webolife wrote:I've been away from TB forum for a while now... this thread has gained some new splices while I was gone, and I have limited time to address them all. But I would like to address the misplaced comparison of the alleged 148,000 km of spreading with the alleged 40,000 km of subduction/collision zones. This comparison simply has no basis... why?
1. The midocean ridges are 3-dimensional zones of upwelling, not just horizontal [linear] spreading. Overlapping flows of upwelling basalt form a large thickening of the rift, as well as the accompanying spreading of the seafloor.
2. The zones of collision are are also 3-dimensional; involving very wide belts of uplift, eg. the Rockies of the US and the Himalayas of central Asia, as well as very high ranges.
3. The increased thickness of the collision zones of the continental crust, ie. the "roots" of the boundary mountain ranges, are more than sufficient to make up for the linear "frontal" figures.
4. If subduction must be invoked [however I feel it is unnecessary to do so], immeasurable amounts of seafloor would have been re-assimulated into the mantle.
The linear comparison is just not appropriate.
The quote came from a sceptical geologist and made perfect sense to me. Assuming basalt is upwelling surely it would flow along a path of least resistance? To do so twice the amount of basalt has to solidify to push apart the two halves of the plate apart. The assumption is that this process is actually occurring; consider the following from the Geological Society:
‘The mechanism by which tectonic plates move is still a subject of much debate among Earth scientists. The Earth is dynamic thanks to its internal heat, which comes from deep within the mantle from the breakdown of radioactive isotopes. This causes convection in the mantle – hot rocks rise and cold rocks descend. This very slow motion in the solid state transfers stresses to the lithosphere, just as convection in a boiling pan of thick soup will cause the skin to buckle where the convection cells meet.
‘As the theory of plate tectonics developed, mantle convection was long thought to be responsible for the movement of tectonic plates across the Earth’s surface. This theory is now largely out of favour, with modern imaging techniques unable to identify convection cells in the mantle sufficiently large to drive plate movement. Instead, it is thought to be caused by 'slab pull'. Newly formed oceanic lithosphere at mid ocean ridges is less dense than the asthenosphere, but becomes denser with age as it cools and thickens. This causes it to sink into the mantle at subduction zones, pulling slabs of lithosphere apart at divergent boundaries and resulting in sea floor spreading or rifting. How plate movement operates in detail, however, is highly controversial.’ (my emphasis)
First we lean of convection currents in the mantle then we learn that this explanation is now defunct. Plate movement is now though to be due to the hypothesised ‘slab-pull’- another unproven concept.
Now, let’s consider a spreading centre.
‘The Mid Atlantic Ridge, like other ocean ridge systems, has developed as a consequence of the divergent motion between the Eurasian and North American, and African and South American Plates. As the mantle rises towards the surface below the ridge the pressure is lowered (decompression) and the hot rock starts to partially melt. This produces basaltic volcanoes when an eruption occurs above the surface (Eyjafjallajökull in Iceland) and characteristic basalt “pillow lava” in underwater eruptions. In this way, as the plates move further apart new ocean lithosphere is formed at the ridge and the ocean basin gets wider. This process is known as “sea floor spreading” and results in a symmetrical alignment of the rocks of the ocean floor which get older with distance from the ridge crest. (my emphasis)
In this explanation notice how we get from the mantle rising having previously been told mantle convection currents do not exist, to not being told how the plates move apart! The MAR just fills in a growing gap between the two plate halves! But where does slab-pull occur in the Atlantic Ocean?
This explanation seems to be popular as it can be found at the NOAA website: (http://oceanexplorer.noaa.gov/explorations/05galapagos/background/mid_ocean_ridge/mid_ocean_ridge.html)
Where we learn that; ‘Like the rest of the deep ocean floor, we have explored less of the mountains of the mid-ocean ridge system than the surface of Venus or Mars, or the dark side of the moon. Use of submersible or remotely operated vehicles to explore the mid-ocean ridge has provided information on less than 0.1% of the mid-ocean ridge! Much of the mid-ocean ridge still remains a mystery, and we will continue to explore it.’
Mantle convection ergo ‘ridge-push’ has been quietly dropped but no doubt explanatory cartoons will not reflect this. Plate Tectonicists are just as inventive as Wile E. Coyote- one ACME spreading centre, no problem!
Plate movement is now reliant on slab-pull at subduction zones, worse still 40,000 km of slab-pull now has to ‘pull’ 148,000 km of spreading centres! Quoting Peter James writing in ‘The Tectonics of Geoid Changes’: ‘A horizontal force acting at the base of a plate cannot physically produce any downward sliding of the plate, particularly where the plate is of slightly less density than the material it is required to penetrate. The most likely effect of the horizontal drag would be to force the oceanic plate up over the edge of the continental plate, as a thrust fault.’
Can it seriously be considered that an oceanic plate whilst sinking is able to push aside denser lithospheric material, push up continental crust or form island arcs and pull thousands of square kilometres of cold oceanic crust all at the same time! Unimaginable!
Will this stop the ever inventive Plate Tectonicists? No, one ACME subduction zone coming right up!