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Hotspots or Not?


Hot spots, semi-permanent regions of Earth's crust that experience continuous upwelling of magma, have always been the ugly sister of plate tectonics. Though elegantly "explained" by Tuzo Wilson and Jason Morgan, seismological and geological evidence for the origins of hot spots remains elusive. In fact, hot spots remain at the center of a hot debate over the mechanisms by which plate tectonics operates. Here we catch a glimpse of that debate.

 

A major puzzle for proponents of the theory of plate tectonics, and a key complaint of those who resisted this theory, was the formation of island chains like the Hawaiian Islands. How could a trail of islands form in the middle of a plate away from its boundaries if the centers of volcanic activity were oceanic ridges?

The answer was provided by a famous Canadian geologist, J. Tuzo Wilson, who hypothesized in 1963 that the plates did indeed move, but that certain regions of the crust are characterized by "hot spots." These hot spots represent regions where magma continuously breaks through the lithosphere, i.e. they represent stationary magma sources in the asthenosphere. Wilson envisioned a person lying in a stream on their back blowing bubbles through a straw. The rising bubbles represent the magma and the movements of the stream represent the plates. As the stream (i.e., the plates) moves, the straw (a hot spot) releases bubbles that rise to the surface and form a line of bubbles (i.e., a chain of volcanic islands). According to Wilson, after a period of millions of years, the island moves beyond the hot spot, cutting off the source of magma, and a new island begins to form. The major leading journals at the time rejected Wilson's first manuscript on this topic. Finally, he managed to get his ideas published in the Canadian Journal of Physics, a smaller journal, but the importance of his work was not appreciated until a few years later.

One piece of evidence that provided support for Wilson's hypothesis was the differing age of the Hawaiian islands. From the big island of Hawaii to the beautiful canyon-filled island of Kauai, the age of the islands gets successively older. The oldest volcanic rocks on Kauai are about 5.5 million years old and are deeply eroded. The oldest exposed rocks on Hawaii are less than 0.7 million years old and new volcanic rock is continually being formed as a result of Mauna Loa. East of the southernmost tip of Hawaii today, a new island is being formed. Called Loihi, this volcanic seamount is still underwater, but it extends 8000 feet (about 1.5 miles) above the sea floor. Keep an eye out for vacation getaways to Loihi sometime in the not-too-distant future!

Interestingly, Hawaiian mythology alludes to the differing ages of the islands, long, long before the theory of plate tectonics was around (unless, of course, those ancient astronauts taught a few courses in geology). Being "attuned" to the land and the sea like most native peoples, the Hawaiians were aware of the differences in vegetation, soil, and rocks in the northwest islands (Niihau and Kauai) as compared to the southeast islands (Maui and Hawaii). It was believed that Pele, the Goddess of Volcanoes, lived on Kauai until her "evil" older sister, Namakaokahai, who is the Goddess of the Sea, forced her to flee further south. Pele moved to Oahu, but after many years, her sister once again forced her southward. This sisterly rivalry has continued to the present time, and Pele now lives on Hawaii, presumably until her future home of Luihi is ready. Clearly, the Hawaiians understood the cycle of formation of the islands. It just took scientists a few thousand years to figure it out.

Hot spots are through to act as "pipelines" for magma, called mantle plumes, that rise from deep within the mantle to the surface of the Earth. They may persist for hundreds of millions of years or they may dry up. Some scientists believe that more than a hundred hot spots have been active within the past 10 million years. Near Midway Island (the site of a major World War II naval engagement that is still the subject of one of my favorite war movies, the Battle of Midway), there is a chain of islands known as the Emperor Seamount Chain, which has also been formed by the action of a hot spot. Other hot spots have been found in Iceland, the Azores (in the middle of North Atlantic Ocean and featuring a "famous" karaoke bar), and the Galapagos Islands.

Nonetheless, seismological studies, especially seismic tomography, have not provided convincing evidence of mantle plumes at many hot spots. Many "former" hot spots, like Yellowstone Park, are no longer considered to be hot spots. Some scientists now recognize a number of different processes that may give rise to "hot spots". Despite more than 30 years of research on hot spots, a great deal of science remains for enterprising scientists to discover.

For a good accounting of an alternative interpretation of hotspots, see http://www.mantleplumes.org

 

   
   
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