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Hot Spots

About 30 years ago a Geophysicist named J. Tuzo Wilson came up with an idea to explain why there was volcanic activity out in the middle of the Pacific Ocean, in the middle of the huge Pacific Plate. At the time, scientists thought that volcanoes only happened at plate boundaries, but nobody could explain why they were happening out in the middle of a tectonic plate. Dr. Wilson said that there are "hot spots", under the earth's crust in some places. These are called hot spots because they are places where a lot of heat is concentrated in a small area. The heat causes the overlying rock to melt. Since the magma is liquid and is lighter than the surrounding rock it "floats" to the surface and forces its way out of fissures in the crust. once magma erupts through the crust it is known as lava. Over time, the continual outpouring of lava can form a sea mount or island volcano if the hot spot is under the ocean floor, as in the case of the Hawaiian Islands. There is just one hot spot that never moves. But the Pacific Plate continually (and slowly) moves north over the hot spot, forming a new volcano on the overlying plate each time.

Doing the Science

Scientists had a lot of questions about why there were volcanic islands way out in the middle of the Pacific plate. It just didn't seem to fit in with their theory of plate tectonics. Dr. Wilson's idea of hot spots helped the island volcanoes to fit into the theory of plate tectonics. If the Pacific plate was moving over a hot spot, then that would explain why a chain of sea mounts and volcanoes had formed as the plate moved. If this was true, then the volcanoes should be of different ages, from oldest to youngest in a single direction.

In order to test his theory, Dr. Wilson took samples of volcanic rock from each of the volcanic islands in the Hawaiian chain and tested them to see how old they were on a geologic time scale. He found that the oldest rocks were from the northernmost island of Kauai, which also had the most weathering of rock. He also found that progressively younger rocks were found on the Hawaiian islands the further south he went (see map). The youngest rocks of all were found on the big island of Hawaii, the southernmost island. In fact, new "rocks" are still forming on the island of Hawaii, making it the youngest volcano in the island chain. There is even more evidence to support his theory because there is a new volcano forming on the sea floor south of Hawaii, called Loihi. Right now it's just a sea mount, but if the lava continues to build up on its slopes, someday it will be a new island.

How did deep ocean trenches help to explain plate tectonics?

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