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).
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Old Rocks Give New Clues about Ancient Earth!
News story originally written on July 11, 2002

Scientists have found 2.5 billion year old rocks in China that help us understand more about what the Earth was like long ago.

These rocks formed when Earth was young, during the Archean Age. During the Archean, there was no ocean and no continents and Earth was a very hot place. Earth cooled slowly and eventually it was cool enough for part of the upper mantle layer (the asthenosphere), below the Earth’s crust, to flow just a little, moving the giant plates that lie above. This is called plate tectonics and it still happens today. In fact, plate tectonics is a very important process on Earth today, making mountains grow high, volcanoes spew lava, and earthquakes rumble.

When did this important process begin to shape the surface of our Earth ? Scientists have been unsure of when plate tectonics actually began. Until recently, there wasn’t any evidence that plates moved before 2 billion years ago. We thought Earth was still too hot for rocky plates to form at the surface and slide on the asthenosphere. But the 2.5 billion year old rocks found in China tell a different story.

The newly discovered rocks formed long ago in the upper part of the mantle layer, underneath an ancient seafloor. These rocks show evidence that, 2.5 billion years ago, the upper part of the mantle layer would melt a little bit and then flow a little bit. When the rocks in this layer did this, plates could move at the Earth’s surface. So, these rocks show that the plates of the lithosphere were active and sliding around long before we thought they could!

Last modified August 6, 2002 by Lisa Gardiner.

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