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 arcs such as 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. As the plates move across these hot spots, volcanic islands are formed. 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. Despite this "hot" theory, the major leading journals at the time rejected his manuscript. 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 act as "pipelines" for magma 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. 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.
Closer to home, the home of Yogi Bear and Boo Boo is believed to be the site of a hot spot. Yellowstone National Park is well-known for its explosive geysers and crystal-clear hot pools. While this hot spot differs from others in that magma is not being produced, it appears to represent a region beneath the North American Plate where a stationary source of geothermal activity exists.
A Few Thoughts
This incredible story of scientific achievement and the excitement it has generated in many different fields of science continues to grow. I would like to leave you with one thought as it pertains to the Gaia hypothesis. The thickness of ocean sediments increases away from ridges. As we mentioned earlier, oceanic sediments can be responsible for generating "explosive lava" as the sediments are melted and "shot" to the surface in subduction zones where back basins are formed. Because these sediments are primarily biological in origin, and because most biological activity in the sea is confined to the continental margins, could it be that organisms are a driving force of continental drift? Could it be that the single-celled "pillboxes" of diatoms and other phytoplankton (who you will recall may also control cloud cover) also are responsible for the movement of the plates? Is that why non-living planets, such as Mars, no longer exhibit signs of tectonic activity? It makes one wonder...