Jurassic Magnetism

Gravity is the force of attraction between any two masses; the study of this force is gravimetry; the instruments used to perform this study are gravimeters (or sometimes gravitometers). In the context of the Earth, when we measure the acceleration due to gravity, the Earth is so massive compared to everything else in the picture that there is little room for variance: we must be able to detect small variations on the scale of 1/1000th of a “gal” (the unit of gravitational acceleration named for Galileo, and equaling 1cm/s^2). Variations in density — the amount of mass per unit of volume — in the Earth lead to subtle variations in the gravitational acceleration at different points around the globe (higher density -> more mass -> more gravity and vice versa). These density variations arise from the composition and structure of the Earth, which are of interest to geologists and geophysicists. Therefore, data on the “gravity anomaly” (the difference between the mathematically expected and actually observed gravitational acceleration) at each location we visit is one of the staples of any marine geophysics cruise.

The portable gravimeter. Taking a reading involves manually moving a needle to the correct spot using the silver dial

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Any time you use a conventional compass to check which way is north from where you’re standing, you are taking advantage of magnetism, specifically the tendency of a magnetic material to align itself, if its motion is appropriately unconstrained, to the direction of the surrounding magnetic field. Imagine a line extending outward from the north and south points of your compass needle; this line will coincide with one of the field lines which designate the direction of the magnetic field of the Earth. These lines all extend outward from the magnetic north pole and loop out and around (one of them going right through your compass needle, another going through every other magnetic compass needle anywhere around the globe) to dive back in at the magnetic south pole.

Because of what we know of the changing nature of the Earth’s magnetic field, it is possible to imagine that if you had very, very acute senses, you would be able to observe, over the course of several years, the direction to magnetic north from your current spot shifting slightly as the Earth’s field gradually oscillates and reorients itself. You could also observe, if you walked along the surface of the Earth, tiny fluctuations in the direction of the needle as local influences — magnetic fields from magnetic material within the rocks constituting the Earth’s crust — momentarily jostled with the global field for control of your compass’ direction.

This observable variation in the magnetic field as we move over different parts of the Earth is exactly the reason why we are in the Pacific Jurassic Quiet Zone; only instead of a compass and impossibly sensitive eyes, we rely on instruments developed for this specific task: magnetometers.

The DeepTow sled, carrying two magnetometers, is deployed overboard off the a-frame

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