Objectives:\u00a0 To determine graphically certain boundaries within the earth.<\/p>\n
Materials:<\/p>\n
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Protractor\u20144 inch or larger<\/p>\n
12\u201d scale divided into 10ths<\/p>\n
Compass capable of drawing a 10\u201d circle<\/p>\n
Calculator<\/p>\n
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Background:<\/p>\n
When an earthquake occurs, vibrations called seismic waves<\/u> are produced.\u00a0 These waves travel outward from an earthquake\u2019s focus<\/u>, its point of origin, in all directions.\u00a0 Two kinds of seismic waves travel through the Earth:\u00a0 P-waves and S-waves<\/u>.\u00a0 The third kind, surface waves, results when P- and S-waves reach the surface and travel outward along the surface from an earthquake\u2019s epicenter<\/u>, the point on the surface directly above its focus.\u00a0 By studying P- and S-waves, scientists have been able to develop a model for the Earth\u2019s interior.<\/p>\n
P-waves<\/u> can travel through solids, liquids, and gases, and they move faster through dense material than through less dense material.\u00a0 Studies of earthquake waves have shown the P-wave velocity increases or decreases at certain depths inside the Earth.\u00a0 A change in velocity suggests the presence of a boundary between materials of different densities.<\/p>\n
The outer layer of the Earth is known as the crust<\/u>.\u00a0 As P-waves travel through the crust, they suddenly increase in speed at a depth of about 50km.\u00a0 The change in speed indicates they are entering a material of higher density than the crust.\u00a0 This layer is called the mantle<\/u>.\u00a0 This boundary between the crust and the mantle is called the Moho or Mohorovocic discontinuity<\/u>.\u00a0 At a depth of about 2,900km, the P-waves sharply decrease in speed, indicating they are entering the core<\/u>.\u00a0 This boundary between the mantle and the outer core is called the Guttenburg discontinuity<\/u>.\u00a0 At about 5100km, P-waves speed up, indicating another boundary, this time between the outer core and the inner core.\u00a0 This boundary is called the Lehmann<\/u> discontinuity<\/u>.<\/p>\n
S-waves<\/u> can travel through solid material only, not through liquids or gases.\u00a0 Like the P-waves, their speed changes according to the density of the material through which they are traveling, speeding up in dense material and slowing down in less dense material.<\/p>\n
Scientists have found the S-waves cannot be detected on the side of the Earth opposite an earthquake; therefore S-waves were not able to travel through the Earth\u2019s core.\u00a0Since S-waves cannot travel through liquids, and the P-waves velocity<\/p>\n
suggested that the outer core was less dense than the inner core, scientists think that the outer core may be somewhat fluid and the inner core solid.<\/p>\n
Shadow zone<\/u>:\u00a0 P- and S-waves are not detected at seismograph stations that are at an angular distance greater than 103o<\/sup> from the epicenter of an earthquake.\u00a0 P-waves are detected at an angular distance of about 143o<\/sup> or more.\u00a0 These findings indicate a zone on the Earth\u2019s surface between 103o<\/sup> and 143o<\/sup> from an epicenter in which no P- or S-waves can be detected.\u00a0 This region is called an earthquake\u2019s shadow zone<\/u>.\u00a0 Its existence can be explained by the way seismic waves bend as they travel through the Earth and by the inability of S-waves to travel through liquids.<\/p>\n When seismic waves travel through material that changes density gradually, they change direction gradually.\u00a0 When they travel from one material to another material of different density, they change direction sharply.\u00a0 As P- and S-waves travel through the mantle, they may bend slightly because of slightly different densities.\u00a0 When P-waves enter the boundary between the mantle and the core, the difference in densities causes them to bend sharply.\u00a0 They are reflected away from the core and reach the surface about 103o<\/sup> from an epicenter, or they are bent inward and travel through the core.\u00a0 When they leave the core, they are bent sharply again and reach the surface at about 143o<\/sup> from an epicenter.\u00a0 When S-waves reach the boundary between the mantle and the core, they are reflected back because they cannot travel through the dense fluid in the outer core.<\/p>\n Modified from:<\/p>\n What Earthquake Waves Tell Us about the Earth\u2019s Interior<\/p>\n Donald G. Korba<\/p>\n From Earth Science Investigations (1990)<\/p>\n American Geologic Institute<\/p>\n Alexandria, Virginia<\/p>\n Pages 59-63.<\/p>\n \u00a0<\/strong><\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n <\/p>\n ______________.\u00a0 It is called the __________________ discontinuity.<\/p>\n <\/p>\n Calculated depth to boundary is ________________ km.<\/p>\n Depth to boundary from Table 1 is _____________ km.<\/p>\n Why the difference?____________________________________________<\/p>\n 7.\u00a0 Transfer the 143o<\/sup> angles to Sheet 4 for all earthquake foci shown.<\/p>\n 8.\u00a0 What geometric shape have you outlined?\u00a0 Draw it.<\/p>\n 9.\u00a0 This is the boundary between the __________\u00a0 ___________ and the<\/p>\n _____________\u00a0 ____________.\u00a0 It is called the ____________ discontinuity.<\/p>\n 10.\u00a0 Complete the following ratio:<\/p>\n Calculated depth to boundary is ________________ km.<\/p>\n Depth to boundary from Table 1 is ______________ km.<\/p>\n Why the difference? __________________________________________<\/p>\n <\/p>\n <\/p>\n Table 1.<\/p>\n <\/p>\n 1.\u00a0 On Sheet 5(the pie shape figure), draw to scale<\/u> the interior of the earth using the data in Table 1.<\/p>\n 2.\u00a0 Place appropriate depths on the left side.\u00a0 Place appropriate boundary names on the right side.\u00a0 Indicate the surface and the center of the earth.\u00a0 Label the 4 zones within the earth.<\/p>\n PART 4.<\/p>\n 1.\u00a0 Assume the earth is a perfect sphere.\u00a0 Using the data in Table 1. and the formula for the volume of a sphere (V= 4\/3\u00a0 p r3<\/sup>) , calculate what percent of the volume of the earth is crust, mantle, and core.\u00a0 Round off to first decimal.<\/p>\n % volume of earth is crust _______________ %<\/p>\n % volume of earth is mantle ______________ %<\/p>\n % volume of earth is core ________________ %<\/p>\n TOTAL _________100%_______<\/u><\/p>\n \u00a0<\/u><\/p>\n <\/p>\n","protected":false},"excerpt":{"rendered":" Objectives:\u00a0 To determine graphically certain boundaries within the earth. Materials: Protractor\u20144 inch or larger 12\u201d scale divided into 10ths Compass capable of drawing a 10\u201d circle Calculator Background: When an earthquake occurs, vibrations called seismic waves are produced.\u00a0 … Continue reading Procedure:<\/h1>\n
PART 1.<\/h2>\n
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PART 2.<\/h2>\n
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Depth to center of the earth in km\u00a0 \u00a0\u00a0 Depth to boundary in km\n----------------------------------- = -------------------------\nDepth to center of earth in inches\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0\u00a0\u00a0 Depth to boundary in inches\n\n\u00a0\u00a06320\u00a0\u00a0\u00a0 Depth to boundary in km\n------- = -----------------------\n 5\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 you measure\n\n\n<\/pre>\n
Depth to center of the earth in km\u00a0 \u00a0\u00a0 Depth to boundary in km\n----------------------------------- = -------------------------\nDepth to center of earth in inches\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 \u00a0\u00a0\u00a0\u00a0\u00a0 Depth to boundary in inches\n\n\u00a0\u00a06320\u00a0\u00a0\u00a0 Depth to boundary in km\n------- = -----------------------\n 5\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0 you measure\n\n\n<\/pre>\n
PART 3<\/h2>\n
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\n Boundary<\/th>\n Depth<\/th>\n Name_____________<\/th>\n<\/tr>\n \n Surface<\/td>\n 0<\/td>\n ——–<\/td>\n<\/tr>\n \n Crust-mantle<\/td>\n ~50 km<\/td>\n Moho(Mohorovicic discontinuity)<\/td>\n<\/tr>\n \n Mantle-outer core<\/td>\n 2900 km<\/td>\n Guttenburg discontinuity<\/td>\n<\/tr>\n \n Outer core-inner core<\/td>\n 5100 km<\/td>\n Lehmann discontinuity<\/td>\n<\/tr>\n \n Center of Earth<\/td>\n 6320 km<\/td>\n ———<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n