Lesson on Theories Explaining the Earth's Movements
Theories Explaining the Earths Movements A theory is reasoned ideas intended to explain facts or ideas. There are 2 theories which explain the earth’s movements namely the Continental Drift Theory and the Plate tectonics theory. i)Theory of Continental Drift -Its proponent was A. Wegener.It explains the origin of 6 continents. It states: • The earth was a single sialic land mass called Pangaea surrounded by a huge ocean called Panthalasa whose floor was a mass of sima. • Pangaea broke into two parts called Laurasia (N. Hemisphere) which lay around equator and Gondwanaland (S. Hemisphere) which lay around south pole which wereseparated by a narrow ocean called Tethys (the present Mediterranean Sea). • Laurasia broke into Laurentian Shield and Fennoscandia (Europe, Asia and N. America) and moved northwards to their present positions. • Gondwanaland broke into Africa, Australia, S. America and Antarctica and India subcontinent.• Africa and India drifted northwards. Evidences Supporting the Theory 1. Fitting of western coast of Africa and S. America into a jigsaw. 2. Discovery of coal 40◦N and 55◦N which was formed by burying of tropical vegetation. 3. Considerable displacement of rocks along some faults e.g. along the Great Glen Fault of Scotland.4. Cape and Buenos Aires folds resemble one another by having east west trend. 5. Red sea shores show evidence of having undergone lateral displacement an indication that it was formed by movement of the earth’s crust. 6. Evidence of ancient Glaciation to the south of equator in Africa in Madagascar and India where there is presence of ancient glacial deposits suggesting these areas were once around South Pole. ii)Plate Tectonics Theory It states that: • The earth’s crust is made of blocks called plates. 7 Large Ones 1. Eurasian plate 2. Australian plate 3. Africa plate 4. Antarctic plate5. N. American plate 6. S. American plate 7. Pacific plate Smaller Ones 1. Indian 2. Arabian 3. Caribbean 4. Cocos 5. Somali plates 6. Juan de Fuca 7. Nazca8. Philippine 9. Scotia • These plates are two types: 1. Oceanic plates which form major areas of the ocean floor including coastal lowland. 2. Continental plates which form the bulk of the continental land mass. • The plates float on molten mantle layer called Asthenosphere.• The plates move relative to each other due to convectional currents in the mantle. • They move away from each other forming extension or constructive boundary called so because magma fills the space between. • They move towards each other forming compressional or destructive boundary calledso because materials between are crushed. The movements of those two types of plates have the following effects: 1. When two oceanic plates meet • There is subduction and the ocean floor is pulled inwards forming a trench e.g. JavaTrench .-Subduction is the passing of edge of one plate beneath the edge of another. • Sediments on the sea floor in the region of subduction are compressed to form Fold Mountains. 2. When an oceanic plate meets a continental plate the edge of the oceanic plate slides beneath the continental plate in a movement called subduction.• Sediments on the sea floor in the region of subduction are compressed to form Fold Mountains. • Fold Mountains are also formed at the edge of the continent when the sial layer is compressed. • The edge of the oceanic plate bends into the mantle forming a trench.3. When two continental plates collide then the sial layer is folded into mountains.- They move past each other forming transform or conservative boundary called so because there is neither construction nor destruction which occurs where the plates are separated by a major fault.Significance of Plate Movements 1. Are sources of earthquakes and Vulcanicity. 2. Causes formation of land forms such as Fold Mountains and ocean trenches. 3. Spectacular landscapes formed are a tourist attraction.4. Eruption of magma can result in formation of valuable minerals.