Graham DeWitt, age 10, of St. Catherines, Ontario, Canada, for his question:

How do icebergs get into the North Atlantic?

We hear a lot of grim tales about these monsters that threaten ships in the North Atlantic. Since they are massive chunks of floating ice, we might expect most of them to skulk around during the winter. But actually, most of the villains prowl the shipping lanes during the summer. This seasonal schedule has a lot to do with where they come from and how they get into the North Atlantic.

An iceberg is a delinquent runaway that once belonged to a glacier. Its parent is an enormous spread of frozen, impacted snow. Through countless ages, the Arctic winters have dumped layers of snow on the mountainous island of Greenland. The summers were too cool and too short to melt it and the frozen snow accumulated. The Greenland icecap is now a thick, massive glacier that covers 666,000 square miles  which is about 13 times larger than all of Pennsylvania.

This is the monstrous parent that sends most of the icebergs into the North Atlantic. Ice is a mineral, though more fragile and more brittle than the usual minerals in the earth's rocky crust. Actually, it is so fragile that a thick glacier cannot support its own weight. Then it gets to be 200 to 300 feet thick, its ice begins to spread outward from the center and the slabs resting on mountains slide down the slopes.

Greenland has plenty of slopes where numerous icy fingers slither down the valleys. Many of these valley glaciers slide down toward the sea at the rate of 50 feet a day. Along the shores, they are pounded by stormy waves and tossing tides. In early summer, currents of warmer water surge northward, under the Arctic ice. This  is when huge chunks start breaking from the shores. Adrift in the water, these floating icebergs are carried away by ocean currents.

Some of the Greenland bergs are captured by the Labrador Current that carries them southward toward Newfoundland. Here the water prows somewhat warmer and the icy monsters melt as they go. But during an average year, about 400 of them last long enough to drift south of Newfoundland and enter the shipping lanes of the North Atlantic. Some seasons are worse than others and more than 1,000 may get through.

Somewhere east of Newfoundland, the Greenland icebergs usually are captured by the warm Gulf Stream. This ocean current floats them eastward to the mid Atlantic, where they melt and add their fresh water to the salty sea.

One of the worst iceberg seasons was 1912, the great ship Titanic collided with one of the monsters and sank. Nowadays, the risk to shipping is less because the monsters are watched by the International Iceberg Patrol, with headquarters in Newfoundland. Every sizeable berg is spotted and its progress charted. The reports are broadcast daily to warn ships in the danger zone.