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
lsua 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 lsua
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 lsua supracrustal belt was once part of
a plate under the ocean, beginning around 3.8 billion years ago.