This image shows a cross section through the earth’s crust and upper mantle showing lithosphere plates (made of the crust layer and the top part of the mantle) moving over the asthenosphere (upper mantle).
Click on image for full size
Old Rocks Give New Clues about Ancient Earth!
News story originally written on July 11, 2002
What was the Earth like a long time ago? To answer this question, some scientists look for very old rocks to find clues. Now they have found rocks in China that are 2.5 billion year old and these rocks have some interesting clues!
These rocks formed during the Archean Age, when the Earth was young. During the Archean there was no ocean and no continents on the Earth. It was a very hot place. After a long time, Earth cooled a bit, and giant plates of rock started to move around the surface of the Earth just like they do today. This is called plate tectonics . Today, plate tectonics is a very important process on Earth. It makes mountains grow high, volcanoes spew lava, and earthquakes rumble.
Scientists want to know when the Earth was cool enough for plate tectonics to begin. They thought that plates started to move about 2 billion years ago. They thought the Earth was too hot for plate tectonics before that time. But the 2.5 billion year old rocks found in China tell a different story.
These rocks formed in the upper part of the mantle layer, underneath a seafloor. They show that a long time ago they used to flow slowly and were able to move the plates of the Earth’s crust that were above them. So, the plates were sliding around long before we thought they could!
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