1. continental drift theory
German Wei Gena (A.Wegener, 1880 ~ 1930) put forward this hypothesis in 19 12. He started with the phenomenon that the outlines of the South American continent and the African continent are very consistent, and it seems that they have split and drifted along the Atlantic Ocean. He collected a lot of information about geological structures, ancient glaciers, rocks and fossils, and found many similarities and splicing places. He boldly put forward the viewpoint of continental drift and initiated a new trend of geotectonics-activity theory. He believes that all continents on the earth were once a huge joint ancient land about 300 million years ago (late Carboniferous), and there was only one "pan-ocean" around them. Later, it gradually separated and drifted in Mesozoic, forming the present state (Figure 13-38, Figure 13-39).
Figure 13-38 The position of continental crust during several drifts.
(Wei Gena, 19 12)
A- Late Carboniferous; B- Eocene; C— Early Pleistocene
Figure 13-39 continental splicing on both sides of the Atlantic Ocean
(According to E.C.Bullard, 1965)
The continental drift theory holds that the lighter granitic continental crust drifts on the heavier basaltic basement. Continental drift follows two directions: one is continental drift from east to west, which is caused by tidal friction; The other is the movement of the mainland from the polar region to the equator, which is caused by the centrifugal force produced by the earth's rotation. Because of the different drift speeds, they split into continents, during which they formed oceans. Fold mountains are formed by extrusion where the continental drift front is blocked by basement.
Limited by the scientific level at that time, the theory of continental drift failed to correctly explain the driving force of continental drift. Because the rigid granite layer will not drift on the rigid basalt layer; The tidal friction resistance and the centrifugal force of the earth's rotation are too small to cause long-distance continental drift. Therefore, although there is a lot of evidence to support it, which shocked the earth science community, it was not accepted by most geologists, especially the fixed theorists (represented by Tantai Theory), and gradually declined in the 1930s.
2. Theory of seafloor spreading
After the Second World War, due to the strategic needs and the development of science and technology, many marine surveys have been carried out, and a lot of achievements have been made, including: the shape of mid-ocean ridge (global rift system), the abnormal distribution of submarine geothermal flow, the abnormal strip of submarine geomagnetic, the age of seabed and its symmetrical distribution, the distribution of submarine seismic zones and sources, island arcs and trenches, the upper mantle asthenosphere and so on. In the early 1960s, American geologists Hess and R.S.Dietz, on the basis of summarizing the theory of convection and continental drift, created a brand-new theory-seafloor spreading theory after comprehensive analysis of a large number of seabed survey data.
According to the theory of submarine expansion, a new ocean floor is formed in the rift zone of the ocean ridge, and it continues to expand to both sides. At the same time, the old ocean floor is inserted into the ground at the trench and returns to the asthenosphere, resulting in material circulation. The expansion rate of the seabed is about one to several centimeters per year, which makes the seabed renew every 300 million to 400 million years. The driving force of submarine expansion is thermal convection of mantle material. Ridge axis is the rising place of convection circle, and trench is the falling place of convection circle. The hard oceanic crust is carried on the asthenosphere and moves passively with the mantle moving towards the fluid (Figures 13-40 and 13-4 1). If upwelling occurs under the mainland, it will lead to the division of the mainland and form a new ocean. For example, if there is a high-temperature brine area in the Red Sea Rift Valley, it is inferred that the Red Sea may be the embryonic form of a new ocean.
Figure 13-40 Schematic Diagram of Submarine Expansion Theory
(quoted from Chai et al., 2000)
Figure 13-4 1 Lithospheric Plate Movement Caused by Mantle Convection (Submarine Expansion)
(According to P.J. Wiley, 1980)
The main evidence of seafloor spreading theory comes from the survey results of marine geophysics, which systematically explains the formation and evolution of seafloor. Although there are still many questions, it has greatly stimulated people's desire for further discussion.
3. The concept of plate tectonics
In a few short years after the theory of submarine expansion was put forward, people have obtained a lot of new research results, such as the annual representative of geomagnetic rotation, submarine geomagnetic tape, submarine age revealed by deep-sea drilling, the discovery of diving in the mid-ocean ridge, the discovery of transform fault, Wilson cycle of the ocean, etc., which confirmed the existence of submarine expansion and led to the birth of plate tectonics.
1968, at an academic exchange meeting, W. J. Morgan of the United States, Lei Bixiong of France and D. P. Mackenzie of Britain all put forward the theory of plate tectonics. The basic principle of seafloor spreading theory is extended to the whole lithosphere and summarized in order to improve the understanding of the general law of lithosphere movement and evolution. Its research has covered the whole area of the earth, so it is called global tectonic theory.
The concept of plate tectonics is that the rigid lithosphere is divided into many huge blocks-plates, which move horizontally in the asthenosphere on a large scale, leading to the interaction between adjacent plates, and the plate edge becomes a zone with strong crustal activity. It is characterized by intense magmatic activity, seismic activity, structural deformation, metamorphism and deep-sea deposition. The interaction of plates fundamentally controls the processes of various internal geological processes and sedimentation.
4. Plate boundary types
There are three types of plate boundaries: discrete boundaries, convergent boundaries and transform fault.
Discrete boundary (plate growth boundary) is mainly represented by mid-ocean ridge (or mid-uplift, rift). Along this boundary, the lithosphere splits and expands, and mantle materials spew out, thus producing the oceanic crust. There are a large number of basaltic magmatic eruptions, frequent shallow earthquakes and graben faults, and the newly-born basalts here are widely subjected to mild metamorphism (basalt becomes greenschist, ultrabasic rocks become serpentine, etc.). ) Due to the high heat flux of ocean ridges. Both the East African Rift and the Red Sea Rift belong to the early stage of this boundary.
Convergence boundary (extinction boundary of plate) is also called Bioff band. Mainly represented by island arc-trench. Along this boundary, two adjacent plates move towards each other, resulting in compression, and the oceanic plate subducts downward, causing strong earthquakes, magmatism, metamorphism and structural deformation.
Due to the deep melting of subduction plates, magma is formed, which leads to the volcanism and intrusion of island arc (mountain arc), and often forms the geomorphic combination of trench, island arc and back-arc basin, which is called trench-arc-basin system. The Pacific Rim tectonic belt is a typical representative of convergence boundary (Figure 13-42).
Figure 13-42 Schematic diagram of magmatic activity and volcano caused by subduction.
(According to A.N.Strahler, 1977)
A- the situation of the volcanic belt on the west coast of America; B- the situation of the island arc area in the western Pacific.
In addition, there is a special convergence boundary-the ground suture line, which is the collision zone between the two continents. When the spreading speed of mid-ocean ridges slows down or the subduction speed of oceanic plates accelerates, the subduction speed is faster than the diffusion speed, which leads to the continuous shrinking and even disappearance of the oceanic crust area, which makes the continental plates originally located on both sides of the ocean collide and squeeze with each other and finally "weld" together, forming a series of mountains at the plate boundary, accompanied by strong structural deformation, magmatic activity and regional metamorphism. For example, the Himalayan fold belt was formed because the Tethys oceanic crust between the Indian plate and the Eurasian plate disappeared and the two continental plates collided. The Yarlung Zangbo River in the northern Himalayas is a ground suture.
Figure 13-43 Schematic diagram of the difference between transform fault and flat inference layer.
A, B- transformation fault; C, d- flat reasoning layer; The double line is the ridge; Black spots are the source.
Transform fault is a special type of plate boundary, along which there is neither plate proliferation nor plate contraction, but two adjacent plates are sheared and dislocated, causing earthquakes and tectonic deformation. The mid-ocean ridge is often vertically fractured, and it can also be connected with trenches and ridges. The famous San Andreas fault in the western United States is a crisscross of the transform fault in the middle of the Pacific Ocean. On the surface, transform fault and flat inference layer are very similar, but there are many differences (Figure 13-43).
5. Division of global plates
According to the plate boundaries, the global plate can be divided into six major plates: Pacific plate, Eurasian plate, Australia-India plate, Africa plate, America plate and Antarctica plate (Figure 13-44):
Figure 13-44 Global Plate Division
The above is a global plate. Except that the Pacific plate is almost all oceanic crust, the other five plates all include oceanic crust and continental crust.
In addition to the above six plates, there are smaller plates with an area less than 1.0× 107km2, including Nazca plate (outside the west coast of South America), Cocoa plate (outside the west coast between North and South America), Caribbean plate (Caribbean Sea and its vicinity), Fokker plate (outside the west coast of North America) and Philippine plate (between the Philippines and Mariana Islands). In fact, most small plates are remnants of ancient plates that have not been completely subducted, such as several small plates on the east coast of the Pacific Ocean, which may be part of the original East Pacific plate.
6. Plate drive mechanism
Although the theory of plate tectonics has long been recognized by geologists, the problem of plate driving force has not been recognized so far, because most of the theories of plate driving force are in the hypothetical stage and cannot be demonstrated by experiments or convincing methods at present.
At present, most geologists believe that the plate moves on the mantle-to-fluid, and mantle convection is the fundamental reason for the plate movement. However, there are still different views on the form (involved depth) of mantle convection: layered mantle convection model and full mantle convection model.
Some scholars believe that the driving force of the plate mainly comes from the gravitational drag caused by the transformation of the subduction plate and the lateral thrust caused by the expansion of the ocean ridge. This view was in the mainstream in the 1980s.
In a word, plate tectonic theory is a geotectonic theory based on the latest achievements of many disciplines. Based on the active mode of the whole earth lithosphere plate, it establishes a worldwide tectonic movement model and gives reasonable explanations to some global tectonic problems. It is the most important and popular earth science theory today.