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EXERCISE EARTHQUAKES AND FLATE BOUNDARIES PURPOSE Describe the method used to find the location and depth of earthquakes on a World Seismicity Map. Describe the relationship between earthquakes and various types of plate boundaries. . Construct a graph showing the distribution of earthquake depths at different locations adjacent to a subduction zone. . Describe the distribution of earthquake depths in an area where continental and oceanic plates converge toward each other. Describe the relationship between depth of earthquakes and major geographic features in . South America. Interpret data to identify the types of plate boundaries located in specific geographic areas. INTRODUCTION Why do earthquakes occur and where they do they occur on the earth? What causes earthquakes? Are earthquakes related to any earth structures? The earths outer shell of rock is believed to be made up of a number of rigid plates, called lithosphere plates, which range in thickness from 80 km to 160 km. These plates are made up of the two upper rock zones of the earth. The lithosphere consists of earths crust and the upper portion of earths mantle (Figure 6.1). The earths asthenosphere is located in the upper mantle below the lithosphere. Some plates include only oceanic crust (mafic composition). Other plates include both oceanic and the less dense continental crust (felsic composition). These plates ride on a zone in the mantle where the rock is partially melted and more fluid. This fluidity enables the lithosphere to float on the more dense asthenosphere An example to help you picture this would be a block of wood floating in a bowl of honey or an iceberg that is floating in an ocean. Water is more dense than ice. This is why ice floats in water. In this exercise you will discover some of the different patterns of earthquakes. The relation between earthquake boundaries and the different kinds of plate boundaries. Plate boundaries are classified as convergent, divergent or transform fault. A great deal of action takes place along adjacent plate boundaries. Large forces build up as one plate moves with respect to another plate. When the force becomes too great, rocks move against each other and fracture. The fracture of rock cause an earthquake. Different types of movement of adjacent plates may occur. Plate boundaries are the sites of much earthquake and other geologic activity. Both the pattern of earthquake distribution and major topographic features are distinctive of the type of plate boundaries and motion (see Figures 6.2,6.3,6.4, and 6.5)
38 Not to scale O km Strong shell ithosphere) 80-160 km ak shell asthenosphere Figure 6.1. Lithosphere plates and their relation to the outer parts of the earth. PROCEDURE 1. Study the World Seismicity Map. Carefully observe the location and depths of earthquakes. Materials: colored pencils, World Seismicity Map. The Physical World map. 2. Answer the following questions after you complete your examination of the 3. The dots on this map represent earthquakes that occurred over a period of time of__ 4. On the World Seismicity Map large earthquakes are plotted ascircles (o), smaller World Seismicity Map. years and months. earthquakes by dots (o). The focus of an earthquake is the point within the earth where an earthquake (fracture) actually occurs. The epicenter of an earthquake is the location point on the earths surface which lies directly above the focus. See Figure 6.6. Which of these two points is represented by the location of the dots on the map showing earthquakes? 5. 6. On the map, what color is used to indicate earthquakes of; a. shallow focus (0-70 km)? b. an intermediate focus (71-300 km)? c. a deep focus (301-700 km)? Scientists believe that zones of frequent earthquake activity are the result of movements along plate boundaries. Many of the earths surface features are associated with plate boundaries. See where the locations of epicenters are located on the World Seismicity Map. Then find the location of ocean ridges and trenches on The Physical World map. List at least two major kinds of ocean floor features which seem to coincide with the location of major zones of earthquake activity.
41 Epicenter FaultFocus Earthquake wave fronts Figure 6.6. Focus and epicenter of earthquake. 7. Where do most of the earthquakes occur in South America? 8. List one kind of continental feature which seems to be related to frequent earthquakes.
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