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. [ILLUSTRATION OMITTED] 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 out·er·most adj. Most distant from the center or inside; outmost. outermost Adjective furthest from the centre or middle Adj. 1. layer, called the crust crust Outermost solid part of the Earth, essentially composed of a range of igneous and metamorphic rock types. In continental regions, the crust is made up chiefly of granitic rock, whereas the composition of the ocean floor corresponds mainly to that of basalt and gabbro. , 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 This is a list of numbered minor planets, nearly all of them asteroids, in sequential order. As of late September 2007 there are 164,612 numbered minor planets, and many more not yet numbered. Most asteroids are ordinary and not particularly noteworthy. 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 planet's 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 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, oozed up 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. "It's a marvelous case of solving a jigsaw A Web server from the W3C that incorporates advanced features and uses a modular design similar to the Apache Web server. Jigsaw supports HTTP 1.1 and provided an experimental platform for HTTP-NG. See HTTP-NG and Amaya. puzzle “Puzzle solving” redirects here. For the concept in Thomas Kuhn's philosophy of science, see normal science. A puzzle is a problem or enigma that challenges ingenuity. ," says one of the researchers, Gustaf Arrhenius of the Scripps Institution of Oceanography Scripps Institution of Oceanography: see California, Univ. of. in La Jolla La Jolla (lə hoi`yə), on the Pacific Ocean, S Calif., an uninc. district within the confines of San Diego; founded 1869. The beautiful ocean beaches, in particular La Jolla shores and Black's Beach, and sea-washed caves attract visitors and , Calif. He notes that the puzzle was "a very difficult one because these rocks are all very old and have been badly mangled."--E. Sohn |
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