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.
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.
Antarctica was part of eastern Gondwanaland along with Madagascar, India and Australia. Eastern Gondwanaland broke off of Africa about 180 million years ago, and not long afterward, India and Madagascar broke off. Finally, Australia and New Zealand broke off around 70 million years ago. As a result, Antarctica is almost entirely surrounded by divergent plate boundaries. The remarkably straight ridge between India and Australia is called the Ninety East Ridge because it runs along 90 degrees east longitude. It is a former plate boundary between India and Australia. The separate collisions of India and Australia with Eurasia are causing the Indian-Australian Plate to break up, but the new break cuts across the Ninety East Ridge nearly at right angles.
Created 20 August 2009, Last Update 14 December 2009
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