The theory of lithospheric plates is one of the most interesting in geographical science. The hypothesis of mobilism (in other words, the drift of the continents), once forgotten, has now revived again, thanks to the discovery of the poles of magnetic anomalies with a variable sign, which are symmetrical to the mid-ocean ridges (their axes), primary magnetization, and also the change in the position of the magnetic poles over time.
Multiple confirmation of the idea of the expansion of the ocean floor along the axes of the mid-ocean ridges to the peripheral regions was obtained during many years of research, as well as as a result of deep-sea drilling. A significant contribution to the research and development of the idea of continental drift (mobilism) was made by seismologists. Thanks to their research, it was possible to clarify the distribution of seismic activity zones on the entire surface of the Earth. It turned out that these zones are extensive, but rather narrow: they pass near the main arcs, along the outskirts of the continents and mid-ocean ridges.
Plate tectonics
This hypothesis of mobilism was called "tectonics of lithospheric plates." There are not many of them - just eight large and one and a half dozen small ones. The latter are also called microplates. The largest plates are located within the Pacific Ocean, they are a thin, easily permeable oceanic crust. Indo-Australian, Antarctic, African, South American, North American and Eurasian plates have a continental-type crust. Lithospheric plates have different boundaries (edges) and move very slowly over the entire surface of the planet. When lithospheric plates diverge, the edges become divergent: diverging, the plates form a rift zone (crack) into which the mantle material enters. On the surface of the bottom, it freezes, and the oceanic crust builds up. More and more new mantle material entering the rift zone expands it and makes the plates move. Where they move apart, the ocean forms, and its size is constantly growing. This type of border exists today along the axes of the mid-ocean ridges and is fixed by rift cracks.
Convergent boundaries form when lithospheric plates converge. When they come closer in the zones of contact, rather complex processes take place, among which two main ones stand out by scientists. The first of these is that in the collision of the continental plate and the oceanic one one of them plunges into the mantle, and this is accompanied by breaking and warping. A deep focus earthquake occurs in the collision zone. After the plate enters the mantle, it partially melts: its lightest components after melting rise again to the surface, becoming volcanic eruptions. And the denser components, gradually plunging into the mantle, sink down to the boundaries of the core. This is how the Pacific Ring of Fire was formed .
When two continental plates collide, hummocking occurs. This can be observed during ice drift, when ice floes, colliding and approaching each other, are crushed. When lithospheric plates collide, they are compressed, and large mountain structures are formed at the edges.
Theory of lithospheric plates
Thanks to long-term and numerous observations, geophysicists have established the average speed of lithospheric plates. In the area of compression of the Hindustan and African plates with the Eurasian plate of the Alpine-Himalayan compression belt, the convergence rate between them is up to 0.6 cm / year in the Himalayas and Pamir and 0.5 cm / year in the region of Gibraltar.
The theory of lithospheric plates has established that Europe is now moving away from North America at a speed of about 5 cm / year. But Australia is "sailing" away from Antarctica at a speed of about 14 cm / year. The highest speeds for oceanic plates - they are 4-7 times higher than continental speeds. The fastest is the Pacific Plate, and the slowest is Eurasian.