Answer and Explanation:
The mantle is one of the four layers that form the earth. It is a very hot and dense rock, that is capable of flow. The crust is the outermost layer of the earth and is located just over the mantle. The crust is divided into many plates, that are able to move over the mantle.
The tectonic plates theory, PTT, is based on the continual movements of the crust and might explain the movement of the different plates, and their directions and interactions.
Plates are limited by three different ridges or borders that differ in the type of movement they produce.
Divergent limits: where two plates get separated, leaving a space among them that make a place for a stream of hot material that comes from the mantle to create a new seabed. This separation occurs because of the sea bottom expansion. As old plates get separated, the new and young crust instantaneously gets formed. There is never an open space left, because the emerging mantle occupies that place. This process occurs along with an underwater mountain range which is known as the mid-oceanic ridge or divergent ridge. An example of this is the ridge located in the middle of the Atlantic Ocean, extending from Greenlander to the southernmost point of South America. These divergent limits are in general associated with the rocky material involved in the expansion of the seafloor.
Convergent limits: These movements occur when two plates collide. It might occur among two oceanic plates or one oceanic and one continental plate. In this last case, the oceanic plate is always the one that subducts. The thicker and older plate subduces under the other plate, and at this point, it starts the volcanic activity. As the thicker plate descends, it is heated and melted and its materials are incorporated into the mantle. As the older plate subducts, all the sedimental material that was on the surface of this plate, accumulate and incorporate into a wedge-shaped mass known as an accretion prism. The fast subduction and accretion prism formation make a place for magma that ascends to the surface by crevices. This produces the formation of the volcanic arches, located behind the accretion prism. If these arches are formed on the continental plate, these formations are known as continental arches. But if they occur in another oceanic plate, the arches are known as volcanic island arches. The volcanic islands are arranged in a circumference arch shape, which is bordered by a fossa. Subduction zones coincide with deep-sea trenches or depressions in the ocean bed. The Mariana Islands in the Pacific Ocean are examples of these volcanic islands. The Nazca plate is another example oceanic plate subducting under the south American plate and originating the Andes mountains.
Transforming limits refer to geological structures related to the seafloor expansion and associated in general with the oceanic ridge, although they might also occur in the continental plate. These limits are known as geological faults, where no rocky material is either destroyed or formed. When the plates move and one of the transforming limits displace from side to side, earthquakes occur. The movement breaks the crust and originates pronounced fractures. The San Andrés fault is an example of this plate ridges.
