In the ocean floor the plates move away and there is a gap between them that is filled with material from the mantle, molten rock (magma) of the asthenosphere, which can flow because it is very hot. As soon as it reaches the surface it undergoes physical and chemical changes when it loses gas and comes into contact with the water at the bottom of the sea. When its temperature drops it becomes a new oceanic crust.
As the plates continue to separate, this new oceanic crust is dragged to the sides of the crest and leaves room for more material to ascend from the mantle. The material that rises is very hot, and transmits part of this heat to the material that is nearby, which pushes the material above it, giving rise to the great elevations above the middle level of the seabed that present the oceanic mountain ranges.
The plates continue to separate and the new bottom, getting colder, passes the highest point and begins a very rapid descent, breaks and new normal faults are created, but now the relative movement of the walls is in the opposite direction to what happens on the same side inside the valley.
As it moves away from the center of expansion, the new oceanic crust cools, which makes it denser and therefore heavier. When weighing more, it puts more pressure on the material of the asthenosphere and makes it descend.
The result of this is that the ocean floor is supported on an inclined surface, and the force of gravity causes it to slide on this surface away from the center of expansion and therefore from the plate on the other side.
If new ocean floor is continually being created and the Earth is not growing, the creation of new surface must be compensated by destroying old surface. On the other hand, if two plates move away from each other, this means that they approach other plates that are in their path, and if they do not move away fast enough they have to compete for the surface they occupy.
At the ends of two plates, one continental and one oceanic, the latter tends to sink, because it is heavier than the asthenosphere, while the continental plate floats because it is lighter. Consequently, the oceanic plate sinks beneath the continental and returns to the mantle, where high temperatures melt it. Oceanic trenches are, therefore, subduction zones where the oceanic plate is consumed.
The gap between the subducted and the subducent plate forms an oceanic trench, where a large amount of sediment is deposited, contributed, above all, by the continental. Sometimes part of these sediments joins the continent and, in this way, the continents grow.
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