The figure of the Earth and effect on gravitational force

• Equatorial bulge - 21 km larger than the polar radius.
• Gravitation higher at poles and depends on latitude
• Does not change uniformly from place to place

Gravity measurements

• Latitude correction due to the bulge of the earth
• Local topography - correction to a "reference ellipsoid" (fixed level to measure from) -generally sea level
  • free air correction
  • Bouger correction - to correct for the amount of material above the reference ellipsoid

Gravity anomalies

• After all corrections are made, the gravity on the earth’s surface is not same everywhere. The differences are called gravity anomalies.

Anomalies track excess or deficiency of mass. Depth to the Moho deduced by seismic data show that crust is thicker under mountains. Crust is lighter than the matrial underneath, so this excess light material is reflected by negative gravity anomalies

Overall earth composition - Mg, Fe, Si, O more than 90% of the earth.

Only 8 elements make up more than 98% of the crust. These are: O, Si, Al, Fe, Ca, Mg, Na, and K. Of these, only two elements (Si, O) make up more than 70% of the continental crust.

Minerals: Building blocks of rocks. Any naturally formed solid element or compound, generally inorganic, with a definite composition and crystal structure - atoms occupying orderly, repeating three-dimensional networks.

Ions: electrically charged atoms or molecules. Charge is indicated by superscript w/ + or -. Number of atoms of a given element in a molecule is indicated by subscript.

• Simple ions: single element
  • Cations - positive charge, e.g., Na⁺
  • Anions - negative charge, e.g., Cl^-
• Complex ions: ions formed of more than one element , eg., carbonate ion, (CO₃)^2-
• Compounds - charge neutral, by a balanced combination of anions and cations. e.g., water 2 H⁺ + O^2-

  Rock-forming minerals: Silicates

  • Oxygen forms a simple anion, O^2-, and silicon forms a simple cation, Si⁴⁺, and these join togeteher the complex ion, the silicate anion, (SiO₄)^4-. This silicate anion is the basic building structure for silicates, the most common rock forming minerals.
  • Minerals can be identified by megascopic features such as color, form or shape, cleavage, etc. and also by optical and structural characteristics.
  • Nonsilicate minerals: oxides, carbonates etc. Important but nowhere near as common in the earth as silicates.

  The three primary groups of rocks

  • Igneous rocks: from cooling of magma
    • Plutonic or intrusive, cool slowly beneath the earth's surface, generally large crystals, coarse- grained
    • Volcanic or extrusive, cool fast at the earth's surface, very small to invisible crystals, fine-grained
  • Sedimentary rocks: produced from either particles or chemical sediments by lithification (cementing together to form a rock from separate particles)
  • Metamorphic rocks: Previously existing rocks affected and changed (but not melted) by high temperature and/or pressure.
    Metamorphism: Changes in mineralogy, texture, and composition. Regional (widespread, e.g. by tectonic plate collision) and contact (localized, e.g. by heating by an intrusive igneous rock) metamorphism.

    The crust is 95% igneous and metamorphic rocks. Sedimentary rocks are minor, but contain nearly all fossils and lots of information about earth history.

  The Rock Cycle: The three groups of rocks are connected by the rock cycle which relates the internal activities of earth to the external activities involving the atmosphere, hydropshere, and the biosphere.

  • Two parts in the rock cycle: one involves the continental crust, the other the oceanic crust.
  • Time scale of the rock cycle: millions to 100’s of millions of years
    • Past rates of internal processes
    • Past rates of external processes

  Magmas: silicate liquids, with minor amounts of dissolved gases

  • Water and carbon dioxide (CO₂) make up 98% of the gases
  • Three distinct type of magmas: based on silica content
    • Basaltic - 50% silica (approximately 80% of all magma, the others are ~10% each.)
    • Andesitic- 60% silica
    • Rhyolitic - 70% silica
  • Properties of magma:
    Viscosity: depends on temperature and composition, especially the silica content.
    • The higher the silica content, the higher the viscosity
    • The higher the temperature, the lower the viscosity.
  
  
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