A number of methods are used to establish the direction and rate of plate motion. Some of these techniques not only confirm that lithospheric plates move, but also allow us to trace those movements back in geologic time.

Geologic Measurement of Plate Motion

Using ocean-drilling ships, researchers have obtained dates for hundreds of locations on the ocean floor. By knowing the age of a rock sample and its distance from the ridge axis where it was generated, an average rate of plate motion can be calculated.

Scientists used these data, combined with their knowledge of paleomagnetism stored in ocean crust and studies of seafloor topography, to create maps that show the age of the ocean floor. Solid lines in Figure 32 indicate plate boundaries. The gradient bands shown in the figure range in age from the present (0) to about (30) million years ago. The width of the bands indicates how much crust formed during that time period. For example, the gradient band along the East Pacific Rise is more than three times wider than the gradient band along the Mid-Atlantic Ridge. Therefore, the rate of seafloor spreading has been approximately three times faster in the Pacific basin than in the Atlantic.

Maps of this type also provide clues to the current direction of plate movement. Notice the offsets in the ridges (the zigzag pattern); these are transform faults that connect the spreading centers. Recall that transform faults are aligned parallel to the direction of spreading. Careful measurement of transform faults reveals the direction of plate movement.

To establish the direction of plate motion in the past, geologists can examine the long fracture zones that extend for hundreds or even thousands of kilometers from ridge crests. Fracture zones are inactive extensions of transform faults and are therefore a record of past directions of plate motion. Unfortunately, most of the ocean floor is less than 180 million years old, so to look deeper into the past, researchers must rely on paleomagnetic evidence provided by continental rocks.

Measuring Plate Motion from Space

You are likely familiar with the Global Positioning System (GPS) used to locate one’s position in order to provide directions to some other location. In it, satellites send radio signals that are intercepted by GPS receivers located at Earth’s surface. The exact position of a site is determined by simultaneously establishing the distance from the receiver to four or more satellites. Researchers use specially designed equipment to locate a point on Earth to within a few millimeters (about the diameter of a small pea). To establish plate motions, GPS data are collected at numerous sites repeatedly over a number of years.

Data obtained from GPS and other techniques are shown in Figure 33. For example, GPS calculations show that Hawaii is moving in a northwesterly direction toward Japan at 8.3 centimeters per year. A location in Maryland is retreating from a location in England at a speed of 1.7 centimeters per year—a value close to the spreading rate established from paleomagnetic evidence from the North Atlantic. Techniques involving GPS devices have also been useful in confirming small-scale crustal movements, like those occurring along faults in regions known to be tectonically active (for example, the San Andreas Fault).

• Data collected from the ocean floor has established the direction and rate of motion of lithospheric plates. Transform faults point in the direction the plate is moving. Establishing dates for seafloor rocks helps to calibrate the rate of motion.

• GPS satellites can be used to accurately measure the motion of special receivers to within a few millimeters. These “real-time” data support the inferences made from seafloor observations. On average, plates move at about the same rate human fingernails grow: about 5 centimeters (2 inches) per year.

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