Chapter Four Earths Structure and Motion

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Chapter FourEarths Structure and Motion
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Topic OneEarths Formation

• Earth formed about 4.6 billion years ago from a
  whirling cloud of dust and gas. It developed
  layers as it cooled and dense material sank to
  its center. Meteorite impacts, the weight of
  overlying material, and the decay of radioactive
  isotopes caused earth to heat up soon after its
  formation. Since then, earth has been losing
  heat. Earth has a magnetic field.

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Topic TwoEarths Rotation

• Earth makes one complete turn on its axis every
  24 hours. Its axis of rotation is tilted with
  respect to earths orbital plane. Effects of
  this rotation include the Coriolis Effect,
  Foucault pendulum behavior, day and night, and
  sunrise and sunset. Earth is divided into 24
  worldwide standard time zones that begin at the
  prime meridian.

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Topic ThreeEarths Revolution

• Earth revolves around the sun in an elliptical
  orbit with the sun as one focus. Combined with
  Earths tilt, revolution causes seasonal changes.
  The summer and winter solstices are the longest
  and shortest days of the year, respectively. On
  the vernal and autumnal equinoxes, day and night
  are of equal lengths.

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Key Terms

• asthenosphere autumnal equinox crust inner
  core
• parallax lithosphere magnetic field mantle
• outer core International Date Line
• prime meridian revolution
• rotation standard time zones summer
  solstice time meridian
• vernal equinox winter solstice

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Topic OneEarths Formation

• The Earth is believed to be 4.6 billion years
  old. The nebular hypothesis states that a slowly
  rotating cloud of dust and gas in space
  eventually formed our solar system. As time
  passed, gravity caused the cloud to shrink and
  flatten into a disk shape, the speed of rotation
  to increase, and hydrogen fusion to form our sun.
  

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• The cloud eventually condensed forming
  planetesimals, masses of rock and ice. These
  would later become planets and moons. As our own
  earth began to spin on its axis, it developed a
  bulge in the center.

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• The Earth is an oblate spheroid, not a perfect
  sphere. The circumference around the poles is
  40,007. Around the equator, it is 40,074. The
  spinning of the earth on its axis causes the
  polar regions to flatten and the equator to
  bulge. The earths axis is an imaginary line
  running through the earth from the north to south
  poles. Seen from space, the surface appears to
  be smooth. The difference between the tallest
  mountain and the deepest ocean trench is only 20
  km. This distance is very small compared to the
  earths average diameter of 12,735 km.

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• By measuring the weight of an object at several
  different places, scientists were able to
  discover this unique shape.
• The weight of an object is less, the farther away
  you are from the center.

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Critical Thinking

• Where do you weigh more, at the top of Mount
  Everest or at the beach? Where would you weigh
  more, at the beach in northern Alaska or at the
  beach in Hawaii? Explain.

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• According to the nebular hypothesis, the original
  earth surface looked like our present day moon.
  It was covered with meteorite impact craters.
  Heat from these impact caused some heavy elements
  such as iron and nickel to melt. Denser liquids
  settled while less dense ones floated to the top,
  forming layers.

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• Direct observation of the earths interior is not
  possible, so indirect measurement must be used.

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• Seismic Waves
• A seismic wave is a wave that travels through the
  Earth, most often as the result of a tectonic
  earthquake, sometimes from an explosion.
• Seismic waves are produced by earthquakes and
  explosions on or near the earths surface. By
  studying how sound travels through the earths
  layers, scientists have determined that the earth
  is made of three major zones.

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CrustThe top layer of the earth. It
is brittle and broken into many pieces that
floaton the asthenosphere.

• It makes up 1 of the earths mass. Under the
  ocean, the crust is called oceanic and is 5 to 10
  km thick. Oceanic crust is thinnest beneath the
  trenches. The crust that makes up the continents
  is called continental and is 15 to 80 km thick.
  Continental crust is thickest beneath mountains.

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Mantle

• The portion of the earth, about 1800 mi. (2900
  km) thick, between the crust and the core.
• It makes up 2/3 of the earths mass and is
  further divided into 2 regions. The upper mantle
  is cool and brittle. This part of the mantle and
  the crust
• make up the lithosphere, a rigid layer 15 to 300
  km thick.

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• Next down is the asthenosphere. It is about 200
  km thick. Because of heat and pressure is has a
  plastic-like consistency and is able to flow,
  like Silly Putty. The ability of a solid to flow
  is called placisity.

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Core

• The central portion of the earth, having a radius
  of about 2100 mi. (3379 km) and believed to be
  composed mainly of iron and nickel in a molten
  state.
• The outer core is a dense layer of liquid about
  2,250 km thick. The inner core is solid and has
  a radius of 1,228 km. The inner and outer core
  together make up 1/3 of the earths mass.

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• Remember
• Mass is the amount of matter in an object.

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• The speed and direction of seismic waves is
  affected by the composition of the materials they
  pass through. The more rigid the material, the
  faster the wave travels.

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Heat in the Earth comes from three sources

• Meteorite Impact
• Weight of overlying material causing compression
• Decay of radioactive isotopes

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Since its original heating, Earth loses heat
slowly by

• Unequal heating of some rock types
• Variations in the thickness of Earths crust
• Percentage of radioactive material varies from
  rock to rock

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• The earth acts as a giant magnet, with 2 poles,
  north and south. The magnetic lines of force
  extend between the magnetic north and south
  poles. These lines of force extend above the
  earths surface, well into the atmosphere. The
  source is believed to be the liquid iron in the
  earths outer core. We know that iron is a good
  conductor of electricity and hypothesize that the
  motions in the core produce electrical currents
  that create the earths magnetic field. A second
  hypothesis is that magnetic fields may come from
  the sun or moon.

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Critical Thinking

• Is a hard-boiled egg a good model of the earths
  different zones. Why or why not?

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Modeling Earths Magnetic Field

• Materials
• Bar magnet card stock Iron filings tape
• Procedure
• 1. Tape the magnet beneath the center of a piece
  of card stock above the magnet.
• 2. Sprinkle the iron filings onto the card stock
  above the magnet.
• 3. Tap the card stock to allow the filings to
  align.
• 4. Draw what you see in the space below.

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• Describe the alignment of the iron filings.
  Earths magnetic field is similar to that of a
  bar magnet. Sketch earths outline over your
  drawing of the filing pattern, and mark the
  location of the poles.

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Homework

• Read Textbook pages 70 74.
• Complete Worksheet
• Earths Formation

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Homework Answers

• Table. See Notes
• 1. T 6. C
• 2. F 7. A
• 3. F 8. D
• 4. T 9. C
• 5. T 10. C

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Topic TwoEarths Rotation
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rotation

• The turning of a body on its axis.
• Evidence that the Earth rotates was provided by
  the Foucault pendulum. The scientist Jean
  Foucault discovered that the direction of
  movement of the pendulum was the same as the
  rotation of the Earth. More evidence is provided
  by observing the wind. Because of rotation, the
  winds appear to be turned, or deflected, to the
  left. This is called the Coriolis Effect. Any
  substance moving above or on the Earths surface
  is subject to the Coriolis Effect.

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• Like other planets in our solar system, Earth
  rotates as it travels around the sun. Earths
  orbit lies on a flat imaginary plane called an
  orbital plane. The axis of rotation is not
  perpendicular (90o), but is tilted at 23 ½ o. As
  a result of this tilt, our axis points to
  Polaris, the North Star. This tilt is always the
  same angle, and this is called parallelism.

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• Earth makes one complete rotation every 24
  hours. That means it rotates at a rate of 15 o
  every hour. The Earths shapes varies, and so
  does the speed of rotation. At the equator, the
  circumference is 40,074 km, making its speed 1670
  km/hr. At the latitude of Albany, the speed is
  1300 km/hr. At the North Pole it is 0 km/hr,
  because the poles are on the axis of rotation.

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• The behavior of a Foucault pendulum and the
  Coriolis effect are both a result of the Earths
  rotation. Another effect is the change from day
  to night. If you look down at Earth from the
  North Pole, it appears to rotate
  counterclockwise. The sun appears to rise in the
  east and set in the west. Half of earth receives
  sunlight at a time.

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Standard Time Zone

• Areas defined by 24-hour, 15º sections of
  longitude. Each is centered on a time meridian
  that established the time of day.

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Time Meridian

• A line of longitude exactly divisible by 15º on
  which each standard time zone is roughly
  centered.
• They often bend to go around political
  boundaries.

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• Areas within a time zone keep the same time.
  Clock time is the average solar time for that
  zone.

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• The sun is used as the basis for telling time.
  When the sun is at its highest point in the sky,
  it is noon. Because the sun moves from east to
  west, it can be found in different places at
  different times. We divide the earth into 24
  standard time zones. In each, noon is set when
  the sun is directly overhead. Because the earth
  is nearly a circle, it contains 360 o. Dividing
  360o by 24, we get 15 o. So as you look at a
  globe, each time zone is made up of 15 o of
  longitude. The time in each zone is 1 hour
  earlier than the time in the zone to its east.

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• The starting point for time zones is the prime
  meridian. As you travel from Greenwich to Hyde
  Park, you move your clock back. As you travel to
  Japan, you move it ahead. When you reach the
  International Date Line, the day changes. As you
  move west, it is one day later. As you move
  east, it is one day earlier. Locations on either
  side of the International Date Line keep the same
  time, but are different dates. In the U.S., we
  are one day behind eastern Asia.

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Homework

• Read Textbook pages 75 78.
• Complete Worksheet
• Earths Rotation

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Homework Answers

• Because of Earths shape, it is wider at the
  Equator.
• Take the number of degrees in a circle and divide
  by the number of hours in a day.
• The length of the suns shadow changes as time
  changes.
• Pendulums, quartz crystals, and radioactive atoms
  can be used.
• 1 sec. 9,192,631,770 cycles of the cesium
  atoms frequency.

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Topic ThreeEarths Revolution
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Revolution

• The movement of one body around another, such as
  the Earth in its orbit around the sun.

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Parallax

• The apparent shift in one objects position
  relative to another caused by a change in the
  location of the observer.

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• January July

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• The earths orbit, or path around the sun, is
  elliptical. Because of this, the earth is not
  always the same distance from the sun.

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Perihelion

• The closest the Earth is to the sun in its
  orbital path.
• The earths perihelion distance is 147 million
  km.

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Aphelion

• The farthest the Earth is away from the sun in
  its orbital path.
• The earths aphelion distance is 152 million km.
  The average distance between the earth and sun
  is 150 million km.

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• The earth tilts on its axis 23.5o from the
  perpendicular to the plane of its orbit. The
  axis points toward the North Star, Polaris. As a
  result, during each revolution, the North Pole
  tips at times toward the sun and at other times,
  its tips away from it. When it tips toward the
  sun, the periods of daylight are longer.

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• As the suns rays reach the earth, they are
  parallel, but because of the curve of the earth,
  they strike different parts of the surface at
  different angles. When the sun is directly
  overhead, the rays strike the surface at 90 o,
  and are more concentrated. As the earth moves
  through its orbit, the angle at which the sun
  strikes it changes.

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• These changes cause seasons. When the North
  Pole is tilted away from the sun, the angle of
  the rays falling on the surface in the Northern
  Hemisphere is greater, causing fewer hours of
  daylight, making the weather cooler (winter). In
  the Southern Hemisphere, the days are longer
  because the suns rays are stronger (summer).

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Arctic Circle
Tropic of Cancer
Equator
Tropic of Capricorn
Antarctic Circle
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Summer Solstice

• First day of summer in our hemisphere. The noon
  day sun reaches its highest point in the sky.

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• Solstice means sun stop. Occurs on June 21 or
  22. The North Pole tilts toward the sun. It
  marks the beginning of summer in the Northern
  Hemisphere. Along the Tropic of Cancer, the
  suns rays strike at 90 o. The Northern
  Hemisphere has the most number of daylight hours.
  Above the Arctic Circle, there are 24 hours of
  daylight. Below the Antarctic Circle, there are
  24 hours of darkness.

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Winter Solstice

• The first day of winter in our hemisphere. The
  noon day sun is at its lowest point in the sky.

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• Occurs on December 21 or 22. The North Pole
  tilts away from the sun. It marks the beginning
  of winter in the Northern Hemisphere. Along the
  Tropic of Capricorn, the suns rays strike at 90
  o. The Northern Hemisphere has the least number
  of daylight hours.

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Autumnal Equinox

• The start of fall in our hemisphere. The noon
  sun is directly over the Equator. One of the two
  days of the year that have equal daytime and
  nighttime hours.

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• Equinox means equal night. Occurs on
  September 22 or 23. Marks the start of Fall in
  the Northern Hemisphere. The suns rays strike
  the equator at 90 o. On this day the pole tilts
  neither toward nor away from the sun. The number
  of day and night hours are the same.

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Vernal Equinox

• The start of spring in our hemisphere. The noon
  sun is directly over the Equator. One of the two
  days of the year that have equal daytime and
  nighttime hours.

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• Occurs on March 21 or 22. Marks the start of
  Spring in the Northern Hemisphere. The suns
  rays strike the equator at 90 o. On this day the
  pole tilts neither toward nor away from the sun.
  The number of day and night hours are the same.

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Precision

• The wobble of a body that orbits and spins on
  its axis.
• It is caused by the forces that act on a
  spinning body. It causes the earths axis to
  spin slowly in a circle, the way a top does as it
  spins on a table. The earths axis completes one
  full circle every 26,000 years, so Polaris will
  continue to be our North Star for many years.

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Another look at ParallaxLook at the five disks
suspended from the ceiling.

• 1. Stand in front of the yellow disk at a
  distance of several meters.
• 2. Close one eye and sketch the position of the
  blue disk relative to the yellow disks in the
  background.
• 3. Take several steps back and to the right of
  the original position. Repeat Sept 2.
• 4. Take several more steps directly back. Make
  another sketch.
• 5. Repeat Step 4.

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• Compare your drawings. Did the yellow disk
  change position as you viewed it from different
  locations. Explain your answer.

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• What kind of results would you expect if you
  continued to repeat Step 4 at greater and greater
  distances? Explain your answer.

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• If you noted the position of several stars with
  a powerful telescope, what would you expect to
  observe about their positions if you sighted the
  same stars several months later? Explain.

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Homework

• Read Textbook pages 80 - 83.
• Complete Worksheet
• Earths Revolution

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Homework Answers

• 1. F 6. F
• 2. T 7. B
• 3. T 8. C
• 4. F 9. B
• 5. F 10. D