# Gravity

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```					               EPS 50: Lecture 20

Gravity

• 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?

1
• 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
material

• 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
line of
gravity
constant
gravity

gravity

2
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

3
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
http://earthobservatory.nasa.gov
- ocean circulation

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

Seafloor mapped using gravity

http://www.ngdc.noaa.gov/mgg/bathymetry/predicted/explore.HTML

4
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

ESA
• 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

5
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                                        http://www.ngs.noaa.gov

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

• 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
measurements
• areas of slightly stronger gravity
affect the lead satellite first, pulling
it away from the trailing satellite
http://earthobservatory.nasa.gov/

6
Local mapping:
magnetic and
gravity features

http://rockyweb.cr.usgs.gov/

Magnetic and Gravity maps
• powerful combination for modern geological study

http://www.geo.umn.edu/mgs/glgmaps.html

7

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 views: 9 posted: 11/28/2011 language: English pages: 7