Although the idea of continental drift has been budding for a long time, Wei Gena, a German meteorologist and geophysicist, comprehensively and systematically discussed the hypothesis of continental drift for the first time. Wei Gena first published the viewpoint of continental drift in 19 12, and further wrote The Origin of Land and Sea in 19 15, which systematically discussed the problem of continental drift. Wei Gena believes that the lighter continental blocks of silicon and aluminum float on the heavier silicon and magnesium layers like giant icebergs, and drift on them; In the Late Paleozoic, continents all over the world became a whole, which is called Pangaea. Among them, the continent in the southern hemisphere is called Gondwana, the continent in the northern hemisphere is called Lauyasia, and the "V"-shaped residual sea area between Lauya and Gondwana is called Tethys Sea, which surrounds the United ancient land. Since Mesozoic, Pan-continent has gradually broken, separated and drifted, which may be due to the influence of some force, forming the basic pattern of modern land and sea distribution.
Fig. 9-1.200 million years ago, a possible schematic diagram of the United ancient continent where all the land was put together.
(quoted from F.Press et al., 1982)
The shaded area around the South Pole in the picture is the estimated glacier coverage area at that time.
Wei Gena's theory of continental drift is mainly based on the evidence of continental shape similarity, stratum, geological structure, paleontology and paleoclimate.
At first, Wei Gena was inspired by the similarity between the curved coastlines of Africa and South America on both sides of the Atlantic. Later, he further discovered that America is closely related to Africa and Europe in stratigraphy, structure and distribution of paleontological fossils. For example, the wrinkled mountain system in Newfoundland, North America echoes the wrinkled mountain system in Scandinavia and belongs to the Caledonian fold belt in the early Paleozoic. The Hercynian fold belt in the Appalachian mountains of the United States disappeared in the Atlantic Ocean at its northeast end, extended to western Britain and central Europe and reappeared; The distribution area of ancient rocks in West Africa (more than 2 billion years) can be connected with the ancient rocks in Brazil, and the rock structure and structure between them are also consistent; The Kepler Mountains at the southern tip of Africa and the mountains near Buenos Aires in South America can be connected with each other in stratum and structure (Figure 9-2). To this, Wei Gena made a very simple metaphor. He said, if two shredded newspapers can be spliced together according to their jagged raw edges and the words printed on them can be connected with each other, we can't help but admit that these two shredded newspapers were torn from a big piece of paper (Figure 9-3). Wei Gena also pointed out that there are also stratigraphic and structural connections between Africa and Indian and Australian continents, and this connection is limited to those before Mesozoic.
Figure 9-2 Rock and Structure Assemblies in South America and Africa
(According to W.K. Hambling, 1980)
1-shielding of metamorphic rocks and igneous rocks over 2 billion years old; 2- Young rock stratum;
3-Syntectonic strike of fold axis (450-650 million years ago);
4-Absolute age and place of radioactive determination
Figure 9-3: The combination of the mainland is like a torn newspaper. The shapes and words can be combined.
(According to W.K. Hambling, 1980)
Paleontologists have long discovered that there is a close genetic relationship between some continents separated by oceans. For example, Zhonglong is a small aquatic reptile that lives in fresh water. It is not only found in Carboniferous-Permian freshwater lake facies strata in Brazil, but also in Carboniferous-Permian similar strata in South Africa. So far, this kind of animal fossil has not been found in other parts of the world, which indicates that there must be a land connection between Brazil and South Africa. Another example is fossil fern, which is widely distributed in the late Paleozoic strata of southern continents such as Australia, India, South America and Africa. In order to explain these phenomena, paleontologists once put forward the "land bridge theory", assuming that in the ocean between these continents, there was once a land or a series of islands connecting distant continents, and then these land bridges sank and disappeared, and the continents were completely separated by the ocean. However, Wei Gena thinks that the similarity of paleontological features between continents is not because there is any land bridge between them, but because these continents were directly adjacent at first, and then they split and drifted away.
In Wei Gena's drift theory, the evidence of paleoclimate plays an important role, especially the distribution of ancient glaciers. In the Late Paleozoic, about 300 million years ago, extensive glaciation occurred in South America, Africa, Australia, India and Antarctica, and some of them can be judged from the scratches of glaciers (Figure 9-4). Judging from the scale and characteristics of the distribution of glacial remains, the glaciers at that time were continental glaciers developed near the polar regions. Moreover, the remains of ancient glaciers in South America, India and Australia still remain on the continental margin, and the direction of glacier movement is from offshore to inland, reflecting that ancient glaciers did not come from the local area. How to explain the distribution and flow characteristics of this ancient glacier has always been a difficult problem in geology. However, it is these characteristics that provide strong evidence for the theory of continental drift. According to the drift theory, continents with ancient glaciers were connected at that time and located near the South Pole. Glacier center is located in southern Africa, and ancient continental glaciers flow radially from the center in all directions, which reasonably explains the distribution and flow characteristics of ancient glaciers. In addition to the remains of ancient glaciers, evaporated salt, coral reefs and red beds, as indicators of ancient climate, can also be used to infer the ancient latitude produced when they were formed. Wei Gena and others have marked the distribution of evaporated salt and coal in Carboniferous on the United ancient continent, in which rock salt, gypsum and desert sandstone are all concentrated in the dry subtropical zone, which is completely consistent with the required paleoclimatic conditions, thus providing evidence for the existence of the United ancient continent.
Figure 9-4 Ancient Glaciers and Continental Drift
(quoted from W.K. Hambling, 1980)
A— the distribution of late Paleozoic glaciers on modern continent, and the arrow represents the flow direction of ancient glaciers; B—— distribution of late Paleozoic glaciers on ancient land before drifting.
Although there are many disputes about the theory of continental drift, the mechanism of drift has not been well solved because of the limitation of the level of understanding at that time. Therefore, the theory of continental drift is opposed by many geophysicists and geologists. In 1930s, the theory of continental drift gradually declined. In 1950s, due to the discovery of some new independent evidence of continental drift, the theory of continental drift became active again. The most powerful evidence is the result of paleomagnetic research.
At present, there is a geomagnetic field around the earth, and there is also a geomagnetic field around the earth in geological history, which is called paleomagnetic field. In the process of its formation, rocks can obtain magnetism because they were magnetized by the ancient magnetic field at that time, and the magnetization direction was consistent with the ancient magnetic field. For example, in the process of magmatic rock condensing into rock, when it cools through Curie temperature point, some ferromagnetic minerals in magma will be magnetized along the direction of geomagnetic field at that time, and this magnetism will remain after magma condenses into rock; In the process of sedimentary consolidation of sedimentary rocks, due to the influence of geomagnetic field at that time, some ferromagnetic mineral particles are oriented along the magnetic field lines and will also obtain weak magnetism. The magnetism obtained in the process of rock formation is called natural remanence or fossil magnetism. Once the magnetism of this fossil is formed, it has strong stability and can be preserved to this day. With the help of fossil magnetism of rocks, we can track the horizontal movement of rocks since their formation.
The observation of modern geomagnetic field shows that the geomagnetic pole moves periodically around the geographical pole. If we look at the time scale of thousands of years or even longer, the average position of the geomagnetic pole can be regarded as coincident with the geographical pole. According to this principle, the paleomagnetic pole in the geological history period can be approximately regarded as the paleogeographic pole; The magnetic meridian direction of paleomagnetic field is regarded as the paleogeographic meridian; The isoclinic line of paleomagnetic field can be regarded as the paleogeographic latitude line, and the magnetic dip angle and latitude value can be converted by simple mathematical formula. The magnetization direction of remanence produced in the process of rock formation can be used to determine the direction of ancient meridian, and the ancient latitude can be determined by measuring the magnetic inclination angle of fossil magnetism in rock.
Figure 9-5 Polar Shift Curves of North America and Europe
(quoted from F.Press et al., 1982)
Paleomagnetic research reached its peak in the 1950s. Blackett and Ron Cohen, famous British scholars, measured a large number of magnetic data of rock fossils, and calculated the paleolatitude and the corresponding paleomagnetic pole position of rock samples in a certain era according to the paleomagnetic elements of fossil magnetism. They found that in some areas or continents, the measured ancient latitudes are often very different from the current latitudes, indicating that these areas or continents have undergone large-scale horizontal displacement, which provides important evidence for continental drift. If we assume that the mainland is fixed at the present position and mark the magnetic pole positions obtained from the magnetism of rock fossils in different eras on the map, we find that the older the geological era, the farther the ancient magnetic pole position deviates from the modern magnetic pole position, and the migration trajectory of the ancient magnetic pole of the mainland can be obtained by connecting the ancient magnetic poles of various eras. But in fact, the magnetic pole is basically fixed near the geographical pole, and the pole shift curve itself reflects the route of continental drift (Figure 9-5). In any geological historical period, there may be only one paleomagnetic pole with a certain polarity (N or S), but paleomagnetic research shows that different polar shift trajectories have been detected in rocks of different continents, which indicates that there must have been relative displacement between these continents. Figure 9-5 shows two pole shift curves measured from rocks in the European continent and the North American continent respectively. In modern times, these two curves intersect at one point. The older the times, the farther the deviation. In order to make the pole shift curve of North America overlap with that of Europe, the North American continent must retreat eastward by about 30 longitude. At this time, the Atlantic Ocean disappeared, and the North American continent and Europe were put together, which just restored the situation of joint ancient land proposed by Wei Gena's drift theory. The paleomagnetic data of other continents further show that if the mainland is roughly restored to the position before drift according to Wei Gena's idea, the positions of the continental geomagnetic poles in the same geological age measured according to the paleomagnetic data are quite close, and the polar shift curves of the continents can also overlap each other. Therefore, the paleomagnetic data convincingly confirmed the continental drift again.
After the paleomagnetic research revived the theory of continental drift, people began to discuss some problems related to continental drift in depth and obtained many new evidences and materials. For example, British scholar Brad and others perfectly spliced the topography on both sides of the Atlantic Ocean (according to about 1000 m isobath) with the help of electronic computers, which provided the most vivid evidence for verifying continental drift; In addition, the new data of paleontology, stratigraphy and structure found in Antarctica and other continents further confirmed the existence of continental drift. Although continental drift declined and revived in the late 1950s and early 1960s, the mechanism of continental drift has not been solved. During this period, the research of seabed geology and geophysics developed rapidly, which finally brought dawn to the solution of continental drift mechanism.