In this assignment we are going to discuss the theory of plate tectonics, its causes and effects and how different geographers have proven it true.
Plate tectonics is the theory that the surface of the earth is divided into a series of plates consisting of continental and oceanic crust. There are different types of plate movements. These include constructive margins such as mid Atlantic ridge, two plates move away from each other. Molten rock or magma immediately rises to fill any possible gap and new oceanic crust. The second type is destructive margins where plates consisting of oceanic crust move towards plates continental crust.
The third type is the collision margins. This is where two plates consisting of continental crust can move together. As continental crust cannot be destroyed, the rocks between them are forced upwards to fold mountains.
The fourth type of movement is the Conservative Margins. These are found where two plates slide past one another. As crust is neither being formed nor destroyed at this plate, boundary new landforms are not created and there is no volcanic activity. However, earthquakes can occur if the two plates “stick”.
The presence of Fold Mountains can lead to a colder climate and can act as a barrier to atmospheric circulation. Tsunamis are giant waves often generated at destructive plate margins that can cross oceans. Tsunamis are rare events but they can cause enormous damage and considerable loss of life. They occur when a sudden, large scale change in the area of an ocean bed leads to the displacement of a large volume of water and the subsequent formation of one or more huge waves. Although Tsunamis can result from a major coastal landslide, their origin is more likely to be seismic either after a volcanic eruption or a shallow submarine earthquake.
Effects of plate tectonics are volcanoes and earthquakes. As plates move they interact. The interaction of plates produces forces that build mountains, rift ocean basins and cause volcanoes and earthquakes. Tension forces associated with diverging plates tend to stretch earth’s crust, causing fault blocks to tilt or slide down and form fault-block Mountains. Generally, the faults that form from tension are normal faults. Once the divergence causes a separation, Rift Valley can form. Examples of Rift Valleys in Africa are the valleys that occur in the mid-ocean ridges, which are mountains that form on either side of rift valleys. Compressed forces produce several effects that take place when plates converge if the converging plates are both continental; the forces generated cause massive folding or rock of layer into mountain ranges such as the Himalayan Mountain or the greatly eroded Appalachian mountains - Neil Ardley etal (1979:320)
Reverse faults may also occur if the forces are great enough. If the two converging plates are oceanic plates, one plate slides under the other and island arcs and volcanoes form. If an oceanic plate converges on a continental plate, melting occurs and volcanoes and entire ranges of mountain can form at this type of convergent boundary - (Feather Sydney : 1997) Earthquakes and volcanic eruptions are caused by movements within the earth. There are many thousand gentle movements of the earth each year. Earth movements cannot as yet be predicted. Scientists do however know which parts of the world are most likely to be affected by these movements even if they cannot say when or with what they will occur.
According to Witherick (1995:11), the presence of Fold Mountains can lead to a colder climate and can act as a barrier to atmospheric circulation. Tsunamis are giant waves often generated at destructive, plate margins that can cross oceans. Tsunamis are rare events but they can cause enormous damage and considerable loss of life. They occur when a sudden, large scale change in the area of an ocean bed leads to the displacement of a large volume of water and the subsequent formation of one or more huge waves. Although Tsunamis can result from a major coastal landslide their origin is more likely to be seismic either after a volcanic eruption or a shallow submarine earthquake.
David Waugh (2009) states effects of volcanic eruptions include human life is destructed. Most of the deaths are caused by the release of poisonous gases which accompanied the blast waves.
Settlements are also destroyed as several houses are destroyed. Rivers and lakes are polluted as ash which falls into rivers and lakes raises temperatures, while sediments and mud also choke channels. The combined effect is the death of all fish.
Communication is also disrupted. Flood waters wash away roads and railway bridges. Falling ash hinders the smooth running of car engines. Forests can be totally flattened and destroyed. Trees being carried down by rivers cause log jam. Services such as electricity supply and communication services are interrupted. Trees are also usually destroyed in the blast. Farming is also disrupted and crops and livestock are destroyed by floods.
The movement of plates either into colder latitudes or at constructive margins where there is an increase in altitude could lead to an overall drop in world land temperature (Waugh 2009).
Plate movement has led to redistribution of land masses and long term effects on climate. These effects may result from a land mass drifting into different latitudes or from sea beds being pushed upwards to form high Fold Mountains.
The presence of Fold Mountains can lead to a cold climate and can act as a barrier to atmospheric circulation. Tsunamis can cause enormous damage and considerable loss of life. They occur when a sudden, large scale change in the area of an ocean bed leads to the displacement of a large volume of water and the subsequent formation of one or more huge waves.
Different Geographers have tried to prove that the theory of plate tectonics is true although different explanations and reasoning have been given. As early as the year 1620, Francis Bacon noted the jigsaw like fit between the east coast of South America and the west coast of Africa. Others were later to point out similarities between the shapes of coastlines of several adjacent continents.
In 1912, a German Meteorologist, Alfred Wegener published his theory that all the continents were once joined together in one large supercontinent which he named Pangaea. Later this landmass somehow split up and the various continents, as we know them, drifted apart. Wegener collated evidence from several sciences.
Wegener’s theory of continental drift combined information from several areas but his ideas were rejected by specialists in those disciplines partly because he was not regarded as an expert himself; perhaps mainly because he could not explain how solid continents had changed their positions. He was unable to suggest a mechanism for drift. Since Wegener put forward this theory, three groups of new evidence have become available to support his ideas. The first one is the discovery of the mid- Atlantic ridge. While investigating islands in the Atlantic in 1948, Maurice Ewing noted the presence of a continuous mountain range covering the whole length of the ocean bed. Ewing noted that the rocks of this range were volcanic and recent in origin; not as ancient as previously assumed was the case for mid-oceans. Later investigations show similar ranges on other ocean floors, the one in eastern pacific extending for nearly 5000 km (Richard White: 2009). The second one is the studies of Palaeomagnetism in the 1950s. During underwater volcanic eruptions, Basalt magma is intruded into the crust and cools. During the cooling process, individual minerals, especially Iron Oxides, align themselves along the earth’s magnetic fields. It was known before the 1950s that the earth’s magnetic pole varied a little from year to year, only then was it discovered that the magnetic field reverses periodically. For example, the magnetic pole is in the south for a period of time and then in the north for a further period and so on. It is claimed that there has been 171 reversals over a period of 76 million years. It is formed when then magnetic pole was in the north, the new basalt would be aligned to the north. After a reversal in the magnetic poles, newer lava would be oriented to the north. After a further reversal, the alignment would again be to the north. Subsequent investigations have shown that these alternations in alignment are almost symmetrical in rocks on either side of the mid-Atlantic Ridge.
The third theory we will discuss is the sea floor spreading. In 1962, Harry Hess studied the age of rocks from the middle of the Atlantic towards the coast of North America. He confirmed that the newest rocks were in the centre of the ocean and were still being formed in Iceland and that the oldest rocks were those nearest to the USA and the Caribbean. He also suggested that the Atlantic could be widening by up to 5cm every year. One major difficulty with this theory or concept of sea floor spreading was the implication that the earth must be increasing in size. This was not so; evidence was needed to show that elsewhere parts of the crust were being destroyed. Such areas were found to correspond to the fringes of the Pacific Ocean, the region where major earthquakes and volcanoes occurred. These discoveries led to the development of the theory of plate tectonics which is the now virtually universally accepted but which may be modified following further investigations and study by Geographers.
In conclusion, the theory of plate tectonics states that the earth’s crust and upper mantle are broken into sections. These sections called plates move around on the mantle. When plates move, they interact un different ways. Tectonic activity has caused huge changes in the earth’s surface. These forces have swifted rock layers and folded the crust as easily as if it were paper. Such forces have built mountains and split continents. They have also caused faults, cracks in the earth’s crust. Shifts along a fault can also cause earthquakes or violent jolts in the area around it. In coastal areas undersea earthquakes can cause huge waves known as Tsunamis to erupt. Plate tectonics cause folding of rock layers into mountains. There are different theories that have tried to prove the theory of plate tectonics true. They include Wegener’s theory of continental drift, also called Pangaea.
David Waugh 92009), An Integrated Approach, Nelson Thornes Ltd, Cheltenham
Feather Sydney (1997), Earth Science, McGraw- Hill companies Inc. New York
Neil Ardley etal (1979), The Universe and the earth, MacDonald Education Ltd, London Michael Witherick (1995), Environment and People, Stanley Thornes, London
Richard White (2009), Africa in Focus; A Physical Human and Economic Geography, Mcmillan Publishers, New York
DISCUSS THE THEORY OF PLATE TECTONICS, ITS CAUSE, EFFECTS AND HOW DIFFERENT GEOGRAPHERS HAVE PROVEN IT TRUE.