I'm just now learning geology so I hope this question doesn't seem idiotic. I always thought that the Earth was not a perfect sphere with conditions everywhere equal in terms of forces acting on its geology and plate tectonics. Gravitational forces, rotation, magnetic, etc might mean geology at the poles might be very different than at the equator. Reading your account brought this old speculation to mind again. Are there observed differences, common to both poles, in plate tectonics and crustal geology? Maybe circumpolar geology is kind of quiet or is it where illuminating insights are to be found? Dumb question maybe,?
It’s certainly not a dumb question, but also not simple to answer. Perhaps most important to realize is that the positions of the continents (and the oceanic crust too, for that matter) have moved drastically with respect to the poles (all of them – geographic, poles of rotation, magnetic poles) over geologic time, so I’d say there are very few (if any) places that were near the poles for long enough for any possible effect to result, and I suspect that if there were any, it would be extremely subtle compared to the forces of heat convection in the mantle that drive crustal plate tectonics.
Yes, when earth has glacial periods, there is more ice at the poles than elsewhere, and its presence (and later absence) certainly affects geologic processes, but in the grand scheme of things, except for the unusual “snowball earth” periods when most of the planet was covered, glacial periods are relatively short-lived. Thus if glacial ice prevented sedimentation and life from operating for a time in the polar regions, it might be enough to leave a record in the rocks (the actual action of the glaciers would do much more) but there would be a return to more typical patterns within (usually) a few million years. Scandinavia is still rebounding (uplifting) as a result of the removal of the most recent glacial ice, and that certainly affects such things as coastal cliffs and other things.
Climate at the poles has varied a lot over time as well, so that for example, in the Cretaceous things were warm enough in near-polar lands that cold-blooded animals thrived. There are coal beds in Antarctica, from a time when Antarctica was no where near the south pole.
The forces you mention, gravity, rotation, magnetic, are certainly different at the poles than at the equators (the magnetic equator is not in the same place as the geographic or rotational equator), but they are really not strong enough to have much (probably not any) impact on tectonics. In early days of understanding continental drift (1920s), there was an idea called the Polfluchtkraft, which means “pole-flight force,” something generated by the centrifugal force of rotation of the earth. Nowadays we know that it does indeed exist, but its strength is far, far too low to affect anything significantly; the strength of the crust, even the relatively weak oceanic crust, is much greater. The overall figure of the earth (the oblate spheroid, 21 km greater in equatorial radius than polar radius), does reflect this force in an overall elastic earth (dominated by the mantle; the brittle crust doesn’t really come into play, but breaks brittlely because of different forces). The elastic solid earth actually has tides, like the oceans, related to the gravitational attraction of the moon and sun, and they are stronger nearer the equator than at the poles. They raise and lower continent-scale regions by as much as a few centimeters every time the moon orbits the earth. There have been studies to see if highs and lows of earth tides have any role in generating earthquakes; I haven’t checked the studies lately but I’m pretty sure there may be some slight suggestion of correlation, but it’s pretty minimal, and far outweighed by normal plate tectonic processes. Beyond that, I don’t see any possible relationship between things like earth tides and other geologic processes.
If the Polfluchtkraft were significant, most mountains would be at the rotational equator because the force is driven south from the north pole and north from the south pole, and they would converge at the equator. Even considering the highly diverse and variable positions of continents, this is not the case, and there is no systematic orientation of plate tectonic motions in north-south or south-north orientations, not today and not over geologic time. (You can certainly find some correlations, such as the dominantly east-west fracture zones in the Pacific ocean, and multiple largely north-south spreading centers, such as the mid-Atlantic; but over time, I think those are mostly fortuitous. I won't quite say NO effect; but likely no significant effect.)
As important as the magnetic field is for keeping us protected from cosmic rays etc., it probably has even less direct impact on global geology than the gravitational and rotational aspects. It DOES affect things, inducing a magnetic orientation in solidifying molten rocks; there are even bacteria that generate magnetite crystals that they appear to use for navigation in muds (there are both north-seeking and south-seeking bacteria). The magnetic poles also exchange positions, on their own and irrespective of the relative motions of the crustal plates. These magnetic reversals are not well understood, either in terms of what makes them happen or what happens when they do it; but there is no correlation with mass extinctions, for example (there do seem to be some pretty subtle, small scale extinctions, but the jury is still out on that IMO).
I hope this provides a start at addressing your question.
More than a start. I will re-read it several times to be sure I've got it down. Thanks sincerely for taking so much time to answer a Geology 101 level learner.
Alaska in the summer is the ultimate vacation, imho. You bring back memories of Uganik lake, river and bay, Kodiak, years ago.
I'm just now learning geology so I hope this question doesn't seem idiotic. I always thought that the Earth was not a perfect sphere with conditions everywhere equal in terms of forces acting on its geology and plate tectonics. Gravitational forces, rotation, magnetic, etc might mean geology at the poles might be very different than at the equator. Reading your account brought this old speculation to mind again. Are there observed differences, common to both poles, in plate tectonics and crustal geology? Maybe circumpolar geology is kind of quiet or is it where illuminating insights are to be found? Dumb question maybe,?
It’s certainly not a dumb question, but also not simple to answer. Perhaps most important to realize is that the positions of the continents (and the oceanic crust too, for that matter) have moved drastically with respect to the poles (all of them – geographic, poles of rotation, magnetic poles) over geologic time, so I’d say there are very few (if any) places that were near the poles for long enough for any possible effect to result, and I suspect that if there were any, it would be extremely subtle compared to the forces of heat convection in the mantle that drive crustal plate tectonics.
Yes, when earth has glacial periods, there is more ice at the poles than elsewhere, and its presence (and later absence) certainly affects geologic processes, but in the grand scheme of things, except for the unusual “snowball earth” periods when most of the planet was covered, glacial periods are relatively short-lived. Thus if glacial ice prevented sedimentation and life from operating for a time in the polar regions, it might be enough to leave a record in the rocks (the actual action of the glaciers would do much more) but there would be a return to more typical patterns within (usually) a few million years. Scandinavia is still rebounding (uplifting) as a result of the removal of the most recent glacial ice, and that certainly affects such things as coastal cliffs and other things.
Climate at the poles has varied a lot over time as well, so that for example, in the Cretaceous things were warm enough in near-polar lands that cold-blooded animals thrived. There are coal beds in Antarctica, from a time when Antarctica was no where near the south pole.
The forces you mention, gravity, rotation, magnetic, are certainly different at the poles than at the equators (the magnetic equator is not in the same place as the geographic or rotational equator), but they are really not strong enough to have much (probably not any) impact on tectonics. In early days of understanding continental drift (1920s), there was an idea called the Polfluchtkraft, which means “pole-flight force,” something generated by the centrifugal force of rotation of the earth. Nowadays we know that it does indeed exist, but its strength is far, far too low to affect anything significantly; the strength of the crust, even the relatively weak oceanic crust, is much greater. The overall figure of the earth (the oblate spheroid, 21 km greater in equatorial radius than polar radius), does reflect this force in an overall elastic earth (dominated by the mantle; the brittle crust doesn’t really come into play, but breaks brittlely because of different forces). The elastic solid earth actually has tides, like the oceans, related to the gravitational attraction of the moon and sun, and they are stronger nearer the equator than at the poles. They raise and lower continent-scale regions by as much as a few centimeters every time the moon orbits the earth. There have been studies to see if highs and lows of earth tides have any role in generating earthquakes; I haven’t checked the studies lately but I’m pretty sure there may be some slight suggestion of correlation, but it’s pretty minimal, and far outweighed by normal plate tectonic processes. Beyond that, I don’t see any possible relationship between things like earth tides and other geologic processes.
If the Polfluchtkraft were significant, most mountains would be at the rotational equator because the force is driven south from the north pole and north from the south pole, and they would converge at the equator. Even considering the highly diverse and variable positions of continents, this is not the case, and there is no systematic orientation of plate tectonic motions in north-south or south-north orientations, not today and not over geologic time. (You can certainly find some correlations, such as the dominantly east-west fracture zones in the Pacific ocean, and multiple largely north-south spreading centers, such as the mid-Atlantic; but over time, I think those are mostly fortuitous. I won't quite say NO effect; but likely no significant effect.)
As important as the magnetic field is for keeping us protected from cosmic rays etc., it probably has even less direct impact on global geology than the gravitational and rotational aspects. It DOES affect things, inducing a magnetic orientation in solidifying molten rocks; there are even bacteria that generate magnetite crystals that they appear to use for navigation in muds (there are both north-seeking and south-seeking bacteria). The magnetic poles also exchange positions, on their own and irrespective of the relative motions of the crustal plates. These magnetic reversals are not well understood, either in terms of what makes them happen or what happens when they do it; but there is no correlation with mass extinctions, for example (there do seem to be some pretty subtle, small scale extinctions, but the jury is still out on that IMO).
I hope this provides a start at addressing your question.
More than a start. I will re-read it several times to be sure I've got it down. Thanks sincerely for taking so much time to answer a Geology 101 level learner.