Looking at the Sea The Changing Oceans

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The face of the Earth is always changing and throughout geologic history oceans have been created and destroyed. Modern geologic evidence indicates that the ocean bottom is moving at a rate from about one-half to six inches a year through a process called plate tectonics.

Three views of the oceans and the drift of the continents over time

Pangaea broke up with part of the continent drifting north and part south. 1) The northern part split to form the North Atlantic Ocean 208-146 million years ago (mya). 2) The South Atlantic and Indian oceans began to form 146-65 mya. 3) The continents continue to drift. Today the oceans are still changing shape; the Atlantic Ocean gets wider by a few inches each year.

Roughly 200 million years ago the Earth's surface was very different from the familiar pattern of land we know today. All of the land masses were grouped together into one vast supercontinent called Pangaea. The rest of the globe was covered by a single great ocean known as Panthalassa.

Slowly, over millions of years, the great land mass split apart. The pieces began to move over the Earth's surface driven by slowly churning currents in the molten rocks beneath the Earth's hard outer layers. The gigantic plates on the Earth's crust move like a conveyor belt. As new areas of ocean floor form at mid-ocean ridges, old areas are dragged down, or subducted, into the Earth's mantle, which explains why the older rocks cannot be found.

Diagram of a mid ocean ridge and subducting plate.

By about 35 million years ago the pattern of land and sea was very much like it is today. But the continents are still moving and as the Atlantic and Indian oceans continue to get wider by a few inches every year, the Pacific is slowly shrinking. At the northeast corner of Africa we can see the start of a new ocean. For the last 25 million years, the Red Sea has been widening. If it continues at the same rate, in 200 million years it will be as wide as the Atlantic is today.

Some related sites of interest

The Pangaea Theory
The Pangaea theory was treated with much skepticism when it was first raised. But since then, there has been much evidence to support this theory.
Hydrothermal Vents
Tour a hydrothermal vent system as scientists might encounter along the Juan de Fuca Ridge in the Pacific Ocean.
Submarine Volcanoes
The most productive volcanic systems on Earth are hidden under the ocean. The magma and lava of submarine volcanoes create the edges of new oceanic plates.
Hot Stuff
Volcanoes, mid-oceanic ridges, and deep-sea vents are all associated with sea floor spreading and plate tectonics. They are also the newest places on Earth. Locate the Earth's "hot spots" in this activity.
Satellite view of the Red Sea.

If the oceans are always changing where does the water come from to fill ocean basins?


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