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Late 20th Century Tests of the Continental Drift Hypothesis 4 – Characteristics of the Ocean Ridges Unless otherwise noted the artwork and photographs in this slide show are original and © by Burt Carter. Permission is granted to use them for non-commercial, non-profit educational purposes provided that credit is given for their origin. Permission is not granted for any commercial or for-profit use, including use at for-profit educational facilities. Other copyrighted material is used under the fair use clause of the copyright law of the United States. What to look for: • • • • • • • • Ridges are the centers of spreading in seafloor spreading. The crust “ages” away from them. In some cases (Atlantic and Indian) they originally formed under a continent. This rifted the continent and subsequently separated the pieces. In these cases the continental edges on either side appear to fit (the earliest ever observation suggesting drift) and they also fit the ridge! In other cases (Pacific and Arctic) they originally formed in oceanic crust and nothing matches anything else. (Except the two sides of the ridge). Intense earthquake activity in the rift is direct evidence of movement there. Normal faults in (active) and parallel (inactive) to the rift indicate both tensional stress are responsible for the earthquakes, and that they have done so for a very long time. (The faults, like the crust they are in, “age” with distance from the ridge.) These faults allow for the formation of mafic magmas in the upper mantle and their rapid transit toward the surface. The resulting mafic rocks replace the crust that the tension moves away from the ridge. Though the initial reaction is always to assume that a continent moves away from a ridge, it is also possible for the ridge to drift away from the continent. If this science had started with the hypothesis that certain continents were once joined, the early scientists should have been able to make a number of predictions. For example, they might have predicted the things that actually were first observed: things that match, same biota, similar climates on now distant continents, and so on. There’s another type of prediction that they might have made, and, in fact, did make after the fact. These predictions involve the oceans between them and the oceans on the opposite sides of them. For example: if two things were once joined and now are not, we might expect to find the place they were originally joined somewhere between them. The shelf edge of a continent directly across the Atlantic is not the only thing that the edge of a continent fits. It also fits the ridge, which is half the distance between them. There is a reason this is called the “Mid-Atlantic Ridge”. (Except north of this large transform fault, where it is called the Reykjanes Ridge. It is still the same spreading center.) This works also in the Indian Ocean, where there is a “Mid-Indian Ridge”, but not in the Pacific (East Pacific Rise) or Arctic (Nansen Ridge) where the ridges are not in the middle. The continents across those oceans do not appear to fit because they were never joined. Base map © by National Geographic Society. If we drew a cross-section of the North Atlantic along the line A-A’ to show the topography or relief of the seafloor, and if we drew it so that the vertical and horizontal scales were the same, we wouldn’t see much. Our profile would look like the upper line below. The distance along the line from about Raleigh, NC to Dakar, Senegal is more than 6500 km (4000 miles) and the maximum depth is only about 5 km (3 mi). In fact, the line is fatter than the ocean is deep, to scale! Base map © by National Geographic Society. A A’ To represent the relief we have to introduce vertical exaggeration – i.e., make the vertical scale far greater than the horizontal. This makes things look both much taller and much steeper than they actually are, but we can see the pattern accurately on a smallscale profile. The lower profile is vertically exaggerated by about 100x. (The map at left has a pretty extreme VE too!) A’ A Mid-Atlantic Ridge Sea Level The ocean ridges all have a similar structure. They stand high above the adjacent abyssal plains, and the central part of the ridge has a deep, steep-sided canyon in it called the rift or central rift. For some distance on either side there are additional steep scarps parallel to the rift, and there are numerous strike-slip or transform faults that cut across the entire ridge system and offset the segments. The central rift is a graben – a topographic low formed between paired normal faults that dip toward each other. The parallel scarps are also normal fault scarps as the diagram shows. Remember how normal faults form. The ridges are clearly points of divergence of two pieces of crust. The divergence creates tension and causes the normal faults. The parallel fault scarps are old rift-bounding faults that have drifted away. If the continents across the Atlantic have diverged then between them we should expect to find a relic of where they were originally joined. Here it is. It still “looks like” the continents, and it is still diverging! Base map © by National Geographic Society. The transforms faults that cross-cut the ridges are simply how the thin, brittle plates accommodate the need to move faster in some places (near equator of rotation) than others (toward poles of rotation). (We are tempted here to imagine Africa and North America both moving away from the M.A.R., but actually N. America and the M.A.R. are both moving away from Africa, which is nearly stationary)! Base map © by National Geographic Society. What other evidence do we have that the ridges are sites of crustal movement? Source of base map uncertain. If anyone recognizes it let me know so I can credit it properly. Check and see if the circled zones of intense earthquake activity are not ridges and transform faults. Each earthquake epicenter is direct evidence for the edges of the plates moving. ALL faulting takes place within the crust and within the rift. Therefore the band of epicenters is very narrow and the depths to foci very shallow Partial Melting The ridges are also sites of intense igneous activity as well as earthquakes. The mantle is made primarily of peridotite, which is mostly olivine. There is also a small fraction of the rock that is not olivine, but lower temperature minerals like amphibole and Ca-feldspar. These minerals are hot enough in the upper mantle to melt, but the extreme pressure keeps them from doing so. Whenever an earthquake occurs in the ridge it is because something has pulled the two plates on either side in opposite directions, opening tilted cracks down which the hanging wall blocks can slip, creating the normal faults of the rift graben. This opening of cracks has the side effect of dropping the pressure in the upper mantle. Suddenly the low temperature minerals find two new conditions: 1) the pressure is now low enough for them to melt, and 2) there is an open conduit up which they can intrude, possibly even reaching the surface and extruding. The result is a set of mafic dikes: gabbro deep in the oceanic crust and basalt near the surface. Voila: new oceanic crust. Older oceanic crust has been moved laterally away from the ridge. Make sure you see how this creates the age pattern we spent so much time and effort to understand. Curious note: Africa is the only continent whose adjacent ridges have roughly the same shape as the continent (red). Along the northern border of its plate are a set of subtle trenches and transforms (black) that accommodate very slow movement of Africa northward, very close to its pole of rotation. (Which is probably in the Atlantic near the ridge. Because the continent is almost completely surrounded by ridges it cannot be moving away from them. That would mean it would be moving eastward from the M.A.R. and westward from the M.I.R., which, I hope, seems absurd to you. Particularly since there are rifts within Africa. and no convergent mountains to speak of Instead, the ridges must be moving away from Africa. Whenever we say “continental drift” we have to remember that the oceans are drifting too – the entire plate, not just the part that Francis Bacon could see. This explains several interesting things about Africa: 1) It sits over warmer mantle than any other continent, with several hotspots. Presumably it has been insulating the mantle and “storing” heat. 2) It has the narrowest shelves of any continent (probably because the warm mantle buoys it up). 3) Despite having only one small mountain chain (the Atlas, on the NW corner) it has the highest average elevation of any continent. The maximum elevation in the Atlas is 13,663’; there are numerous peaks in Colorado alone higher than that. Also, presumably, because the warm mantle buoys it up. So while it is tempting to think of the continent moving away from a ridge, remember that it can work both ways. YES! NO! Take-home Message: • • • • • • • • Ridges are the centers of spreading in seafloor spreading. The crust “ages” away from them. In some cases (Atlantic and Indian) they originally formed under a continent. This rifted the continent and subsequently separated the pieces. In these cases the continental edges on either side appear to fit (the earliest ever observation suggesting drift) and they also fit the ridge! In other cases (Pacific and Arctic) they originally formed in oceanic crust and nothing matches anything else. (Except the two sides of the ridge). Intense earthquake activity in the rift is direct evidence of movement there. Normal faults in (active) and parallel (inactive) to the rift indicate both tensional stress are responsible for the earthquakes, and that they have done so for a very long time. (The faults, like the crust they are in, “age” with distance from the ridge.) These faults allow for the formation of mafic magmas in the upper mantle and their rapid transit toward the surface. The resulting mafic rocks replace the crust that the tension moves away from the ridge. Though the initial reaction is always to assume that a continent moves away from a ridge, it is also possible for the ridge to drift away from the continent.