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Transcript
What’s on your plate? A teacher briefing on plate tectonics This teacher briefing is for teachers and other educators to give young people an understanding of the structure of our planet and how the physical geography of Abu Dhabi today displays the consequences of dramatic geological activity. If for a moment we could look inside the Earth, we would see it’s made of four concentric layers(2): The inner core The Earth’s inner core is in the centre and is the hottest part of the Earth(3). It is like a solid ball made up of the elements iron and nickel(4). With immense temperatures of up to 5,500°C(5), the temperature at the inner core is thought to be similar to that of the surface of the Sun(6). The outer core The Earth’s outer core is a liquid layer(7) that surrounds the inner core. This layer is about 2,250 kilometers(8) thick and it is believed that this outer core is made up of super-heated liquid molten lava(9). This lava is believed to be mostly iron, and nickel(10). The mantle The next layer is the Earth’s mantle(11). It is the widest section of the Earth(12). It has a diameter of approximately 2,900 km(13) and is about 85%(14) of the total weight and mass of the Earth. The mantle is made up of semi-molten rock called magma(15). In the upper parts of the mantle, temperatures reach 500°C(16) and the rock is hard, but lower down at the boundary with the core, the rock is soft and beginning to melt with temperatures up to 4,000°C(17). The crust The Earth’s crust is the solid outer layer of the Earth(18). It is a thin layer between 0-60 km thick(19) and makes up less than 1%(20) of the Earth’s mass. The crust is the solid rock layer upon which we all live. When you look at our planet, you see that the surface of the Earth consists of a lot of water (71%)(21). The other 29%(22) consists of land. You can divide this land into six(23) big pieces, which are called continents. The different continents - arranged in decreasing order of size are: Eurasia (Europe and Asia together), Africa, NorthAmerica, South-America, Antarctica and Australia(24). There are two different types of crust: Oceanic crust The oceanic crust is the crust below the oceans. It is made up of extremely dense rock which scientists call Sima(25). Sima is made out of two basic substances. These substances are silica and magnesium(26). Here the crust is 6-11 km thick(27). The rocks of the oceanic crust are very young compared with the rocks of the continental crust. The rocks of the oceanic crust are not older than 200 million years(28). Continental crust The continental crust carries land. The rocks found in the crust of the continents is called Sial(29). Sial is made of silica, and aluminum(30), and is much lighter than the Sima beneath it. The continental crust is older than the oceanic crust, some rocks are 3.8 billion(31) years old. Plate tectonics The Earth’s crust is divided into a series of plates that are continually moving, colliding or pulling apart relative to each other. (32) The Earth’s crust consists of nine large plates (33) and twelve smaller ones(34). The continents are in the continental plates and the oceanic plates make up much of the sea floor. Heat rising and falling inside the mantle creates convection currents generated in the Earth’s core(35). The convection currents move the plates. Where convection currents diverge near the Earth’s crust, plates move apart(36). Where convection currents converge, plates move towards each other. The movement of the plates and the activity inside the Earth is called plate tectonics(37). The movement and collision of the Earth’s plates has had a profound impact on the physical geography of Abu Dhabi and the wider region. Modern World Present Day 20 million 20 Million years ago 100 million 100 Million years ago 200 million 200 Million years ago 300 million 300 Million years ago 400 million 400 Million years ago 500 million 500 Million years ago 600 million 600 Million years ago 650 650million Million years ago Plate tectonics cause earthquakes and volcanoes(38). The point where two plates meet is called a plate boundary(39). Earthquakes and volcanoes are most likely to occur either on or near plate boundaries(40). There are three types of boundaries that we can observe. These plate boundaries are: Divergent boundary The word divergent is a big word used by scientists to describe what happens when two objects move away from each other. So, a divergent boundary is a boundary where two tectonic plates are moving away from one another(41). Where plates pull apart, hot molten rock emerges as magma(42) and so new matter is added to the plates. This is also accompanied by earthquakes. When the magma reaches the surface, it cools and solidifies to form a new crust of igneous rock. This process is repeated many times, over a long period of time. Eventually the new rock builds up to form a volcano. These boundaries tend to be found under the sea. Convergent boundary Convergent is another big word that scientists use to describe two objects that come together. In the case of plate tectonics, the two objects coming together are plates of the Earth’s crust(43). As these two plates push against each other, one is eventually forced down beneath the other. In many places the huge plates of the Earth’s surface are slowly moving together with unimaginable force. Sometimes the edge of one plate is gradually destroyed by the force of collision, sometimes the impact just crimps the plates’ edges, thereby creating great mountain ranges(44). When one tectonic plate bends beneath the other, it is called subduction(45). Most of the time this happens because a dense oceanic plate collides with a lighter continental plate. The oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. The point at which this happens is called the subduction zone(46). As the oceanic plate is forced below the continental plate it melts to form magma and earthquakes are triggered. The magma collects to form a magma chamber. This magma then rises up through cracks in the continental crust. As pressure builds up, a volcanic eruption may occur. As the plates push together, the continental crust is squashed together and forced upwards. This is called folding(47). The process of folding creates fold mountains(48). Fold mountains can also be formed where two continental plates push towards each other. Transform boundary A transform boundary is where two plates slide against each other in a sideways motion(49). These boundaries between plates is referred to as transform boundaries(50). As two plates slide past one another, in a transform boundary, neither plate is added to at the boundary, nor destroyed. The result of two massive plates pushing against one another is that massive amounts of energy build up. Occasionally this energy is released suddenly in the form of large earthquakes(51). Abu Dhabi The physical geography of Abu Dhabi today displays the consequences of such dramatic movements, which still continue – the Emirate is located close to an active plate boundary between the relatively small Arabian Plate and the massive Eurasian Plate(52). The following definitions are provided for the educator’s reference. Young people should have the opportunity to explore definitions through individual activities. Some definitions may have been deliberately simplified for a young audience. Boundary A line which marks the limits of an area, a dividing line like a plate boundary. Dense Closely compacted in substance, having the parts crowded closely together. Magma Magma is molten rock often located inside a chamber beneath the surface of the earth. Concentric Having a common centre; “concentric rings”, the larger often completely surrounding the smaller. Diverge To move or draw apart. Molten Reduced to liquid form by heating; “a mass of molten rock”. Convection currents Convection currents in the hot, solid rock of the Earth’s mantle help to drive the movement of the rigid plates making up the Earth’s surface. Converge To come together so as to form a single thing. Crimp To compress something into small folds or ridges. Elements There are more than one hundred substances that cannot be broken down into simpler substances and are the simplest forms of matter. Igneous To have solidified from lava or magma. Lava Lava is molten rock which due to its high temperature, it is quite liquid when it is first released from a volcano or fissure, but soon solidifies into rock. While it is still below the earth’s surface it is called magma. Sial The material of the upper or continental part of the earth’s crust, it relatively light and rich in silica and alumina. Sima The material of the lower part of the earth’s crust, underlying both the ocean and the continents, it is relatively heavy and rich in silica and magnesia. Subduction The sideways and downward movement of the edge of a plate of the earth’s crust into the mantle beneath another plate. For more teacher resources including Teacher Briefings, Lesson Plans, Activity Worksheets, and Fact Sheets, go to www.environmentalatlas.ae. References: 2) S. Marshak (2007) Earth: Portrait of a Planet. W. W. Norton & Co. 3, 35) A. Bell (1997) How the Earth Works: How Plate Tectonics Works: Block 2 (Course S267). 4, 36, 37) A. Cox (2008) Plate Tectonics: How it Works. 5,6) F. P. Miller (2010) Geothermal Gradient: Earth, Internal Heat, Gravitational Binding Energy, Potassium, Uranium, Radioactive Decay, Thorium, Mantle Convection, Plate Tectonics, Komatiite, Geothermal Power. 7) N. Oreskes (2003) Plate Tectonics: An Insider’s History Of The Modern Theory Of The Earth. 8) R. Harman (2006) Earth’s Changing Crust: Plate Tectonics and Extreme Events. 9) P. Keary (2009) Global Tectonics. 10) J D. Winter (2009) Principles of Igneous and Metamorphic Petrology. For full references, go to www.environmentalatlas.ae.