Why there is a fault line in the Earth's crust?

I wouldn't call it one fault line. the earth's crust consists of several pieces, compare them to jigsaw puzzle pieces, and they don't lie still. there's incredible forces under the earth's crust, which cause them to move about. the earth's crust never has consisted of one whole piece. millions of years ago, when earth as we know it was still being formed, the surface wasn't solid yet, but it was all lava. then the surface started cooling down gradually and it solidified, but before it could become one whole plate, the pieces which had solidified already started moving about, as they were floating on a stream of molten rock.