"That puts us over the Fullerton Fracture Zone," Pitt said slowly.
A fracture zone is a linear oceanic feature--often hundreds, even thousands of kilometers long--resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on either side of an active transform fault move in opposite directions; here, strike-slip activity occurs. Fracture zones extend past the transform faults, away from the ridge axis; seismically inactive (because both plate segments are moving in the same direction), they display evidence of past transform fault activity, primarily in the different ages of the crust on opposite sides of the zone.
In actual usage, many transform faults aligned with fracture zones are often loosely referred to as "fracture zones" although technically, they are not.
"I've got a solid reading on the magnetometer."
A magnetometer is a measuring instrument used to measure the strength or direction of a magnetic field either produced in the laboratory or existing in nature. Some countries such as the USA, Canada and Australia classify the more sensitive magnetometers as military technology, and control their distribution.
The International System of Units unit of measure for the strength of a magnetic field is the tesla. As this is a very large unit, workers in the earth sciences commonly use the nanotesla (nT) as their working unit of measure. Engineers often measure magnetic fields in Gauss. 1 Gauss = 100,000 nT or 1 Gauss = 100,000 gamma.
The Earth's magnetic field (the magnetosphere) is a potential field. It varies both temporally and spatially for various reasons, including inhomogeneity of rocks and interaction between charged particles from the Sun and the magnetosphere.
The earth's magnetic field is relatively weak. A simple magnet that may be purchased in a hardware store produces a field many hundreds of times stronger than the earth's field. The earth's magnetic field varies from around 20,000 nT near the equator to 80,000 nT near the poles. It also varies with time. There is a daily variation of around 30 nT at mid latitudes and hundreds of nT at the poles. Geomagnetic storms can cause much larger variations.
Magnetometers, which measure magnetic fields, are distinct from metal detectors, which detect hidden metals by their conductivity. When used for detecting metals, a magnetometer can detect only magnetic (ferrous) metals, but can detect such metals buried much deeper than a metal detector. Magnetometers are capable of detecting large objects like cars at tens of meters, while a metal detector's range is unlikely to exceed 2 meters.
"Keep it down to bare steerageway."
The rudder of a vessel can steer the ship only when water is passing over it. Hence, when a ship is not moving relative to the water it is in or cannot move its rudder, it does not respond to the helm and is said to have "lost steerage." The motion of a ship through the water is known as "making way." When a vessel is moving fast enough through the water that it turns in response to the helm, it is said to have "steerage way." That is why boats on rivers must always be under propulsion, even when traveling downstream.
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