match up with the direction of north. When do we finally get around to telling them the “truth?” 
Amen to that. In 9th grade Earth Science we do actually discuss the declination angle between magnetic north and true north, and we do a lab discussing polar wandering, but it seems every year they dumb down the curriculum a little more. Soon we’ll be talking about Santa’s home!
Two things:
- Coastal California (south of SF) is consist of a few things: Salinain [spelling?] Granite and the Franciscan Mélange (Note: this is way, way, way oversimplifying).
The granite, of course, would be of the continental variety.
The Franciscan Mélange is understood to be the result of the piling of upper sediments in a subduction zone which eventually broke above sea level. As one goes further inland, one can actually find the visible remains of the original subduction zone fault (long since “dead”) in certain areas of California.
(If there’s one thing I’ve notice about these above level marine sediments, they erode quite easily without vegetation ground cover).
Is the area west of the SA fault part of the Pacific plate? You bet it is. Remember, the continental crust (along with the Mélange) is attached and somewhat overlays part of the basaltic oceanic crust. As the Pacific plate slides north, that part of California will slide along with it. The basaltic part will be subducted at the Aleutian trench (because among many reasons, as the basalt ages, it becomes denser). That sliver of continental California will eventually accrete on as another piece of suspect terrain in Alaska. As for the Franciscan Mélange, that’s really nobodies business.
Take a look at geological map of Alaska. You’ll quickly make out a stripe after stripe of various geological formations sort of following the coastline. This is evidence that other “islands” in the Pacific have already been vacuumed of their "basalt” and are now buddied up with other island chunks who made earlier & later treks.
- The Indian-Eurasian collision will result in partial subduction of the Indian’s continental crust. But not very much. Let me use a grade Z analogy. Take two people lying on air mattresses in a swimming pool. Have one lay person place the narrow edge of his/her air mattress on the narrow edge of the other. Note that the one of the air mattresses is forced underneath the water. But not very much. And there in lies the point. Each side can try force the mattress down via there weight, but the buoyancy of air mattress (aka continental crust) is going to prevent it from sinking very deep. As I said its a grade Z analogy; you could argue, “Well the granite will melt”. Yup, but it will rise up again as either an inland volcanic feature or as subsurface pluton. Again, it ain’t going to sink into James Hutton’s locker.
Hope this helps
That’s some outstanding information, user.
I bet Cecil loves how we keep this thread title near the top…
He probably appreciates the irony
Thanks, Gumbercules
I’m no professional…my interest grew out taking an adult-ed class at Santa Barbara Community College. I took four classes altogether in Geology over the last ten years. I also spent a lot of time reading about the subject as a hobby (along with “geology hikes”) when I lived California and DC. But it’s been a while since I read up on it (+ 5 years).
:dubious: