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Question: How did the ring of fire form?
The "Ring of Fire" refers to the circle of earthquakes and volcanoes that surround the Pacific Ocean. For the last few hundred years it was known that earthquakes and volcanoes occurred in these narrow bands but until the discovery of plate tectonics in the 1950s and 1960s, no one knew why. The movement of these plates causes mountains, ocean trenches, and most other formations of our earth.
Try to find a map or globe where the water of the earth‚s oceans has been "drained" to expose the ocean floor. Just as in the continents, you will see mountain ranges and deep valleys. Try to picture how these were formed as the plates of the earth moved ever so slowly but over long periods of time.
Plates made of continental material such as North America are lighter than oceanic plates such as the floor of the Pacific Ocean. Therefore, when these plates collide, at a very low speed, the oceanic plate dives underneath the continental plate causing what is known as subduction. Since the deep earth is hotter than the earth near the surface, the oceanic plate becomes hotter with the result that magma (molten rock) is formed. This magma is lighter than the surrounding rock and makes its way to the surface to produce volcanic eruptions. Places that this occurs are island arcs such as the Aleutian Islands and the Japan Islands.
At the same time that subduction is going on, the movement of the plate downward causes earthquakes where the continental plate and oceanic plate slide past each other. These earthquakes can be very deep, as much as 600 km (400 miles), and can be very large. The earthquake near Sumatra in the Indian Ocean on December 26 that caused the huge tsunami was a subduction earthquake although it was only 30 km (20 miles) deep.
As the Pacific plate moves in a generally northwest direction, it creates the volcanic arcs described above. But it interacts with the North American plate in California in a very different fashion. Instead of one of the plates diving under the other, they slide past one another. This is seen in California as the famous San Andreas Fault. Do you think these earthquakes would be as deep as those produced by a subducting plate?
Another part of the Ring of Fire is the western coast of South America where an oceanic plate is diving under South America. You should ask yourself, "If the Pacific Plate is moving to the northwest with respect to North and South America, how can it be diving under South America?" The answer is that the plate that is interacting with South America is not the Pacific Plate but a smaller plate known as the Nazca Plate, which is moving to the east. For this to be possible, there must be a place out in the Pacific Ocean where new crust is being formed. Look at the map of the ocean floor mentioned above and see if you can find where this is happening.
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