Mary Sargent, age 14, of Rochester, N.Y., for her question:
How are islands formed?
The biggest island is the continent of Australia. Hawaii, Indonisia and the Philippines are made up of many medium and small sized islands. But most of the world's uncounted islands are too small to appear in an atlas. Some of these midgets have local names and many are too small to have names at all. Yet every island has its own story to tell.
There are atolls and coral islands, volcanic islands and soggy isles that form in muddy deltas. These and many other islands were formed in different ways. The continent of Australia, We suspect, broke apart from a larger land mass in the remote past and drifted away over the Sea. Many of the world's smallest isles appear and disappear, here today and gone tomorrow.
Some of the most spectacular islands were formed by volcanoes. In the remote past, eruptions started on the floor of the sea. Lava cooled and formed cones which grew taller after each eruption. At last the volcanic mountains reached above the waves and formed islands of solid rock.
The ocean floor has its mountains with ranges of lofty peaks. Out in the Atlantic there are lonely islands which are peaks sitting upon the shoulders of a massive range of underwater mountains. Other islands may form in the wide mouth of a river. Muddy silt is dumped in this delta area, arid the deposits often become high enough to create a mud flat, a stretch of land above water level.
Some islands are formed by corals, though these small sea dwellers usually need some help. The soft, spidery coral animals like warn, surface water. They often build their stony houses atop underwater mountains and volcanoes. If the underwater platform is near the surface, the teeming sea dwellers may build enough coral to carry it up above the waves.
Some islands form because of changes in sea level. In an Ice Age, the water level sinks, and the tops of undersea mountains break the surface and become islands of dry land. When the sea invades the land, hills and high points are often left sticking above the water, and the new coastline is fringed with a row of off shore islands.
Islands are formed in many different ways. Some are new growths of rocky minerals which reach up from the deep ocean to pierce the surface. Some are deposits of silty mud and sand and schale are made by corals. Others are hilly humps left above the water when the sea invades the land.