Transcript for Exotic Terrane, segment 05 of 12


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As studies progressed in the Pacific Northwest, scientists embarked on geological studies in the Pacific Ocean, working day and night from specially-equipped research vessels. They explored the geology of the Earth's crust beneath the sea. They also studied the volcanic island chains that dot the Pacific Ocean. Nobody could predict that this research would solve many geologic mysteries on the North American continent.

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This is Anatahan Island. It is really just the top of a giant underwater volcano. Anatahan rises near the equator in the western Pacific Ocean. It is one of the Mariana Islands, which includes the islands of Saipan and Guam.

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Anatahan is an active volcano. In these pictures it is dangerously close to erupting. A small village on the island was evacuated because of the eminent danger. When this simmering volcano finally erupts, it will probably be sudden and extremely powerful.

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Just to the north of Anatahan, the island of Pagan had already come to life. Explosive eruptions like this one are typical of Mariana Island volcanoes. To learn what causes these volcanoes, we must first understand a geologic concept called plate tectonics. The Earth's crust is made of separate pieces called plates that fit together like a puzzle. The plates are floating on a hot plasticlike material below. Some of the plates are pulling away from each other, and some are colliding.

The Mariana Islands are located where two of the Earth's crustal plates are colliding. If we remove the ocean water around the islands, the collision is revealed on the sea floor. The plates come together just east of the Mariana Islands. By looking beneath the sea floor we have an even better view of the colliding plates. The collision formed a deep canyon near the islands. This is the Mariana Trench which bottoms out at thirty-six thousand two hundred feet below sea level, the lowest spot on Earth. Mount Everest would easily fit in the trench with over a mile to spare before reaching sea level. Plate tectonics is driving the two plates together at the rate of a few inches per year. The plate on the right, which is the heavier of the two, is sinking below the lighter plate. This is called a subduction zone. As the plate sinks deeper, it melts. Magma is created which can force its way up through the plate above. Volcanoes erupt on the sea floor. Eventually they might become islands like Pagan and Anatahan.

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It is common for a chain of volcanic islands like the Marianas to rise above the zone where crustal plates collide. The chain typically forms along a curved line which is called a volcanic island arc.

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In addition to volcanoes, other parts of a sea floor are pushed up where plates collide. When they rise to a depth just below sea level, coral reefs begin to colonize. This reef grows offshore of Saipan, the capital of the northern Mariana Islands. The sea floor underneath is being pushed up very slowly and may eventually rise out of the sea. Saipan itself was lifted over fifteen hundred feet above sea level by the collision of the plates below it. Banzai and Suicide Cliffs dominate the north end of Saipan. They are the uplifted remains of old coral colonies. A tragic incident occurred here during World War Two. Japanese forces lost a fierce battle for Saipan to American forces. Rather than surrendering, thousands of Japanese people committed suicide from these limestone cliffs. Peace memorials commemorate the tragedy.

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On voyages to the Pacific Islands, scientists learned a great deal about volcanic island arcs like the Mariana Islands. When they returned to North America, they discovered that the rocks of modern island arcs were strikingly similar to ancient rocks in the Pacific Northwest. For Tracy Vallier and his colleagues, the present became the crucial link to the past. They could see firsthand how the older rocks in the Hells Canyon area formed.

Those folks who were out there dealing in the deep ocean hardly had any idea, I suspect to begin with, that they were going to be solving problems on the continents which were so complicated like what Tracy was working with. Well, it did happen just that way, and that's, that's progress.