Carol Niasci, aged 11, of Somerville, Mass., for her question:

Will you please explain what a tidal wave is?

Some tidal waves are unexpected. They are caused by unexpected events. A hurricane can cause a tidal wave. Its furious winds whip the sea into monstrous waves. Water piles up in hills and is tossed ahead of the storm. One heaving wave starts another. The churning water spreads out like giant ripples from a giant stone in a giant pond. Masses of heaving water bounce upon a distant shore. The tidal waves from a hurricane sometimes arrive days ahead of the hurricane that caused them.

An ocean volcano can also cause a tidal wave. Sometimes a volcano erupts on the floor of the ocean. Sometimes it shakes the ground along a coastline. '"his churns up millions of tons of ocean water. Before this crater settles down it often slops over the land in a tidal cave. Waves ring out in all directions. Water piles up in hewing mounds. Each wave pushes at the wave ahead of it. Whole towns have been drowned before this water goes back to the bounds of the sea.

There are also tidal waves that are expected. In fact, most tidal waves arrive right on schedule. For they are part of the monthly tidal system that washes up and down the beaches. High tide reaches a different level each day of the month. The highest high tide of each month is called the spring tide. The lowest high tide of each month is called the neap tide.

A scheduled tidal wave can be expected, if it is to come, with the spring tide of the month. The highest spring tides of the year come in spring and fall. So that is when the biggest tidal waves are expected in tidal wave areas.

The shape of the land and the sea bed determines the size of the tides. Along flat beach may have a smaller tide than a rocky core ten miles away. This is because the flat beach offers no resistance to the incoming surge of tidal water,

Certain shorelines are shaped in such away that they hold back the advancing tides, for a time at least. Tidal water piles up in a great heap and comes crashing through. Such a tidal wave may look like a cliff or wall of advancing water. The biggest of these tides build up in the mouths of rivers and in fingers of sea which encroach into the land. There the tidal water piles high as it tries to break into a funnel. Such tides are called bores.

Find the river Ganges on your map of India. See the funnel of water at its mouth. Tidal waters pile up here in walls seven feet high. When they break through, they may rash down the river for 70 miles at almost 40 m.p.h. The river Amazon also has a tidal wave. This South American river empties into the Atlantic at the equator. Here the ocean tides battle with the river current and build up a tidal wave 15 feet high.

The highest tidal waves occur regularly in the Bay of Fundy which is between New Brunswick and Nova Scotia. Find this funnel of land and ocean on your map of Canada. The bore of this tidal gave advances in a wall of water maybe 60 feet high. Spectators watch it from the shore.