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Inside the Earth

Inside the Earth. I. Earth’s Dimensions.

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Inside the Earth

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  1. Inside the Earth

  2. I. Earth’s Dimensions • Eratosthenes(200 B.C.) - great scientist, Alexandria, Egypt. He knew the earth was round! He measured the angle of the sun at Alexandria (29.7o N) and Syene (Tropic of Cancer - 22.5o) at noon on Jun. 21 to estimate the earth’s circumference at 40,000 km. His estimate was very close to the actual figure. (See below.) • Due to centripetal force caused by the earth’s rotation, the earth is not really round! It is slightly flattened at the poles. This “egg-like” shape is called an oblate spheroid. • Earth’s diameter is 12,756km at the equator and 12740km at the poles • Circumference - 40,076km at equator; 40,008km at poles ( 0.17 % difference )

  3. Oblate spheroid

  4. II. The Layers of the Earth a. The crust is the thinnest layer of the earth: 65km or less Thickestunder the continents - 30 - 40 km thick (up to 65 km under biggest mountains)Thinnestunder the oceans - only 5 - 10 km thick.Oceancrust is made of basalt and is slightly denser than the granite of the continents.

  5. Seismic Wave Travel

  6. II. The Layers of the Earth b. The next layer down is the mantle. It is the thickestsingle layer: 2,900km thick It is made of iron-rich silicates and oxides (magnetite, olivine, etc.), which are much denser than the rocks and minerals of most of the crust. Parts of the mantle are plastic: still solid, but able to bend and flow and change shape like a liquid (“Silly Putty”). The mantle is separated from the crust by a sudden, sharp boundary called the MohorovicicDiscontinuity, or “Moho” for short.

  7. II. The Layers of the Earth c. The inner and outer cores are thought to be made of iron and nickel. Shearing seismic S-waves won’t travel through the outer core. Since shear waves won’t travel through liquids, this tells scientists that the outercore is liquid. Compressional seismic P-wavesspeedup as they pass through the inner core, so it must besolid. The outer core has a thickness of 2,200km. The inner core has a thickness of 1,200km ( It is a 2400km diameter ironcrystal!) The Core-Mantle Boundary is very irregular and somewhat changeable

  8. III. Earth’s Magnetic Field • The inner core of the Earth spins independently of the Earth. (slightly faster.) • The Earth's magnetic field is generated mainly at the core, as the highly conductive metallic fluid of the outer core rotates with respect to the solid inner core, generating electric currents and an accompanying magnetic field.

  9. III. Earth’s Magnetic Field c. The magnetic field shields the Earth from the Sun’s cosmic rays. If not for the magnetic field, cosmic rays would strip away the ozone layer. (The ozone layer protects us from UV rays.)

  10. III. Earth’s Magnetic Field d. The interaction of the magnetic field and cosmic rays from the sun can be seen in the Northern and Southern latitudes as the Aurora Borealis and Aurora Austrialis.

  11. II. The Layers of the Earth

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