In John McPhee’s Assembling California, the continents ride on moving plates that divide in spreading zones in the ocean, and right now Africa is pushing Italy into Europe (raising the Alps), and India has cruised up and bashed into Asia and is still shoving at 2 inches a year.
The plates may subduct into a trench such as at the Philippines or the Washington/Oregon coast, plunging deep where the pressure and heat melts the material which then comes up as magma and makes volcanoes.
My description here is a simplification. Try the book!
The American continent once only extended as far west as Utah, before chunk after chunk came in from the west, adding to the continent. Nevada and California were made up of at least four island arcs smushed eastward against the prior ones, one after the other.
This book is copyright 1993. McPhee didn’t go much into the mechanism, and when he was writing, plate tectonics was still being developed. So why causes all this massive sailing around? How did Pangaea form, and how did it split up?
Is it the same effect you get when you have a stew on the simmer, or split pea soup with ham pieces in it–you see spots where plumes of hot juice rise to the surface and spread, and the liquids must be sinking somewhere else to make room. If that’s it, where does the Earth’s heat come from? Why does a line appear and become a spreading zone, and why does it stop spreading?
Earth’s core is heated mostly by radioactive decay. Technically some heat is left over from the formation of the earth, the monster collision that formed our moon and such but mostly it is the radioactive decay that keeps it toasty down there.
Yeah, that’s basically it: large convection cells in the Earth’s molten mantle. Much of the heat that drives this process actually comes from radioactive decay in the core as unstable heavy elements undergo fission into lighter elements and releasing large amounts of heat in the process.
I am not 100% certain but I think most plate movement is caused by divergent boundaries. This is where two plates spread apart such as at the mid-Atlantic ridge. This makes for a constant pushing force and the other types of boundaries result (convergent where one plate dives under another and transform where two plates slide sideways against each other).
I am guessing the energy for all of this comes from the earth’s core. Hot things want to expand so in effect the earth’s core is like a boiling pot with a cover on it. Eventually enough energy is present inside the pot (steam) to push the lid up and let some steam out. If you sealed the pot tightly (pressure cooker) the pressure rises and rises till eventually you’d blow the lid off. So too with the earth’s core. If you sealed the crust tightly energy would not radiate away fast enough so eventually the pressure inside builds enough to crack the crust and let some out. The earth of course is doing this on a continual basis so there is a sort of equilibrium at the divergent boundaries resulting in a fairly steady “push”.
It’s actually giant Techton Men (and Women) inside the earth holding the plates with their arms above their heads and pressing them aginst the plates held by other Techton Men (and Women).
(Don’t ask about volcanic eruptions. That’s serious Techton TMI.)
Although the general consensus of geophysicists is that Rayleigh-Benard convection cells in the mantle are the driving force, this site presents a non-nutjob minority view, in which crustal asymmetries with consequent relative thermal instability are the active component, with mantle convection the passive element.
I’m pretty sure I saw Superman pushing on the plates back in the first movie.
– CalMeacham, who’s actually done geophysics work on the nature of the subduction of the Indian subcontinent and the rise of the Himalayas.
We’ve discussed this before, I think (or maybe it was on another board, I’ll look around). I’m pretty sure it’s the other way around, or at least one prominent theory has it that the majority is from the heat of formation and heat of formation of the core.
The tectonic plates are moving because of the constant expansion of Hell. When there is no more room in Hell, the tectonic plates shall expand wider across the Earth.
Last I heard, the prevailing theory for the formation of the Moon did, indeed, involve a monster collision between the proto-Earth and another planet-sized object, which kicked up debris from the Earth which eventually formed the Moon.
Meanwhile, the amount of energy you’d get from forming the Earth can be calculated precisely, and I imagine that the rate of heat loss from the core is pretty well-known as well. So I don’t think that this is a case where there can really be conflicting models: Either the heat of formation is enough, or it’s not. I’m pretty sure it’s not, but I don’t have the numbers handy at the moment.
Does the moon have a molten core? Certainly we do not see volcanoes or plate tectonics on the moon but deep down is it molten?
If not why not? If the moon was formed from the same stuff as the earth then one would think it would have radioactive material to provide heating. Granted the moon is smaller with no insulating atmosphere but given that most of the earthis molten material I’d be surprised if those differences were all it took to make the difference.
Not as far as we know. The Moon is much smaller than the Earth, and as a result it cooled off significantly faster due to its higher surface-to-volume ratio. This is also why the Moon lacks a magnetic field–actually, that is the main observational evidence for the lack of a molten lunar core.
Pop science article on the formation of the Moon and her core. If the Moon has a molten core, it’s a small and slow moving or highly turbulent one, owing to the lack of a magnetic field. (Since the Moon rotates very slowly–29 odd days–and combined with the tidal loading by the Earth, the core, if it exists, may not turn consistently in one direction.)
The Earth’s internal heat is almost entirely due to radioactive decay. Heat from original condensation would have been radiated away long ago. I don’t have specific numbers (and am to tired to look up the figures and make a BOE estimate right now) but you can figure radiative emission to the 3K background of space, even accounting for radiation absorbed from the Sun, would result in equilibrium on the order of something like 50M years or less; this was a problem for supporters of Darwin (whose theory of “descent with modification” required enormous timespans to explain highly complex organisms) and a boon for Lord Kelvin’s geological estimates prior to the discovery radioactivity, was a significant argument against the “Old Earth” hypotheses versus some relatively literal forms of Biblical Creationism. When radioactivity came into the picture, however, the argument was skewed the other direction.
As for tectonic theory, it is one of the youngest of modern, widely regarded scientific theory, even fresher than QED or “Big Bang” theory. That it satisfies so many otherwise disparate standing questions in science gives strong validity to the theory, but our actual understanding of what goes on more than a few miles below the Earth’s crust is limited to informed speculations and order of magnitude (at best) estimates.
This post is a remarkable series of observations, inferences, and questions. As to the last one, there are a number of places that it does not stop spreading. One of the most remarkable is the mid-Atlantic ridge, where the material keeps coming out, spreading across the ocean floor and then dives back down under the continental plates, presumably to enter convection cells that eventually circulate back to the surface someplace else, eons later. I want to recommend a wonderfully literate discourse on the matter - A Short History of Nearly Everything, by Bill Bryson. Not only is it an elegant discussion of the current state of understanding of plate tectonics, but it explains why our understanding is so limited, so recent and what has interfered with our clarity of thought over all the years that the issue has perplexed us. It’s a remarkable book. Mary Foo and anyone else interested in understanding this and any other mystery of nature would do themselves a big favor by thumbing through this tome. It has become one of my bibles. It should be required reading in every Freshman science course - high school and college. Also history. Maybe philosophy. If I were still teaching in middle school, I’d be excerpting from it left and right.
Sorry, but this book is full of urban legend and misapprehension; to excerpt from it would be perpetuating inaccuracies and misunderstandings. Bryson, despite claims to have conferred with “experts in the field”, seems to have failed to have the manuscripted throuroughly vetted before publication. A recent discussion of Bryson’s techincal merits can be found [thread=356705]here[/thread], but if you search back you can find plenty of criticisms of Bryson’s allegeded knowledge of science. I don’t specifically recall his coverage of plate techtonics, but his explainations of natural selection and modern physics are so badly mangled that hey may do far more damage than good. To say that his knowledge in these areas is limited is an understatement.