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Transcript
Geography 10 Earth’s Internal Structure Ms. Ripley Name:_______________________ Instructions: Read the following passage and answer the questions 1. What are seismic waves? What are seismographs? 2. Describe 2 ways in which the structure of the rocks in the Earth affect the movement of seismic waves. 3. Why is the Earth’s structure made up of layers and how are the layers different? 4. Why is the Earth’s core so hot? 5. Describe 3 characteristics of: a. the Earth’s core b. the Earth’s mantle. 6. What causes the rock movements in the Athenosphere and how do these movements affect the crust? 7. The Earth’s crust is either sima and sial. Describe 2 characteristics of: a. sima. b. sial 8. Explain isostasy. L ittle was known about the internal structure of the earth until recently. New technology has enabled scientists to probe deep beneath the earth’s surface by studying vibrations in the different layers of the lithosphere. Earthquakes and underground nuclear tests send shock waves through the many layers of the lithosphere. These seismic waves are measured using electronic sensors called seismographs. Vibrations move through the layers of the lithosphere differently, depending on temperature. Cooler layers are generally more rigid than hot layers. When these cool layers vibrate, they send out seismic waves at a higher frequency than the shock waves that travel through hoc layers. Rock density’ also affects how shock waves travel. Sometimes the layer is under so much pressure that it is plastic and absorbs the shock waves. In geological terms plastic does not mean it is made of the same material as toys. It means that it is soft and pliable. The substance is solid but it behaves like a liquid. It flows like a liquid and rebounds when pressed down. Vibrations through the layers of the earth are altered by differences in temperature and plasticity. Piecing together evidence from seismic readings, scientists have determined what the interior of the earth is like. Why There Are Layers When the solar system was created 4.6 billion years ago. The earth and the other planets formed from a cloud of dust, gas, and ice. Gravity pulled the different bits and pieces of the solar system into huge chunks of solid matter. One such chunk became earth. As the new planet spun, gravity pulled heavier elements to the centre while lighter objects floated to the surface. This is why the earth is made up of layers. Each layer has different properties. They are made of different Geography 10 Earth’s Internal Structure Ms. Ripley Name:_______________________ elements and have different thermal characteristics. Seismic evidence supports theories that geologists have long had about the earths structure. But more detailed information about transition zones between layers is increasing our understanding of geology. The Core At the centre of the earth is the core. It is mad up of two parts, the inner core and the outer core. While the core appears quite small compared with the outer layers, it actually makes up a third of the planet’s total mass. When the planet was formed the elements were pulled to the centre of the planet. The inner core is believed to be made of iron, with some silicon and pockets of oxygen and sulphur. The density of the inner core is estimated to b 12.7 to 13 g/cm3, about the same density as the element mercury. Estimates of the temperature of the inner core vary from 4000°C to 6650°C. That is even hotter than some estimates of the sun’s temperature! What makes the earth’s core so hot? Scientists speculate that this incredible heat is the result of decaying radioactive material. Each time an alpha ray is emitted from radioactive matter, nuclear energy is given off. So the same radioactive isotopes that help geologists date rock layers create the heat of the earth’s inner core. All of this heat is held in by the insulating effect of the rock layers that lie above. You might think that these temperature would melt the core, but this is not the case. The incredible pressure of the rock pressing down makes the layer solid rather than liquid. The earth’s outer core is cooler and molten because the pressure is less. This fluid layer generates about 90 per cent of the earth’s magnetic field. It also forms the magnetosphere, a force field that protects the planet from cosmic radiation and solar wind. Scientists think that electric currents deep in the outer core are caused by the earth’s rotation. These currents produce the magnetic field that surrounds the earth. The outer core is less dense than the inner core, about 10.7 glcm3. Like the inner core, it is made up primarily of iron. The transition zone between the inner and outer core is approximately 5200 km below the earth’s surface. The Mantle Above the core is the mantle. This layer makes up about 80 per cent of the earth’s total volume. It is less dense than the core, averaging 4.5 g/cm, but denser than the layers above it. Iron, silica, and magnesium are the mantle’s main elements. Like the core, the mantle is divided into two parts: the lower mantle and the upper mantle. The lower mantle starts about 2900 km beneath the earth’s surface and Geography 10 Earth’s Internal Structure Ms. Ripley Name:_______________________ extends upwards almost 1200 km to the much narrower upper mantle. As you move towards the surface, the temperature of the mantle gradually cools. Until recently, the discontinuity, or boundary, that separates the lower mantle from the outer core was believed to be a smooth one. Recent seismic studies have shown that the discontinuity is in fact uneven, with many peaks and valleys much like the surface of the earth. The Asthenosphere Technology has enabled scientists to discover a new layer in the upper mantle. Called the asthenosphere, this layer is thought to affect plate tectonics and mountain building. Unlike the rest of the mantle, the rock in this layer sometimes acts like a liquid. Convection cells, most likely caused by decaying radioactive elements, sometimes occur making the rock less dense. Convection currents distribute this heat throughout the asthenosphere. As the material moves through the layers, continents sitting on the crust directly above the asthenosphere also move. This movement causes the crust to be folded or faulted to create mountains. When the hot spots reach the upper limit of the asthenosphere, volcanoes may erupt through the crust, forming new mountains. The disco ery of the asthenosphere helps to explain mai of the processes that shape the lithosphere. The Crust The outer layer, the earth’s crust, is found ne the surface. The crust forms the continents and underlies the oceans. This relatively thin layer (less than 0.1 per cent of the earth’s volume) consists of two parts. The part that underlies the oceans is often called the sima. It is made up of a rock called basalt. Comprising iron, magnesium, and silica minerals, this rock has the same composition as the mantle. It is less dense, however, at 2.9 g/cm3. This layer of the crust moves with convection currents in the asthenosphere. Where the sima collides with and plunges beneath flows under the continental crust, friction occurs. Volcanoes and earthquakes are often found along these continental margins. The part of the crust that underlies the continents is mainly granitic. This rock is made u of many different elements, including silica, potassium, and aluminum. It is less dense than the asthenosphere at 2.8 g/cm3 and floats on the denser plastic layer beneath it. The continental crust, often called the sial, varies from 20 to 80 km in thickness. Where there are mountain ranges it is thicker, sinking deep into the mantle. As the mountains erode over millions of years, the crust rises. It gradually becomes thinner until the mountains are nothing more than deeply eroded upland regions like those found in the Canadian Shield. This Geography 10 Earth’s Internal Structure Ms. Ripley Name:_______________________ process in which the sial rebounds is called isostasy. It is important to understand the structure of the earth because it explains many of the geological processes that occur in the lithosphere. As scientists learn more about the inner workings of the planet, our knowledge of the complex world in which we live will increase.