external phenomena affecting earth: gravity?

[Guest kozko]

After watching a sort of documentry in my geology class I was wondering 2 things about gravity:

1. how do we measure gravity?

2. how do we detect gravity and changes in gravity?

 

any feedback on this would be nice, since no matter how well my teacher tries to explain it...he still cant explain it.

 

thanks

[coquina]

After watching a sort of documentry in my geology class I was wondering 2 things about gravity:

1. how do we measure gravity?

2. how do we detect gravity and changes in gravity?

 

any feedback on this would be nice' date=' since no matter how well my teacher tries to explain it...he still cant explain it.

 

thanks[/quote']

 

Gravity maps indicate what kind of rocks are below the surface. What is measured is not gravity itself, but differences in gravitational acceleration.

 

If there is a very dense rock (like basalt) beneath the surface it has a greater gravitational acceleration than a less dense one, like granite. But, there are all kinds of other things that affect the gravity of a particular area. - Mountain ranges, tides, stuff like that. However, those amounts are known or can be calculated, so their effects can be removed from the equation.

 

You're trying to get a "flat slab" measurement, with nothing else affecting the measurement but the differences of the rocks in the sub-surface.

 

The measurements are taken with a gravitometer - look here:

http://geophysics.ou.edu/gravmag/measure/relative.html

 

The idea is that a hunk of basalt or other dense rock will attract the weight on the end of the spring more than the granite or some other not so dense rock.

 

I'm not sure, but I think some of the more sophisticated instruments can be calibrated so that external effects are removed before the measurements are taken.

 

Check out Dick Gibson's primer on gravity and magnetics.

http://www.gravmag.com/gmprimr.html

 

Down at the bottom is a gravity map of the US, with explanations about the reasons for the highs and lows.

 

Hope this helps..

[Guest kozko]

the information that you provided cleared up all the problems i was having in understanding how gravity is measured...or not measured for that matter...

thanks