The Assumption on The Formation Process of Ancient and Modern Continents

Since ancient times, humans have always been full of curiosity about the earth. In fact, humans began to observe and describe geological phenomena, especially, the composition of continents from the middle of the 16th century. In 1596, Abraham Ortelius, a Belgian cartographer and geographer, proposed that the Americas had been interconnected with Europe and Asia before the prehistoric flood, which was appeared in the ancient mythology, and it might occur around 20-30 million years ago, that occurred (L RUNDIĆ - Bulletin of the Natural History Museum, 2012, para#12). Moreover, in 1620, Francis Bacon had noticed the shape of the east coast of South America and the west coast of Africa on the map and he put forward the possibility that the Western Hemisphere used to connect with Europe and Africa. After that, in 1668, R.P.F.Plesay of france believed that the Ameicas were not separated from the rest of the earth before the great flood. The similar continental plates had depicted on the map that attracted more attention to the history and causes of the plate.

Based on previous theoretical research, humans used existing technologies and equipment to continue to explore the research of the original state of the earth. For instance, by the end of the 19th century, the Austrian geologist Eduard Suess(1885) noticed that the rock formations on the continents of the southern hemisphere were very consistent, therefore, combined them into a single continent, called Gondwana. In addition, Suess also proposed another idea in 1893 that there was a shallow inland sea between the Laurasia and Gondwana continents, then named the Tethys Sea by comparing the fossil record of the Alps and Africa (AMC Şengör - Nature, 1979) Most importantly, Alfred Lothar Wagner, a German meteorologist and geophysicist, assumed the continent of the world was a unified whole before the Paleozoic of the Carboniferous, surrounded by vast oceans in 1912, called “Pangaea”. Nevertheless, Alexander du Toit disagreed Wagner’s hypothesis; he proposed two original super continents separated by the Tethys Ocean, a northern/equatorial Laurasia and a southern/polar Gondwanaland in1937 (Alex du Toit plate tectonics and people, n.d). Furthermore, in “Supercontinents in Earth history” (JJW Rogers& M Santosh, 2003), the author proposed the period of constituting three supercontinents, during the Mesozoic period; Laurasia and Gondwana were parts of Pangaea. No matter which scientist made the conjecture about the original continent, they all believe that the continent they proposed split up to form the seven continents of the world today, although they did not master sufficient evidences to support their assumptions.

This essay will focus on what kind of force caused the ancient continents to rupture and form the present seven continents.

Due to the crustal activities, continents formation was affected. There are two opposing and long-standing arguments about geotectonics theory. One is called ‘fixism’ that was based on the hypothesis that the earth used to be a hot sphere, then the earth later became hardened with cooling and thus had a fixed ocean and continent, simultaneously, with the contraction of the Earth’s cooling process, pressure was generated, and there was no weak edge of the continent or a deep-sea basin filled with soft sediments, intermittently extruded into mountains, which was first advocated by Isaac Newton. Fixism does not recognize the existence of large-scale horizontal movements, like ‘Mobilism’. It only believed that the force on the continents depending on the vertical movements of the continents and ocean. As well, geosyncline-platform theory is a prominent behave of the fixism which was first developed by J.Hall(1859) and James Dwight Dana(1873) during the classical studies of an investigation and study of the Appalachian mountains (Geosynclinal Theory, n.d).

The argument of geosyncline- platform theory was the theoretical pillar of the classical earth structure theory. It had always regarded the geosyncline (ocean) and the platform (land) as the basic geological structural unit, and believed that the activities of the trough and the platform are in the situ subsidence and ascending motion. The trough was transformed into a platform by motion and metamorphism, which was a process of developing from a movable oceanic crust to a stable continental crust. Therefore, the theory assumed that the movement of the earth's crust was gradually stabilizing and stationary. At the end of all conversions, the geological movement was terminated and the crust is rigid. Because this theory’s working method was mainly to study underground paleontological, which studied the law of the development of the crust from time, so it was also called “history school” or “traditional school”. Also, from the 19th century to mid-20th century, the fixism had been predominant on the geotectonics until the mobilism was first published by A.L.Wagner in 1912.

Mobilism is another one of those two arguments; it is also an exception of the fixism. In fact, the concept of continental drift is the most notable one of theory in the mobilism. Moreover, the previous proposal about Pangaea was also advocated by A.L.Wagner. A lecture (JC. Maxwell, continental drift and dynamic earth, 1968) presented that, A.L.Wagner inferred that the dynamic mechanism of Pangaea continental drift was related to the two components of the Earth’s rotation: the tidal force that drifted westward and the detachment force that pointed to the equator. The lighter silicon-aluminum continental block floated on the heavier viscous silicon-magnesium layer (ocean block). Owing to the tidal force and the extreme force, the Pangaea continent ruptured and separated from the silicon-magnesium layer, and the westward and equator activities that drifted horizontally and moved nearby.

However, Edward Irving has written a situation in his lecture “The role of latitude in mobilism debates” (2003) that there was a fierce debate in the late 1920s. Because the source of the motive force that caused the continental drift was not found, so it was impossible to prove Wagner’s arguments to be correct. Although this hypothesis aroused deep interests from all geologists of countries like C.Techert(1931) said in his book (Journal of the Washington Academy of Sciences ). Additionally, the lack of positive evidence of large-scale horizontal displacement in the crust caused the theory to be left out. But thanks to rapid development in marine science and geophysics and paleomagnetic of the 1955s, there was a lot of reliable evidence to support continental drift to re-emerge the attention of geographers, such as, in the 1955s, the study of paleomagnetics measured the variation of the magnetic pole position in various geological times like the continents of each period had been moving slowly along the magnetic poles before the formation of a new ocean, which just confirmed the possibility of continental drift (Irving. E, Drift of the major continental blocks since the Devonian, 1977).

In the 1960s, Harry.Hess and R.S.Deitz proposed sea-floor spreading hypothesis to provide further evidence for the continental drift theory and to improve it (H Frankel - Scientific discovery: Case studies, 1980). Seafloor spreading theory thought the smaller density oceanic crust floats on the dense mantle asthenosphere, and heterogeneity of the mantle material density, which caused the convection of the material in the mantle or the asthenosphere. In the two upward circulations, the oceanic crust was sretched to form the mid- ocean ridge. The magma emerged from the ridge and cooled to form a new oceanic crust was continuously growing, along with the mantle flow. It would push on both sides, causing the plate to move. (Peter A. Rona, Kurt Boström, Lucien Laubier, Kenneth L. Smith, Hydrothermal Processes at Seafloor Spreading Centers, 2013, page2-4)

In 1965, the famous Canadian geophysicist J.T.Wilson proposed the concept of “converting faults”, which was a strong evidence for verifying seafloor expansion. First of all, the transform fault was giant horizontal fault zones in the mid-ocean ridge, and It was characterized by the simultaneous separation movement of the geological bodies on the left and right sides with the ridge axis as the boundary. Moreover, the relative offset distance of the mid-ocean ridge axis on both sides of the fault could reach hundreds to thousands of kilometers. Also based on the earthquake data, it is fully stated that the transform fault was one of the joint boundaries of the crust component (later called ‘plate’), and its direction of displacement indicates the direction of movement of the plate (J.T.Wilson, science, 1965).

From 1967 to 1968, Le Pichon, X. Francheteau, J. & Bonnin, J and others advocated the plate tectonics theory, it was believed that the lithosphere that makes up the surface of the earth was not a whole part, but consists of some active “plates” surrounded by mid-ocean ridges, trenches and horizontal faults. The tectonics of the theory of continental drift proposed by Wagner, and compared the arguments of the fixed theory with the new research results, and found that it was completely inconsistent, as well as, the activity theory had replaced the fixism position in geology ( Le Pichon, X. Francheteau, J. & Bonnin, J, plate tectonics, 2013).

In conclusion, since ancient time, human curiosity about the unknown had become their driving force for studying the earth. Humans explored the place where they lived and drew it on the map. Since then, according to the similar continental shape on the map, it had been assumed that three supercontinents had appeared on the Earth before the Mesozoic. Then geographers began to study how the continent formed or split, and proposed fixed-state conjectures (J.Hall). Furthermore, due to the passage of time and advances in science and technology, the arguments of the fixism were refuted, and people’s sights focused on the continental drift (A.L.Wagner) to stand of the mobilism.

However, it was pity that there is no sufficient evidence to sustain the continental drift theory, so it was left out until paleomagnetism (Edward.Irving) and marine science provided favorable evidence for the continental drift theory in 1950. After that, geologist proposed seafloor spreading hypothesis (H.Hess&R.S.Deitz) and plate tectonics (Le Pichon, X. Francheteau, J. & Bonnin, J), which were more adequate evidence to against the fixism, so that the fixism lost its foothold and instead take the mobilism. However, each theory is not absolutely correct, the development of science and technology, and the way of research are different. Human studies on the earth have not stopped.