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How GPS Works
GPS is a real-world application of relativity and uses a constellation of 24 satellites, each with a transmitter and a very precise clock. The transmitter broadcasts the satellite's orbital parameters and the time of the broadcast. On the ground, the GPS receiver picks up the broadcasts from multiple satellites (at least three, but preferably four) and uses the time signatures and orbital information to determine the time in transmission. Specialized algorithms pull that data together and find the position of the receiver relative to the satellites.
GPS and Continental Drift
Geologists measuring plate tectonics put receivers on different fixed points and record their position carefully. The GPS receiver constantly records its positional data over several months (and in some cases, years), gathering the rate and direction of movement of the piece of land under the receiver. GPS data shows the velocities of plates in three dimensions: North-to-South, West-to-East and changes in altitude.
GPS and Vulcanism
GPS receivers' ability to generate a record of altitude changes are used by geologists to predict volcanic eruptions. As volcanos build up in pressure in ways that can lead to an eruption, they swell and can cause GPS receivers to move -- when these motions are over a short period of time, vulcanologists will put seismometers and gas spectrometers on a volcano to try to pinpoint the exact eruption time frame.
Other Advantages of GPS Measurements
Because GPS measurements can be very precise, the reference points used for GPS-derived tracking of plate tectonics can be several thousand kilometers apart and their positions and motions relative to each other measured with great precision. This has helped identify multiple plate subduction zones and allows geologists to identify known expansion rates for the ocean floors. The fastest expansion rate is the Pacific ocean, which is expanding by about 6 inches per year, while the Mid-Atlantic Ridge is expanding by about an inch per year.
Other Plate Tectonics Measurement Methods
GPS systems have only been available since the 1970s and only available for civilian use since the 1990s. Earth's magnetic field "flips" polarity at intervals that range from 100,000 to 1,000,000 years. The main driver of tectonic plate movement is the expansion of new rock welling from the Earth's mantle. When this rock cools, it records the polarity and field strength as a permanent record of the Earth's magnetic field over time, essentially recording a long term record of plate tectonic motion over geological epochs.