Sunday, December 16, 2012

Measuring Gravity at Mt. Erebus

Measuring gravity changes at Mt. Erebus will help measure the "plumbing" of the volcano or, how the magma flows within the volcano.  Our gravity man, Kevin Mickus, uses a gravimeter to measure local gravitational fields at many points around Mt. Erebus.

Gravity Principles

Though it's often drilled into our head during Physics I,II,III etc that the acceleration of gravity on Earth is 9.81 m/s^2, gravity at specific points on the earth changes due to various parameters.  Gravity changes with elevation, latitude, earth tides of the crust from the moon, and air pressure, to name a few.  In addition, the density of an object affects its gravitational acceleration and direction. 

Here at Mt. Erebus, gravity caused by the Earth is greater because we are at a pole rather than the equator.  Reasons for this include the "equatorial bulge," and the increased inertia caused by Earth's rotation.  The "bulge" results in a greater distance between and object and the Earth's center as compared to at a Pole, and therefore that object experiences a weaker gravitational pull.  The same rationale is applied to gravity at high altitudes.  The effect of altitude can be quantified by the following equation:

For studies of Mt. Erebus (we are currently at 11,500 ft. above sea level), we want to remove all these effects in order to obtain gravitational anomalies related to different densities of the volcano rock.  Denser material causes higher local gravitational fields, whereas the opposite is true of less dense material.  The two material extremes on any volcano are dense rock and less-dense magma.  Measuring gravity at Mt. Erebus can help locate where magma chambers exist.  Higher gravity measurements mean dense, cool volcanic rock, whereas lower gravity measurements mean less dense, hot magma.  The differences in gravity between these two extremes are very small.

The location and flow of magma reservoirs within a volcano are key to understanding its dynamics, including magma "plumbing system," volcano deformation, temperature distribution, and eruption mechanisms. 

Measuring Gravity

Gravity anomalies are measured with a gravimeter, and measured in milligals.  A Gal is a unit of acceleration that is used only in the field of gravimetry, and is 1 centimeter per second squared (cm/s^2).  When measuring the densities of rocks, precision is on the order micro-gals.

Though its cost is significant, a gravitometer is essentially a spring and a mass system.  The spring counteracts the force of gravity pulling on the test mass. Then change in length of the spring may be calibrated to the force required to balance the gravitational pull. 


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