The first breath-taking pictures of the Earth
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The first breath-taking pictures of the Earth taken from space showed it as a solid ball covered by brown land masses and blue-green oceans. We had never seen the Earth from that distance before. To us， it appeared as though the Earth had always looked that way and always would. Scientists now know， however， that the surface of the Earth is not as permanent as we had thought.
Scientists explain that the surface of our planet is always moving. Continents moves about the Earth like huge ships at sea. They float on pieces of the Earths outer skin. New outer skin is created as melted rock pushed up from below the ocean floor. Old outer skin is destroyed as it rolls down into the hot area and melts again.
Only since the 1960s have scientists really began to understand that the planet Earth is a great living machine. Some experts have said this new understanding is one of the most important revolutions in scientific thought. The revolution is based on the work of scientists who study the movement of the continents—a science called plate tectonics.
The modern story of plate tectonics begins with the German scientist Alfred Wegener. Before World War One， Wegener argued that the continents had moved and were still moving. He said the idea first occurred to him when he observed that the coastlines of South America and Africa could fit together like two pieces of a puzzle. He proposed that the two continents might have been one and then split apart.
