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old hand
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old hand
Joined: Nov 2000
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Mid ocean ridges are predominantly places of tension, resulting in cracking of the Earth's crust. There is no pushing of the adjacent plate - in fact they move apart at a combined speed of up to a metre per year.
The ridge itself is a product of volcanic activity. Just like any volcano, there's a build up of solidified lava around the mouth. The upwelling magma also pushes the ridge up - rather like inflating a balloon under a blanket.
stales
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Carpal Tunnel
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Carpal Tunnel
Joined: Nov 2000
Posts: 3,146 |
I certainly defer to you in this area, stales. As I said my knowledge of the modern theories of plate tectonics comes almost entirely from once-over-lightly-with-feeling Discover Channel programmes. Mind you, I did experiment, dish-astrously, with plate tectonics when I was loading the dishwasher last night. I bet the earth's crust doesn't get told off for destroying crusty plates the way I did! 
The idiot also known as Capfka ...
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Joined: Apr 2000
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Carpal Tunnel
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Carpal Tunnel
Joined: Apr 2000
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In reply to:
geology is destiny! after all the Bronx is gneiss, Manhattan is schist*, and NY is not with out its faults!
That was excellent Helen, even if I don't know anything about the neighbourhoods in question.
Bingley
Bingley
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Carpal Tunnel
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Carpal Tunnel
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Dear stales: I have read that much of the heat in core of Earth is from decay of radioactive elements. Can the hot spots be due to fact that the radio-active material arrived in large lumps that have not become homogenized in four billion years?
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old hand
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old hand
Joined: Nov 2000
Posts: 866 |
wwh
Although far from an expert on the matter, I don't believe hot spots are localised pods of higher radioactive decay. Let's play visual games...
Hold your hands up level, palms down, in front of you to represent two plates. Bring your fingertips together (convergenet crustal movement), then slide one hand under the other (subduction). The leading edge of the plate that's been subducted (fingertips of your lower hand) starts to melt - note this occurs below and to one side of the impact zone. Because it's hotter (simply through being closer to the fire in effect), the newly melted part is less dense than the surounding upper mantle and crust. Being less dense, it rises towards the earth's surface (ie up towards the middle of the palm of your top hand). Once it hits the earth's surface you've got a hot spot.
The upwelling may never make it to the surface if (frinstance) it is not significantly hotter than the surrounding material or if it cools before it gets there.
A lava lamp is an excellent example of how it all works. The waxy stuff is denser than the surrounding liquid when cool so initially sits at the bottom of the flask. Once heated (and thus less dense), gobs of the stuff rise to the surface where they cool and sink again.
When I last studied the subject, it was thought that the molten part of the interior earth is a function of radioactive decay and gravity. It's a hangover from the earth's molten beginnings. The fact that we're still hot and some other planets aren't is (I think) a product of the earth's size (and thus surface area). Surface area relative to volume increases in inverse proportion when talking of spheres - that's why babies lose so much heat through their heads. Thus the smaller planets have all undergone a greater rate of cooling and the bigger ones are still boiling clouds of gas.
stales
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