Lisa Weinstein, age 10, of Pittsburgh, Pennsylvania, for her question:

What holds an island up?

When you look out over the vast ocean, there is nothing but water as far as you can see. You know that its waters are deep and perhaps you suspect that they go down down down in a bottomless pit. That's just what the sailors thought when Columbus crossed the vast Atlantic. But they were wrong. The sea is not bottomless. Its water is held inside enormous basins made of solid rock. There is a solid floor under every part of the ocean.

This solid ocean bed is not very different from the dry land. It happens to be wet and oozy and the patches of land happen to be high enough to poke up above the watery waves. Continents are just large patches of the earth's solid crust, high enough to stand above the ocean. Islands also are humps of the earth's crust, high enough to poke above the waves. The solid earth's crust enfolds every part of our round globe. Some parts of it are the deep hollows that hold the seas. Other parts are higher humps that form the continents and islands of dry land.