Daniel Pizzo, age 11, of Cape Elizabeth, Maine, for his question:

What exactly is the continental slope?

The continental slope and the continental shelf are geographical features of the ocean floor. Some people refer to them as though they were one and the same, but they are not. However, the difference may surprise you. In most places, the continental shelf is a gentle slope from the shores out to deeper water. The continental slope plunges almost straight down to the ocean abyss.

Picture a map of the world and remove the shining waters from the deep blue seas. The floor of the oceans is revealed as a continuation of the solid crust that forms the continents. And the sea bed has more ups and downs, more stupendous geographical features than the dry land. Among the most interesting ones are the continental shelves and the continental slopes, which go together.

Normally the average height of the bumpy continents is about half a mile above the seas which slop over the continents' edges. The borders of the continents are like doorsteps, submerged by fairly shallow tidal waters.

These doorsteps slope gently from the beaches out to sea. where at some point they plunge down the steep walls of the great ocean basins. No, the sloping sills around the continents are not the continental slopes. They are the continental shelves, vast regions of rather shallow sunlit water that teem with seaweeds and multitudes of marine animals. The continental slopes are the stupendous walls that contain the great ocean basins.

Geographically, the shelves belong to the continents. They are submerged because the world's supply of ocean water brims over its basins and slops over the edges of the land. Geographically, the continents end where the shallow shelves plummet one, two or three miles down the steep walls of the great ocean basins. These sudden drops in the seabed are called the continental slopes.

Off the ancient coast of Maine, the continental shelf slopes gently out to sea for 150 miles or so, and then the sea bed dips down the slope to the huge basin of the Atlantic Ocean. America's Pacific shores are bordered with youngish mountains and here the shelves reach out only 20 miles or so to where the slopes plunge down the basin walls.

In some regions, the restless waters of the shallow shelves wash masses of debris down into the basins. This material may pile up at the base of the steep slopes. But in many regions, the continental slopes are sheer escarpments, with majestic straight sided cliffs, cut with enormous canyons.

Sunlight filters through the surface waters but it cannot pierce down through the deep ocean abyss. Hence, the stupendous continental slopes exist forever in total darkness. If we could remove the water and see them, they might remind us of the edges of the Grand Canyon ¬only more so.