Not long before 3.8 billion years ago, lots of asteroids (小行星) were pummeling Earth, keeping its crust in a hot, molten state. After the hard crust formed, much of it sank at various times into the planets hot insides. There, it melted before returning to the surface as lava.

　　In some places, however, the crust never sank. One of the oldest such places is in Greenland, in an area called the Isua supracrustal(上地壳) belt. The rocky crust there is between 3.7 and 3.8 billion years old. The belt was once part of the seafloor, but now it is exposed to air.

　　The researchers recently look at the Isua supracrustal belt. They noticed long, parallel cracks in the rock that have been filled in with a type of volcanic rock.

　　To explain this structure, the scientists propose that tension in the crust caused the seafloor to crack open long ago. Hot, liquid rock, called magma(岩浆), flowed up slowly from deep inside Earth to fill the cracks. Finally, the area cooled, forming what we see today.

　　That explanation, plus chemical clues inside the rock, suggests that the Isua supracrustal belt was once part of a plate under the ocean, beginning around 3.8 billion years ago.

　　16 The ground beneath our feet is indeed still.

　　A Right

　　B Wrong

　　C Not mentioned

　　17 The shape of lands and oceans are slowly changed with the movements of plates.

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