Plates grow by pulling apart at the mid-ocean ridges and sinking back into the earth's interior at the tranches, mostly around the rim of the Pacific. As plates grow, they create strips of progressively older crust on either side of the ridge. We can determine the age of the ocean floor by drilling into it an retrieving samples, but mostly because newly formed crust is slightly magnetized by the earth's magnetic field, and we can compare the magnetism of the sea floor with the history of changes in the earth's magnetic field.
On the colored diagrams, land is brown, and submerged continental crust is tan. The bluish gray area, labeled Submarine Volcanic Plateau, includes large areas of sea floor where thick accumulations of lava flows built up. This color also indicates areas where the exact age of the sea floor is uncertain. On these diagrams, pairs of parallel lines show boundaries where plates are spreading apart, and simple lines show all other types of boundary.
The earth has two kinds of crust. The continents are mostly made of thick granite. When continents pull apart, the gap is filled by thin crust made of basalt. In plate tectonics, a continent is any piece of continental crust surrounded by oceanic crust or plate boundaries. New Zealand is a continent. Unlike most continents, most of New Zealand is submerged. Also, unlike Africa and South America, it does not fit neatly against Australia. The submerged northern portion, called the Lord Howe Rise, is actually a collection of smaller fragments that moved in a complex way so the present shape is not the original shape.
The ocean floors show a remarkable degree of regularity. The crest of the mid-ocean ridges is at an average depth of 2700 meters, and the 3000 meter contour on the map reveals most of the crest. As newly formed ocean crust cools, it becomes denser and sinks, until it finally sinks to an average depth of 5500 meters. The broad deep areas about 5000-6000 meters below sea level are called abyssal plains and are the flattest areas on Earth. The very deepest parts of the ocean, up to 11 kilometers deep, make up a very tiny part of the sea floor. They are not out in the middle, like one might expect in a bowl. Instead they are long, narrow trenches close to a continent or island chain. The trenches mark convergent plate boundaries where oceanic crust sinks back into the earth's interior.
The Pacific Plate is unusual in that it is almost entirely oceanic crust. Only in a few places like Baja California and southern New Zealand are small slivers of continental crust attached. The continents are moving away from the Atlantic and Indian Oceans and converging on the Pacific. Most of the Pacific is surrounded by subduction zones. Note how wide the age bands are in the Pacific. The Pacific, Nazca and Cocos Plates are among the fastest moving plates on Earth, moving at up to 15 centimeters per year. In general, the larger the portion of plate descending back into the earth, the faster the plate moves. This is good evidence that the weight of the descending plate helps pull the rest of the plate along.
The mid-ocean ridge in the Pacific, the East Pacific Rise, is bounded on the east by the Cocos and Nazca Plates. When North America was further east, the East Pacific Rise extended all the way to Alaska and the Nazca, Cocos and Juan de Fuca Plates were a single large plate, which geologists call the Farallon Plate. About 30 million years ago North America came into direct contact with the Pacific Plate and the Juan de Fuca Plate was cut off from the rest of the Farallon Plate. Today only a small remnant of it remains.
Created 10 August 2009, Last Update 20 January 2010
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