Activity 4: Tracking a Hot Spot
 
A hot spot is a localized region of active volcanism on the Earth’s surface. It may be located in the interior of a plate or along a plate margin (Iceland). Those within plates may occur on either continental (Yellowstone) or oceanic (Hawaii) plates. Over 100 hot spots have been identified on the surface of the Earth (Fig. 1).
 

 

Fig. 1: Hot spot locations around the world.

  

In many cases, a hot spot sits at the end of a long chain of extinct volcanoes. Volcanoes increase in age the further they are from the active hot spot (Fig. 2). These volcanic chains are known as hot spot trails, tracks, or chains and are best developed on oceanic plates. Hawaii is at the southeastern terminus of a very famous hot spot trail, but the track created by the Yellowstone hot spot is less obvious.

 
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Fig. 2: A hot spot trail.

 

The existence of hot spot trails suggests that lithospheric plates move with respect to hot spots and that the magmatic source for hot spots lies below the lithosphere. As plates move over a magmatic source, the active volcano on the surface is displaced from its source and eventually cut off from its magmatic source. A new volcano then forms over the hot spot. Some scientists believe that plumes of hot material rising from the core-mantle boundary cause hot spots.
Lava and pieces of mantle rocks brought to the surface at hot spots provide petrologists and geochemists with rare samples of the Earth's interior. These samples are particularly important in understanding the Earth's interior because they have ascended through only a short distance of oceanic crust. Thus their chemistry is likely to reflect that of the mantle where they were formed.
Because they appear to be stationary for long periods of geologic time, hot spots provide an absolute framework against which to measure plate motions. Their trails are valuable clues for reconstructing past plate tectonic motion (Fig. 2). Hot spot trends show us the directions of sea floor spreading. Abrupt kinks in a hot spot track reflect past changes in the direction of plate motion. By dating lavas along the hot spot trail (Fig. 2), the velocity of plate motion along individual segments of the trend can be calculated from the following equation:
 

 
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