Plate tectonics: theory, mechanisms, and geological evidence

¶ … plate tectonics is responsible for changing continental landmasses through geological occurrences.

Thousands of years ago the earth's surface has been hypothesized as one big landmass. The Earth's surface has been constant motion. "Fragmented into giant sheets of solid rock that glide atop a layer of hotter, more pliable material, the globe's appearance is forever changing." [Cowen, 1999]. These plates are semi-rigid, floated on flow of mantle. The plates measured around 50 miles, thickness of 25 miles on land and 4 miles thick under the oceans. Their movement was categorized by an average of a few inches a year. Even today, geologists and tectonicists hypothesize the earth to continue this movement in similar slow manner, even if we do not notice it. The plates' movement is called the Plate Tectonic Theory, a theory presented in 1912 but upheld in 1960.

STATEMENT OF PURPOSE

The main controversy in the Plate tectonic theory is that whether the movement of the plates has displaced landmass across the continents or not and whether it is the sole reason for the change of the landmass or it is through atmospheric influence. This paper will help explain how Plate Tectonics has caused changes in continental landmasses through geological occurrences.

ANALYSIS

Plate Tectonics

The concept of plate tectonic is derived from moving plates or the earth crust from its original location to some other parts of the world. The dynamic system of the plate tectonic have been responsible for the huge recycling system of the earth, by spreading the centers, thrusting the crust down and melting the mantle to increase or subdue some parts of the crust to change the surface [Austin et al., 1994].

Most plate tectonics evidence is found where volcanic eruptions or earthquakes occur. The chances of the formation of crust from the earthquakes is often embedded in the ocean floor as molten rocks, rising out into the surface when the plates are separated from the crust and not as a result of deposits. For example in areas around the Mid Atlantic Ridge, the plates are seen to have drifted apart but new oceanic crust is formed when molten rocks rise from the mantle [Price, 1999]. The Western side of South American is another example of change of land surface where the boundary of two plates comes together to form a heavier crust. When this occurs, it forces the landmass to depress, under the lighter continental crust. A similar example of landmass changes is the San Andreas Fault, belonging to the North American Plate and Pacific Plate. In this case the stick slip motion of the plates is responsible for the eruption of earthquakes in California. Yet another instance of plate tectonic is the boundary in the Himalayas. The mountains are uplifted from the impact of the Asian Plate and the Indian sub-continent.

Scientists around the world however dispute the existent evidence of the hypothesis that the landmass formations of the continents are the results of plate tectonics. For instance it has been said that this theory is "A hypothesis that is appealing for its unity or simplicity acts as a filter, accepting reinforcement with ease but tending to reject evidence that does not seem to fit" (Pratt, 2000). The argument posit that although the logic flow of the plate tectonic do give convincing evidence, but it cannot "over ride and over rule all other hypothesis" (Pratt, 2000) which pose the argument that the disturbing dogmatism of atmospheric development responsible for the change, tearing and wearing of the earth's surface.

To understand the evidence one must delve into the structure, conception of plate tectonics before accepting it wholly. Hence, the following arguments attempt to present the factual evidence of the earth's change of landmass and attribute to plate tectonics.

Types of Plate Tectonics

Plate boundaries are identified and defined mainly on the basis of earthquake and volcanic activity. The close correspondence between plate edges and belts of earthquakes and volcanoes is therefore to be expected and can hardly be regarded as one of the "successes" of plate tectonics" (McGeary and Plummer, 1998 qtd. From Pratt 2000).There are three types of plate movements that are recognized at the boundaries of the plates. These are known as convergent, divergent and transform-fault.

Convergent boundaries of plates move toward each other and collide. When continental plates collide with each other, they usually form mountain chains. Take for instance the case of Himalayas, where the oceanic plates of South America and Nazca Plate boundaries collided and formed what is called subduction of the boundaries.

In the case of divergent boundaries, the continental plates move in opposite directions. For instance the Mid Atlantic Ridge whose plates "diverge" lava rocks which later cooled to add more substance to the edges of the oceanic plates [Monastersky, 1999]. This process is also known as sea floor spreading, where the ocean bed decrease its height by increasing its mass [Harish, 2001].

In some cases the plates move past each other without any kind of collision. This is known as the transform fault boundaries. An example of the transform fault is the San Andreas Fault zone where the boundary of the Pacific Plate that categorize Los Angeles move slowly northward towards the North American plate to San Francisco but it does not collide. Due to these actions, the plate tectonics is considered to be the major contributor to landmass change.

When plate tectonic occurs it is common to have different kind of results. In that the different movement of the geological layers often result in different kind of reaction in the earth's surface. The movement of convergent, divergent and transform fault are all instances of such differential geological reaction. These are divided into subduction, sea floor spreading and continental drift.

Subduction and spreading

Subduction is the result of two plates moving towards each other. In this case the heavier plate often get depressed or subducted. It forms a deep trench in the ocean floor. The depth of the trench could result in earthquake activity and volcanic islands because of the resultant rocks that emerge from the surface collision [Blood, 1991]. An example of such a subduction is the Pacific Ring of Fire in Japan, where the land continents collide, resulting in the land being raised or uplifted and form mountain ranges, also the case of the Himalayas ranges of India, Andes Mountains in South America etc. The evidence of the subduction is found in the shape of the mountain chain. Furthermore the geological up thrust of the molten rocks from the seabed could also be found in these mountain ranges [Monastersky, 1998].

However, despite these evidences, according to one scientist Beloussov (1980, 1990) plate tectonics theory is still premature to act as the basis for structuring of the ocean floor, mountain ranges and formation of the geological activity. He wrote that:

It is... quite understandable that attempts to employ this conception to explain concrete structural situations in a local rather than a global scale lead to increasingly complicated schemes in which it is suggested that local axes of spreading develop here and there, that they shift their position, die out, and reappear, that the rate of spreading alters repeatedly and often ceases altogether, and that lithosphere plates are broken up into an even greater number of secondary and tertiary plates. All these schemes are characterized by a complete absence of logic, and of patterns of any kind. The impression is given that certain rules of the game have been invented, and that the aim is to fit reality into these rules somehow or other." (1980, p. 303).

Yet even if one were to refer to the different spheres of the earth like oceanic lithosphere, midocean ridges and upwelling of molten material from the mantle it is seen that once magma cools it spreads in different ridges. When this happens the horizontal plates start to move and plunge into the mantle, displacing other geological features like subduction zones. This in turn is responsible for formation of the earth's later crust. If this evidence is not enough, then how can one explain the spreading of seafloor, where do the magmatic volcanic arcs go, why do trenches form and how are boundaries created even at the ocean floor level.

David Pratt [2000] in his research identified the ocean floor as far from having uniformity in its shapes and sizes. It is seen that the "ocean floor's lithosphere is symmetrical in relation to the ridge axis" and that "increased in thickness with distance from the axial zone, more detailed seismic research has contradicted this simple model." This notion in Pratt's study show that there are several "low velocity" zones in the oceanic mantle that are slower then the other crust area where the depth of the zones varies in their distance from the midocean ridge. This is the reason why the Atlantic, Indian, and Pacific Oceans "have shown the extensive distribution of shallow-water sediments ranging from Triassic to Quaternary. The spatial distribution of shallow-water sediments and their vertical arrangement in some of the sections refute the spreading mechanism for the formation of oceanic lithosphere" (Ruditch, 1990 qtd. From Pratt, 2000). In this context, it would mean that the ocean floor underwent subsidence and elevation. The ridges, height of the Romanche fracture zone in the equatorial Atlantic for instance is considered to be the model of spreading. But there is no evidence that its formation is derived from plate tectonics. The height of these ridges range from 1-4 km above the sea floor. The formation of these ridges, it has been predicted were faster then the plate tectonics only 5 millions years ago [Pratt, 2000].

Continental Drift

However, if the above was the case then how can scientist justify the hypothesis for continental drift. According to Ann G. Metzger and Jill Stevens Johnston of Center for Earthquake Research and Information, University of Memphis [1998], plate tectonics is the basis of reasoning for the theory of continental drift. This theory believes that around 175 million years ago the surface of the earth was one large mass of land. Despite the differential in opinions among scientists regarding the formation of the landmass of today, all have conjugate in their opinion on the fact that rock formation, geological distribution of these rocks as well as the different evidence of land support like earthquake, faulting and folding and ocean floor spreading cannot be reasoned if plate tectonic is ignored. Some rock composition may have changed but the overall layers and connection of rock formation from continent to continent are similar. One explanation for difference in composition is that the plates have collided into each other several times before it came to its present state.

The earth and its contraction of the different layers of crust cannot be treated as pieces of puzzles and according to the "best fit" theory. Since it is hypothesize that the landmass had collided several times, it is not necessary that each and every puzzle corner should match the other. It is not necessary that the east coast of Brazil in South America should match those of southwestern coast of Africa, nor does it mean that the North eastern coast of American match with Western Europe. The very fact that the plates are different in its boundaries, its fossils and rock geological composition show that these boundaries have been subjected to extreme force, collision and deterioration. Using the puzzle method would not give the desired answer. Instead, the plate tectonic theory provides the best answer for the different kind of life form common to most of the plates on earth. Fossil rocks for instance is the evidence of this theory. During the paleonthic age for instance coastlines of the continents were the hubs for changing landscape. Animals living in these areas, evident from fossils, migrated to different plates for survival. These fossils show the trace of the changing landmass [Tarbuck and Lutgens, 1984]. The magnetic traces of the rocks and its minerals for instance characteristics of volcanic or molten rocks all show the how rocks cooled, their particles dispersed to various levels and areas of the plate. When the plates collided the rocks got displaced but their composition remained in tact even though they became part of different landforms like mountains, ocean floor or valleys depending on how forceful the impact of the tectonic was. In America the evidence of such changing landmass is evident in the Ouachita Mountain folds, where the ocean crust meets the continental crust. Similar evidence of continental shifts is evident by the faulted sheets of land, overriding the land of the Northern area. When this process occurs, often the rocks crack and collide with new sets of mountains, displacing sedimentary layers to another level and hence form mountains, plateaus and uplifts.

There is no doubt that weathering and atmospheric pressure act as the changing factor for the landmass. However, unless the landmass itself changes, fragments of the whole landscape cannot change so drastically. For instance in North America the ocean floor gets shallower and the South America plate gets closer to the north is not the result of weathering. A shift of the landmass of this nature could only be attributed to plate tectonics. Even though the movement is slow but it is evident that the movement have been going on since age old. The traces of sedimentary rocks, the squeezed rock walls through folds as well as the fragments resulting from faulting all contribute to change in geological change. Unless large sheets of land over ride each other, one cannot really say that the formation of such geological composition the result of some weathering or even change in mantle level. Indeed the change of such massive nature through cracks, faults and folds could only be done through tectonics.

In this regard tectonicists admit that although the velocity of the movement is slow but it nevertheless exist. Furthermore, the argument that the lithosphere is thick and the plates cannot move with such speed is baseless since the plates glide above the plastic flow of the mantle and does not depend on the depth of the earth's crust. "Plate tectonicists expected seismic tomography to provide clear evidence of a well-organized convection-cell pattern, but it has actually provided strong evidence against the existence of large, plate-propelling convection cells in the upper mantle" (Anderson, Tanimoto, and Zhang, 1992 qtd. From Pratt 2000). It is through this reasoning that one come to the conclusion that the motion of plate could only be the sole reason for the shallow and deep mantle depths when the plate move to and fro, flowing over the surface of the mantle (McGeary and Plummer, 1998 qtd. From Pratt, 2000).