单选题
Earth Rocks on
Most of the time, the ground feels solid beneath our feet. That's comforting. But it's also misleading because there's actually a lot going on underground. Masses of land (called plates)slip, slide, and bump against each other, slowly changing the shape of continents and oceans over millions and billions of years.
Scientists know that Earth formed about 4.5 billion years ago. They also know that our planet was hot at first. As it cooled, its outermost layer, called the crust (地壳), eventually formed moving plates. Exactly when this shift happened, however, is an open question.
Now, an international group of researchers has an answer. They've found new evidence suggesting that Earth's crust started shifting at least 3.8 billion years ago. The new estimate is 1.3 billion years earlier than previous ones.
Not long before 3. 8 billion years ago, lots of asteroids (小行星) were hitting Earth, keeping its crust in a hot, melted state. After the hard crust formed, much of it sank at various times into the planet's hot insides. There, it melted before returning to the surface.
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 took a close 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 whole 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.