Jeff Thornton, age 14, of Phoenix, Arizona, for his question:

What are the sediments on the ocean floor?

The bedrock of the ocean floor is mostly hard, thick basalt. There are gentle slopes and smooth piainsy massive ridges and plunging rifts. The plains may be three miles below the waves, the rifts may be six miles. But no part of the seabed is sealed off peacefully from the rest of the world. Rivers wash down tons of silty clay and dump it onto the shallow slopes around the continents. Through the ages, it forms silty sediments of clay on the ocean floors. Undersea volcanoes pour out lavas and pile ashy sediments upon the basic bedrock. From the teeming surface waters, fragments of decaying plants and animals constantly filter down, down to the bottom. Near the continents, the assorted sediments are stirred by tides and rivers. Farther out to sea, deep ocean currents sweep tons of sediment into the rifts and canyons.

Ocean sediments contain most of the minerals found on land and often nuggets of metals. Some form vast areas of blue or brown clay. Some are oozy with the fatty organic material from soft bodied sea dwellers. Other vast sediments contain hard substances from crusty shellfish and the countless shells of microscopic diatoms. The seabed has been gathering layers of assorted sediments for billions of years, and every century the layers grow thicker.