by Chris Peterson » Fri Jun 11, 2010 1:41 am
dougettinger wrote:Tectonic plate movements certainly will cause mountain ranges at subduction zones and crashing of continents to cause mountains. The high spots that I refer to are not mountains, but the actual continents sitting high above the level of the Earth's average sea basins. The extent of these surface differences is not seen on other solid surface bodies in the solar system.
That is not clear. Mars, for instance, has a very wide range from highest to lowest surface height with respect to the average geoid. The Moon appears to have a crust that varies considerably in thickness from one side to the other. Except for the Moon, we don't really know much about the crustal thickness of other bodies; we work with what we infer from surface height only.
The question remains: What caused the unusual high spots, the continent(s), on Earth's surface ? The answer simply may be that no one knows or has speculated on this topic.
Again, I'd emphasize that we have no way of knowing if the Earth's crust is unusual. But the matter is something that geologists are aware of. The high crustal regions unassociated with tectonic uplift (typically, cratons) have lower density than other continental crust, and both have higher density than oceanic crust. Consequently, they float higher on the underlying mantle. To understand this, it would be necessary to explore the geologic history of the Earth in more detail than we are able to manage. But broadly, we think of the crust originating early on as a thin structure that was repeatedly broken up by tectonics and by impacts. It isn't hard to imagine that this resulted in a degree of lateral differentiation, with different crustal zones having very slightly different densities- which is all you need to have high and low spots.
There are other mechanisms, as well. Where I live here in Colorado, the land is quite high- something not associated with local tectonics (since we are in the middle of a large plate). But uplift is generally understood in terms of the expansion rate at the mid-Atlantic ridge exceeding the subduction rate at the edge of the Pacific. The timing of uplift here coincides with the formation of the Atlantic, and the forces involved are theoretically capable of lifting the center of an entire continent.
The way I see it, there are a variety of mechanisms, some primordial and some current, that result in the Earth having an active, somewhat chaotic crust building process, which in general is more active than weathering processes. I would expect a system thus described to produce a surface similar to what we actually have.
[quote="dougettinger"]Tectonic plate movements certainly will cause mountain ranges at subduction zones and crashing of continents to cause mountains. The high spots that I refer to are not mountains, but the actual continents sitting high above the level of the Earth's average sea basins. The extent of these surface differences is not seen on other solid surface bodies in the solar system.[/quote]
That is not clear. Mars, for instance, has a very wide range from highest to lowest surface height with respect to the average geoid. The Moon appears to have a crust that varies considerably in thickness from one side to the other. Except for the Moon, we don't really know much about the crustal thickness of other bodies; we work with what we infer from surface height only.
[quote]The question remains: What caused the unusual high spots, the continent(s), on Earth's surface ? The answer simply may be that no one knows or has speculated on this topic.[/quote]
Again, I'd emphasize that we have no way of knowing if the Earth's crust is unusual. But the matter is something that geologists are aware of. The high crustal regions unassociated with tectonic uplift (typically, cratons) have lower density than other continental crust, and both have higher density than oceanic crust. Consequently, they float higher on the underlying mantle. To understand this, it would be necessary to explore the geologic history of the Earth in more detail than we are able to manage. But broadly, we think of the crust originating early on as a thin structure that was repeatedly broken up by tectonics and by impacts. It isn't hard to imagine that this resulted in a degree of lateral differentiation, with different crustal zones having very slightly different densities- which is all you need to have high and low spots.
There are other mechanisms, as well. Where I live here in Colorado, the land is quite high- something not associated with local tectonics (since we are in the middle of a large plate). But uplift is generally understood in terms of the expansion rate at the mid-Atlantic ridge exceeding the subduction rate at the edge of the Pacific. The timing of uplift here coincides with the formation of the Atlantic, and the forces involved are theoretically capable of lifting the center of an entire continent.
The way I see it, there are a variety of mechanisms, some primordial and some current, that result in the Earth having an active, somewhat chaotic crust building process, which in general is more active than weathering processes. I would expect a system thus described to produce a surface similar to what we actually have.