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

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 early in Earth’s history, during the Archean Age, when there was no ocean and no continents and Earth was a very hot place. It was not until Earth formed layers and cooled down that plate tectonics started to move giant rocky plates around the surface of the Earth as it does today. Today, plate tectonics is a very important process on Earth, making mountains grow high, volcanoes spew lava and earthquakes rumble. But before plate tectonics started, the world was a very different place.

Scientists have been unsure of when plate tectonics actually began on Earth . Until recently, there wasn’t any evidence that lithosphere plates moved before 2 billion years ago. The Earth was thought to be still too hot for plates to form at the surface and slide on the asthenosphere layer in the upper mantle. However, the 2.5 billion year old rocks found in China tell a different story.

The minerals in the newly found rocks formed in the upper part of the mantle layer, underneath a seafloor. These minerals show evidence that the upper part of the mantle partially melted and then flowed a little bit. When the upper mantle layer does this, plates are able to move at the Earth’s surface. So, these rocks show that the plates of lithosphere were active and sliding around on the upper mantle (the asthenosphere) long before we thought they could!

When plate tectonics is happening, hot volcanic vents often form on the sea floor and these vents may have provided the nutrition and conditions that were needed for the first life to develop and flourish on Earth . Today deep-sea vents provide home and food for many organisms and they may have done so 2.5 billion years ago as well.

Last modified August 6, 2002 by Lisa Gardiner.

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