Sharon Cooke, age 13, of Summerland, British Columbia, Canada, for her question:

How can an island rise from the sea?

An island may be formed by an undersea volcano. The event begins in a weak zone in the ocean floor, where restless crustal blocks are forming mountains. These stresses generate tremendous pressures and reservoirs of molten magma form deep in the crust. Upheavals open vents and the seething mixture erupts to the surface    which is the ocean floor. Through the ages, the undersea volcano erupts again and again, each time adding to its volcanic cone. At last the top of the cone rises above sea level and becomes an island of dry land.

Many islands are built by undersea volcanos, but not all of them. Sometimes the sea invades the land, swamping the lowlands and leaving a few hills above the water level to become islands. This also happens when a large slab of a continent sinks below sea level. These islands and those built by volcanoes take ages to form. But a big river may work faster. It carries tons of silt and dumps it where its fresh water meets the salt sea. Flat muddy islands build up wide river deltas.