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All of Earth’s plates are in motion. For this reason, it’s often easier to
describe a plate’s motion relative to a neighboring plate.
For example, if you were standing on the eastern edge of the African
plate looking at your dog Ralph (sitting very patiently) over on the
bordering Indian-Australian plate, Ralph would appear to be moving
away from you. To Ralph, he is the one sitting still while you move away
from him. The motion is relative to the observer’s point of view, or
frame of reference. In actuality, however, this type of motion would
occur over a very long period, geologically speaking.
Absolute motion and hot spots
When talking about entire plates, scientists often prefer to describe the
plate motions relative to a common frame or point of reference. Ideally,
this reference point is not moving like the plates but is stationary, or
fixed, deep within Earth.
There are approximately 20 places around the Earth where the mantle
below the lithosphere is unusually hot. These hot spots partially melt the
plates above them, creating topographic bulges and chains of volcanoes.
Although their origin is not completely understood, hot spots appear to
be stationary and provide a frame of reference for measuring absolute,
or true, plate motions.
Two of these hot spots are located in the United States. One is under the
Big Island of Hawaii at the southeast end of the Hawaiian Island chain.
The other is in Yellowstone National Park and is responsible for the
volcanic and geothermal activity in that region.
Studying plate motions in a fixed frame of reference provides scientists
with a better understanding of the forces that drive plate tectonics.