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					               EPS 50: Lecture 20


• Magnetic anomalies

• The Geoid

• Magnetic mapping

                   What is Gravity?
• the force that pulls two masses towards each other, and in
  the case of Earth, pulls mass towards the center of Earth

• as an object’s mass increases, the gravitational attraction
  of that object increases (Newton)

• decreases as a function of (distance)2

• infinite range, always attractive

• acceleration due to gravity is
  9.8 m/s2 … or is it?

 • Isaac Newton calculated, from studies of planets and the
 force of gravity, that the average density of the Earth is
 twice that of surface rocks

 • Earth's interior must be composed of much denser

 • if Earth had perfectly uniform composition then gravity
 would decrease linearly with distance, reaching zero at
 the center

 • gravitational field peaks within the Earth at the core-
 mantle boundary where it has a value of 10.7 m/s²

 …gravity provides constraints on Earth’s structure!

        Gravity is perturbed by density
• density is a measure of how much mass is concentrated
  in a given space

• Earth’s topography is highly variable
• different rock types have variable densities

• fluctuations in density cause slight variations in the gravity
  field that can be measured and studied
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            An almost spherical Earth
• local variations in the strength of the Earth's gravitational
  field arise because the earth is not a perfect sphere and
  is not of uniform density

• main deviation from sphericity is Earth's equatorial bulge

• causes gravity to be weaker at the equator than the poles

• local topography (such as the presence of mountains)
  and geology (the density of rocks in the vicinity) also
  influence the gravitational field

                Gravity Time Scales
• although Earth’s surface is not uniform, most variations
  are constant over very long time intervals (mountain)

• gravity influence of large features is pretty much the same
  over a very long time and is known as the mean (or long-
  term average) gravity field

• some mass variations occur on much smaller time scales -
  mostly due to variations in water content as it cycles
  between the atmosphere, oceans, continents, glaciers,
  and polar ice caps

• shorter-term mass fluctuations contribute to what is known
  as the time-variable gravity field

                              Gravity Anomalies
•A gravity anomaly is the difference between observed gravity
and its theoretical value (based on a modeled Earth surface)
• mean gravity field
   - structure of the solid Earth
    - subsurface geologic structures
    - mineral exploration and assessment
    - ocean circulation

• time-variable gravity
    - ground water fluctuations
    - sea ice
    - sea level rise
    -deep ocean currents
    - ocean heat flux

                  Seafloor mapped using gravity

                           The Geoid
• hypothetical reference surface of
  Earth’s gravity field that most
  closely approximates the mean
  sea surface in the absence of
  winds, currents, and most tides

• defines the horizontal everywhere
  and gravity acts perpendicular to it

                     The Geoid Surface
 • the geoid surface is described by geoid heights that refer the
   dfference from an Earth reference ellipsoid (Earth covered in water)
 • -106 meters (S. Indian Ocean) to 73 meters (New Guinea)
 • more than any other data set of the Earth the Geoid shows us the
   dynamic structure of the Earth's deep interior

                The Geoid for North America
•   Yellowstone Hot Spot
•   San Joaquin Valley of California
•   mid-continent rift or suture zone
•   eastern deep suture structure
•   Midwestern postglacial Geoid low                              

Gravity Recovery and Climate Experiment
                                                 2002 - 2007: cm-scale
                                                    monthly gravity

• two GRACE satellites act in
  unison as the primary instrument
• changes in the distance between
  the twin satellites (~220 km) are
  used to make gravitational field
• areas of slightly stronger gravity
  affect the lead satellite first, pulling
  it away from the trailing satellite

                                         Local mapping:
                                         magnetic and
                                         gravity features

                                           Gravity map of Colorado
Aeromagnetic map of Colorado

                Magnetic and Gravity maps
• powerful combination for modern geological study



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