IN ORDER to grasp the meaning of the things seen by researchers on the Alvin, we need a little insight into the way the earth is made. The ground beneath our feet is understood to be made up of a rigid layer (called the lithosphere) that rests on a mass of molten, slow-flowing rock. Apparently, this rigid outer layer averages some 60 miles [100 km] in depth and makes up only about 0.6 percent of the volume of the planet. The outermost part of it, the crust, is uneven, thicker beneath the continents and as thin as 3.5 miles [6 km] beneath the mid-ocean ridge system.

Furthermore, this solid outer shell is not one piece, like the shell of an undamaged egg. Instead, it appears to be broken into a number of large, rigid plates and many smaller ones, all of which are called tectonic plates. These make up the continents and ocean basins. The plates move in relation to one another. Where they draw apart, they thin out and form the rifts of the mid-ocean ridges. Worldwide, plates move at an average of about one inch [3 cm] per year.

[Diagram/Picture on page 4, 5]

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Earth’s crust

Mantle (partly molten)

Trench

Subduction zone

Tectonic plate

Rift

When plates move apart, rifts form

[Picture]

The mid-ocean ridge system weaves around the earth like the seam on a tennis ball

[Credit Line]

NOAA/Department of Commerce