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Continental Drift

#3: Recycling

By the early 1960’s it was established that the earth was making new crust, on the ocean floor, all the time. However, since the earth is not expanding, where is this new land going?

The answers began to appear in 1964, following a major earthquake in Alaska. Geologists visiting the area immediately after the quake were, at first, confused by reports about changes in sea level. Some areas were claiming that the sea level had fallen, and others that it had risen. Despite searching, the geologists could find no evidence of a crack in the earth's surface. A quake with a magnitude as great as this had to have an associated fault line (crack), but where was it?


A particularly observant geologist noticed a strange phenomenon. Along the entire coastline, there was a line of barnacles, still living, way above the highest level that the sea reached. In some places it was as much as 20 feet above the present levels of water. The fact that the barnacles were still living meant that they had been in seawater recently. From this, scientists were able to state that the sea level at the coast had definitely fallen, or more accurately, the land had risen.

Inland Flooding

Further inland though, the story was the complete opposite. Areas, which had been dry for years were suddenly sitting under water. There was no doubt that these areas had dropped during the earthquake.

The Fault Line

By drawing diagrams, and making models, the scientists came to the conclusion that the fault line must lie under the earth's surface. The areas of uplifting and sinking indicated that the line ran from an area just off the coast, diagonally downwards, under the land. They also noticed that just where their theoretical fault line was, there was a deep trench in the ocean floor. Trenches like it can be found all the way around the Pacific Ocean rim.

Putting it Together

Now the scientists had the following information:

  • The earth makes new crust on the ocean floor
  • The crust moves outwards as newer crust is formed
  • The earth is not getting larger
  • There is a trench all the way around the Pacific Ocean
  • There is a fault line running from the trench, under the land mass

It didn't take them very long to decide exactly what was happening to the extra land. It was disappearing down, under the continents, at the trenches. The trenches are like a huge crack in the ocean floor, where the newer rock slides under the older rock. As it slides, the weight of the older rock pushes it downwards. Eventually it reaches the outer core of the earth, which is extremely hot and molten. Here it heats up and becomes molten, joining all the rest of the molten rock, to become magma again.


In a way it is like a natural recycling plant. At the mid ocean ridge, hot molten rock, or magma is forced upwards. As it comes into contact with the cool ocean it solidifies, becoming new solid rock. The rock is gradually pushed outwards my more rock as it forms. The entire ocean floor is being pushed outwards towards the continents. At the edges of the continents, the new rock is pushed downwards, under the continents. The solid rock heats up and becomes magma again. Then the cycle begins all over again

Is That the End of the Story?

It might seem that the story is finished now. We have successfully explained how the continents were pushed apart. We have proved that the ocean floor is constantly being renewed. We have seen where the extra ocean floor goes. However, it isn't quite the end of the story. The theory of continental drift once it got this far, became known as the theory of Plate Tectonics. So now we need to look at Plate Tectonics, and also check those facts about disappearing earth. Not all of it goes down under the continents, some of it goes up!

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